Available online at www.sciencedirect.com Acta Materialia 57 (2009) 5667–5680 www.elsevier.com/locate/actamat High-temperature stability, structure and thermoelectric properties of CaMn1Àx Nbx O3 phases Laura Bocher a, Myriam H Aguirre a, Rosa Robert a, Dmitry Logvinovich a, Snejana Bakardjieva b, Jiri Hejtmanek c, Anke Weidenkaff a,* b a Empa, Solid State Chemistry and Catalysis, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland Department of Solid State Chemistry, Institute of Inorganic Chemistry of the ASCR, CZ-250 68 Rez, Czech Republic c Institute of Physics of the ASCR.s, CZ-162 00, Prague 6, Czech Republic Received 13 February 2009; received in revised form 25 July 2009; accepted 29 July 2009 Available online 15 September 2009 Abstract Polycrystalline perovskite-type CaMn1Àx Nbx O3 phases (with x ¼ 0:02; 0:05; 0:08 and 0.10) were investigated with regard to their structure, microstructure and thermal stability as a function of temperature The studied phases revealed a complex microstructure at room temperature with 90 twinned domains At high temperatures, the manganate phases underwent a structural transition from orthorhombic to cubic symmetry, as confirmed by in situ high-temperature X-ray powder diffraction and electron diffraction data Thermogravimetric heating/cooling studies showed a reversible thermal reduction/reoxidation process that occurred above a defined transition temperature A possible mechanism relating the high-temperature structural transition and the thermal reduction process of slightly substituted CaMnO3 phases was proposed The thermal reduction process resulted in a change in the Mn3þ =Mn4þ concentrations in the Mn sublattice, and therefore in a modification of the transport properties A comprehensive study examined the impact of both phenomena on the electrical and thermal transport properties Ó 2009 Acta Materialia Inc Published by Elsevier Ltd All rights reserved Keywords: Perovskites; Thermoelectricity; Structural transition; Thermal reduction; Twinned domain microstructure Introduction Transition metal oxides are currently of significant interest for the development of renewable energy technologies such as solid oxide fuel cells, thermoelectric (TE) modules and high-temperature superconductors [1,2] Manganates with a perovskite-type structure represent a family of oxides with various remarkable properties, such as ferromagnetism, metallicity and spin/charge ordering phenomena [3] Several CaMnO3 -related perovskite-type phases have been the subject of numerous studies exploring diverse cationic substitutions and/or oxygen deficiencies [4–6] and studying their physical, chemical and thermoelectric prop- * Corresponding author Tel.: +41 79 751 6883; fax: +41 44 823 40 41 E-mail address: anke.weidenkaff@empa.ch (A Weidenkaff) erties [7–10] CaMnO3 crystallizes at room temperature in the orthorhombic with pffiffiffi Pnma S.G., i.e with the pffiffisymmetry ffi cell parameters 2ac à 2ac à 2ac (ac refers to the cubic crystal structure), involving successive cooperative rotations of MnO6 octahedra around the h1 1i crystallographic axis At high temperatures (T > 1180 K), a structural transition of CaMnO3 yields the cubic crystal structure, i.e a higher space group symmetry [11] One main structural divergence between the cubic and the orthorhombic symmetry concerns the buckling bond angles (B–O–B), expressed as h, which decrease below 180° in the orthorhombic crystal structure while remaining equal to 180° in the cubic symmetry [12] The deviations of the (B–O–B) bond angle from 180° modify the transition metal d bandwidth characterized by the integral hopping W, since W / cos h [13] For instance, a lower bending of the (B–O– B) bond angle from 180° implies a smaller overlap of the O 2p and the transition metal 3d orbitals This results in a 1359-6454/$36.00 Ó 2009 Acta Materialia Inc Published by Elsevier Ltd All rights reserved doi:10.1016/j.actamat.2009.07.062 5668 L Bocher et al / Acta Materialia 57 (2009) 5667–5680 narrowing of the d band and therefore influences the electronic transport properties of perovskites Twinned domains are frequently observed in orthorhombic perovskite-type structures [14–16] Twinning phenomena arise when orthorhombic unit cells grow in different directions accommodating the slight discrepancy between lattice parameters, i.e ao ’ co in the case of Pnma S.G (ao and co refer to the orthorhombic crystal structure), associated with a low octahedral rotation angle u value [17] Hence, thermally induced structural transitions which involve changes in space group symmetry results in modifications of the electronic structure and of the microstructure in the perovskite phases In manganate phases, high-temperature structural transitions are often related to the formation of oxygen vacancies upon heating [18,19] Oxygendeficient manganate phases have been studied in detail regarding their structures [20,21] and microstructures [4], emphasizing the role of the oxygen vacancies in the modification of the crystal structure The electrical transport properties of CaMnO3Àd phases vary with the extent of the oxygen deficiencies, as previously reported by Taguchi [22] Among the manganate phases, A- and B-site substituted CaMnO3 exhibit a rich phase diagram [23], implying mixed-valence Mn4þ and Mn3þ cations with 3d and 3d electronic configurations, respectively CaMnO3 is an antiferromagnetic insulator and exhibits large Seebeck coefficients, i.e S ¼ À800 lV KÀ1 at 300 K [24] A- or B-site aliovalent substitutions in the CaMnO3 system induce the creation of Mn3þ cations in the Mn4þ matrix, resulting in a n-type conduction Good TE materials should display a high figure of merit Z; Z ¼ S =qj, i.e a large Seebeck coefficient S, a low electrical resistivity q and a low thermal conductivity j [25] Recent studies on new promising ntype manganate phases, CaMn1Àx Nbx O3 (with x 0:08), reveal a ZT value of 0.32 at 1070 K as a result of a low lattice thermal conductivity [26] The study shows that specific internal boundaries and interfaces, which are induced by the presence of twinned domains, might decrease the thermal conductivity due to phonon scattering while keeping the electronic properties largely undisturbed To our knowledge, the electrical and thermal transport properties of both A- and B-site substituted CaMnO3 have been largely studied only up to T ’ 1000 K [8,24,27–31]; the higher temperature range (T > 1100 K) has not been explored so far with regard to their TE properties Since oxide materials are evaluated regarding their high-temperature thermoelectric applications, the thermal stability of the studied phases is of main interest The present paper reports on the study of the CaMn1Àx Nbx O3 phases (with x ¼ 0:02; 0:05; 0:08 and 0.10) with respect to its crystal structure, microstructure and thermal stability at high temperatures The study focuses on the influence of the hightemperature structural transition, the oxygen vacancy formation, and the effects on the electrical and thermal transport properties in the temperature range of 600 K < T < 1250 K Experimental Polycrystalline CaMn1Àx Nbx O3 (with x ¼ 0:02; 0:05; 0:08 and 0.10) perovskite-type phases were synthesised by a ”chimie douce” synthesis method (abbreviated SC, for soft chemistry method) [32] The SC synthesis process has already proven to be a successful method for synthesizing cobaltate- [33], titanate- [34] and manganate- [10] phases at relatively low temperatures (T < 1000 K) The SC synthesis route is based on the thermal decomposition of complex polymeric precursors, where the cations are homogenously premixed in aqueous solution with citric acid acting as chelating agent The detailed experimental procedure is described elsewhere [26] Completeness of reaction and phase purity were controlled at room temperature by X-ray powder diffraction (XRPD) using a PANalytical X’Pert PRO MPD H–H diffractometer equipped with a linear detector X’Celerator The high-temperature laboratory data were collected on a PANalytical X’Pert system equipped with a Johansson monochromator (Cu Ka1) and a linear X’Celerator detector An Anton Paar HTK 1200 environmental heating chamber was used for the in situ XRPD studies The data were collected from 298 to 1273 K under air atmosphere during isothermal steps at the required temperature Each scan was recorded in an angular range of 20 < 2h < 83 , with a 0.008° step size and a counting time of 40 s per step Prior to each measurement, the sample was heated at the desired temperature with a heating rate of 20 K minÀ1 followed by a isothermal step to perform a fast control scan The high-temperature crystallographic parameters were obtained from Rietveld refinements of the XRPD data acquired at different temperatures during in situ measurements Rietveld refinements [35] were performed using the FULLPROF program [36] The reflection shape was modelled by a Thompson–Cox–Hastings pseudo-Voigt profile function [37] corrected for asymmetry [38,39] and the background by a six-coefficient polynomial function Electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM) studies were carried out using a Philips CM30 electron microscope, operating at 300 kV (double tilt ± 30°) The high-temperature ED studies were performed on a JEOL JEM 3010 electron microscope, operating at 300 kV, using a Gatan model 652 double tilt heating sample holder (±30°) TEM samples were prepared by dispersing ground powders in ethanol and depositing them onto a holey carbon film supported by a Cu ring for the room-temperature experiments and a Mo ring for the high-temperature study The polycrystalline powders were pressed into barshaped pellets and sintered in air at 1473 K for h The resistivity and thermopower measurements were performed on a bar-shaped sample ð1:5   mm3 Þ between 320 and 1240 K under air atmosphere using a RZ2001i unit from Ozawa Science (Japan) The detailed experimental procedure was previously described [10] L Bocher et al / Acta Materialia 57 (2009) 5667–5680 The thermal stability of the materials was studied using a Netzsch STA 409 CD thermobalance In situ thermogravimetric analysis was performed upon two successive heating/cooling cycles under a synthetic air atmosphere (20 vol.% H2 /He) During each thermal cycle, the sample was first heated up to 1253 K at a heating rate of 10 K minÀ1 , maintained at the desired temperature for a h isothermal step and finally cooled down to 500 K at a cooling rate of 10 K minÀ1 The specific heat capacity was determined by differential scanning calorimetry (DSC) using a Netzsch DSC 404 C Pegasus The polycrystalline sample was heated from room temperature to 1273 K at a heating rate of 20 K minÀ1 under synthetic air atmosphere (20 vol.% O2 /He) with a gas flow rate of 50 ml minÀ1 Three experiments were performed successively to determine the specific heat by the ratio method: the baseline, the standard sample with a known Cp (sapphire), and the polycrystalline sample Results and discussion 3.1 Structural characterization Figs 1a and b present the refined high-temperature XRPD patterns of CaMn0:98 Nb0:02 O3 at 773 and 1173 K, respectively The reflections observed at 773 K can be indexed in the orthorhombic unit cell (Pnma S.G., N° 62), while the XRPD data recorded at 1173 K can be refined using the cubic structural model (Pm3m S.G., N° 221) The orthorhombic perovskite structure is generally described as a pseudo-cubic framework of corner-sharing BO6 octahedra with the A cations in cuboctahedral coordination[12] The relationship between orthorhombic and cubic crystal pffiffiffi structures can be defined as follows: ao ’ co ’ ac and bo ’ 2ac , in case of the Pnma S.G (For convenience, information related to the orthorhombic and cubic structures are labeled with ”o” and ”c” sub- (a) scripts, respectively.) In the studied temperature range, the main phase is assigned either to the orthorhombic or to the cubic structure A small amount of CaMn2 O4 ð’ 2:3%Þ is detected as secondary phase for all the studied temperatures The marokite-type phase, CaMn2 O4 , crystallizes in the orthorhombic structure (Pbcm S.G., N° 57) with lattice parameters of ˚ bo ¼ 9:99A ˚ and co ¼ 9:68A ˚ [40] Fig 2a depicts ao ¼ 3:15A; the temperature evolution of the CaMn0:98 Nb0:02 O3 orthorhombic phase The angular range of 36 < 2h < 43 is emphasized in Fig 2b since this region includes superlattice reflections sensitive to the orthorhombic structure and its related tilting system With increasing temperature, the ð2 1ÞPnma ; ð1 2ÞPnma reflections and the ð2 1ÞPnma ; ð1 2ÞPnma ; ð1 1ÞPnma triplet decrease in intensity, disappearing at 1173 K The ð2 0ÞPnma and ð0 2ÞPnma doublet merges to a single ð1 1ÞPmÀ3m reflection at 1173 K These results indicate that CaMn0:98 Nb0:02 O3 undergoes a structural transition from an orthorhombic to a cubic crystal structure at 1073 K < T S < 1173 K in air (hereafter, T S corresponds to the structural transition temperature) Previous studies on CaMnO3 [11] reported the existence of successive structural transitions from orthorhombic (Pnma) to tetragonal (I4/mcm) at 1169 K ðo ! tÞ and further to cubic (Pm-3m) at 1186 K ðt ! cÞ (The tetragonal structure is labeled with a “t” subscript.) In the present case, all orthorhombic superlattice reflections disappear simultaneously at the transition temperature without any intermediate tetragonal phase being present The apparent absence of such intermediate transitions for the CaMn0:98 Nb0:02 O3 phase confirms a limited stability range of the tetragonal phase Thus, the structural transition temperature determined from the experimental data, i.e 1073 K < T S < 1173 K, is consistent with the literature regarding the well-known transition from an orthorhombic to a cubic crystal structure upon heating in perovskite-type phases The refined structural parameters, atomic positions and (b) at 773 K, Pnma S.G 40 -3 20 -3 Intensity x 10 (Counts) 30 10 20 at 1173 K, Pm-3m S.G 40 Intensity x 10 (Counts) Experimental Fitting Diff Exp - Fitting Bragg positions 5669 Experimental Fitting Diff Exp - Fitting Bragg positions 30 20 10 30 40 50 60 Angle / 2θ (degrees) 70 80 20 30 40 50 60 Angle / 2θ (degrees) 70 80 Fig Examples of Rietveld refinements from in situ high-temperature XRPD data of the CaMn0:98 Nb0:02 O3 phase recorded at (a) 773 K and (b) 1173 K under air atmosphere (b) Pnma * (220) (022) (a) (211) (112) (131) L Bocher et al / Acta Materialia 57 (2009) 5667–5680 (201) (102) 5670 298 K * Intensity (a u.) Intensity (a u.) 773 K 298 K 773 K 973 K 1073 K 1173 K 973 K 1273 K Pm-3m 1173 K 20 30 40 50 60 70 (111) 1073 K 80 37 38 Angle / 2θ (degrees) 39 40 Angle / 2θ (degrees) 41 42 Fig (a and b) In situ high-temperature XRPD patterns of the CaMn0:98 Nb0:02 O3 phase from 298 to 1173 K under air atmosphere The arrows in (a) indicate the reflections disappearing during the phase transition The angular range of 36 < 2h < 43 is enlarged in (b), emphasizing the evolution of three main reflections, assigned by red circles, upon heating The minor reflections indicated by (Ã) belong to the secondary phase, i.e CaMn2 O4 The red dotted circles are only intended to emphasize the reflections of main interest (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) agreement factors from the high-temperature XRPD data are summarized in Table The evolution of the lattice parameters and cell volume upon heating are presented in Fig 3a The thermal expansion causes a near linear increase in lattice parameters for the whole temperature range studied As a consequence, a progressive expansion Table Refined structural parameters and atomic positions of the CaMn0:98 Nb0:02 O3 phase recorded at different temperatures during in situ high-temperature XRPD measurements Temperature 298 K 773 K 973 K 1073 K 1173 K S.G ˚) a (A ˚) b (A ˚) c (A ˚ 3Þ Va ðA Pnma 5.2884 (1) 7.4639 (1) 5.2748 (1) 52.05 Pnma 5.3125 (1) 7.5239 (1) 5.3153 (1) 53.12 Pnma 5.3288 (2) 7.5463 (1) 5.3310 (2) 53.73 Pnma 5.3426 (1) 7.5595 (1) 5.3363 (1) 53.88 Pm3m 3.7901 (1) 54.44 Ca x y z ˚ 2) Biso (A occ 0.0311 (3) 0.25 À0.0082 (9) 0.45 (3) 0.5 0.0196 (6) 0.25 À0.0038 (15) 1.543 (4) 0.5 0.0099 (8) 0.25 À0.0042 (20) 1.783 (4) 0.5 0.0052 (8) 0.25 À0.0023 (2) 1.736 (3) 0.5 0.5 0.5 0.5 2.41 (3) Mn/Nb x y z ˚ 2) Biso (A occ 0 0.5 0.30 (2) 0.49/ 0.01 0 0.5 0.77 (2) 0.49/ 0.01 0 0.5 0.60 (3) 0.49/ 0.01 0 0.5 0.55 (3) 0.49/ 0.01 0 0.92 (3) 0.98/ 0.02 Oap x y z ˚ 2) Biso (A occ 0.4875 (10) 0.25 0.0765 (19) 0.37 (2) 0.5 0.4817 (19) 0.25 0.0686 (25) 2.60 (3) 0.5 0.4704 (23) 0.25 0.0279 (27) 1.07 (3) 0.5 0.4675 (23) 0.25 0.0175 (30) 0.931 (3) 0.5 0.5 0 Oeq x y z ˚ 2) Biso (A occ 0.2861 (10) 0.0281 (10) 0.7084 (10) 0.50 0.2835 (22) 0.0295 (9) 0.7155 (22) 0.50 0.2866 (21) 0.0472 (9) 0.7205 (23) 0.439 (18) 0.2840 (19) 0.0421 (7) 0.7181 (17) 0.623 (16) 4.1 3.12 1.9 4.04 3.08 4.06 2.98 1.91 3.96 3.1 1.82 Rwp Rp v2 3.9 2.95 1.87 Atoms are located at the following Wyckoff positions: for the Pnma S.G.: Ca: 4(c), Mn/Nb: 4(b), Oap : 4(c) and Oeq : 8(d); for the Pm3m S.G.: Ca: 1(a), Mn/ Nb: 1(b), O: 3(c) a The cell volumes are calculated based on the cubic cell L Bocher et al / Acta Materialia 57 (2009) 5667–5680 Pnma (a) Pm-3m 5.37 55.5 5.36 55.0 (c) Orthorhombic (Pnma) 5.35 54.5 5.34 5.33 54.0 5.32 53.5 53.0 Mn O Mn bo 5.31 Cell volume (Å ) Lattice parameters (Å) 5671 5.30 52.5 5.29 Orthorhombic cell ao bo/√2 co 5.28 5.27 300 400 500 600 700 800 Cubic cell ac*√2 ao 52.0 co 51.5 900 1000 1100 1200 Temperature (K) Pnma (b) 1.925 Average (Mn-O-Mn) angles Average (Mn-O) bond lengths 180 (d) Cubic (Pm-3m) 1.920 1.915 175 1.910 170 1.905 165 1.900 160 Mn Average (Mn-O) bond lengths (Å) Average (Mn-O-Mn) angles (°) Pm-3m O Mn bc ac cc 1.895 1.890 155 300 400 500 600 700 800 900 1000 1100 1200 Temperature (K) Fig Temperature dependences of (a) lattice parameters, cell volumes, (b) average (Mn–O–Mn) angles and (Mn–O) bond lengths of the CaMn0:98 Nb0:02 O3 phase Schematic representations of (c) the orthorhombic (Pnma) and (d) the cubic (Pm-3m) crystal structures, respectively The images on the left depict the (c) distorted and (d) undistorted unit cells, respectively The drawings on the right illustrate the overlap of the O 2p and the Mn 3d orbitals, pffiffiffi i.e the pffiffiffi (c) curved and the (d) linear (Mn–O–Mn) orbitals, respectively (The lattice parameters are presented along the orthorhombic frame, where ao = ’ co = ’ ac and bo =2 ¼ ac The cell volumes are calculated based on the cubic cell.) of the cell volume is observed corresponding to a thermal expansion of 4.40% upon heating from 298 to 1173 K Table presents the Mn–Oapical=equatorial distances and the distortion angles at the different temperatures Fig 3b reports on the evolution of average bond lengths (Mn–O) and bond angles (Mn–O–Mn) upon heating A continuous increase in both structural parameters is observed until the structural transition temperature At higher temperatures, i.e above T S , refined bond lengths and angles correspond to values characteristic of the cubic symmetry It should be pointed out that both symmetries, i.e orthorhombic and cubic, present divergences regarding their structural parameters, i.e atomic displacement variations, and changes in bond lengths and angles The ideal cubic perovskite structure, with Pm3m S.G., rarely exists; often the perovskite structure distorts from the ideal cubic frame Table Bond distances and distortion angles of the CaMn0:98 Nb0:02 O3 phase recorded at different temperatures during in situ high-temperature XRPD measurements Temperature 298 K 773 K 973 K 1073 K 1173 K S.G ˚ Mn À Oap  ðAÞ ˚ Mn À Oeq1  ðAÞ ˚ Mn À Oeq2  ðAÞ Mn À Oap À Mn ð Þ Mn À Oeq À Mn ð Þ Pnma 1.910 (2) 1.882 (6) 1.920 (6) 155.43 (9) 158.3 (2) Pnma 1.919 (3) 1.905 (11) 1.912 (11) 157.19 (13) 159.7 (4) Pnma 1.899 (2) 1.957 (11) 1.899 (11) 166.77 (7) 155.6 (4) Pnma 1.900 (2) 1.939 (10) 1.922 (9) 168.15 (5) 155.8 (4) Pm3m 1.895 (1) 180.00 (1) 5672 L Bocher et al / Acta Materialia 57 (2009) 5667–5680 arising to orthorhombic or rhombohedral symmetry The distorted derivative phases result from different mechanisms One of the most common distortion mechanisms is based on the rotation or tilting of the BO6 octahedra Due to the small radius of the A site cation with respect to its surrounding frame, the BO6 octahedra tilt and buckle to accommodate the size of the cubo-octahedral void The cooperative rotation of BO6 octahedra around different axes yields the orthorhombic, so-called GdFeO3 -type structure In the present case, successive cooperative rotations of the MnO6 octahedra around the h1 1i crystallographic axis lead to the orthorhombic crystal structure with Pnma S.G Fig 3c and d depict the low-temperature distorted orthorhombic and the high-temperature undistorted cubic structural model, respectively Evidently, structural deviations emerge between the orthorhombic and the cubic symmetry Schematic representations of the bent and linear (Mn–O–Mn) orbitals are presented in Fig 3c and d (at right), corresponding to the orthorhombic and the cubic symmetry, respectively Consequently, the ðo ! cÞ structural transition induces certainly changes in the overlap between the O 2p and Mn 3d orbitals Such variation in the electronic band structure leads to modifications of the electrical and thermal transport properties The XRPD investigations yield a general overview of the crystal structure and its evolution with increasing temperature In addition, TEM studies give a better insight into the local microstructure of the studied materials As recently reported, nano-sized twinned domains in polycrystalline manganate phases can strongly influence their related thermal properties, and therefore their TE activities [26]; TEM studies are thus of major importance Fig illustrates the presence of nano-sized twinned domains in the CaMn0:98 Nb0:02 O3 phase Typical crystallites of 200– 300 nm size were studied The experimental ED pattern (Fig 4b) can be considered as the superposition of two orthorhombic zone-axis ½1 1Šo and ½À1 1Šo , rotated at 90° from each other ED patterns of each domain area were calculated by a Digital Micrograph 3.8.2 Gatan software in fast Fourier transform (FFT) mode The FFTs of the A1 and B1 domains are given in the inset of Fig 4a The experimental ED pattern can be indexed with a near cubic 2ac  2ac  2ac lattice but does not obey cubic symmetry relations Forbidden reflections, i.e (0 k l) with k þ l ¼ 2n þ 1, have emerged, as indicated by the blue circles The presence of an additional set of (forbidden) diffraction spots facilitates the identification of the twin orientations, as indicated in Fig 4a The FFT of the B1 domain is almost identical to that of the A1 domain, except that the (0 k 0) row of reflections has rotated clockwise about the normal to (1 1) by 90° (on the ED pattern the blue cirlces become the green ones) This finding indicates that moving from A1 to B1 across the domain boundary is similar to rotating the orthorhombic unit cell by 90° with respect to ½0 0Šo , defined by ½0 0Š90 This specific microstructure is commonly described as rotation twins across f1 1go [15] The orthorhombic lattice parameters deviate slightly from pffiffiffithe equivalent cubic ones, i.e pffiffiffi ac  2ac  ac in the orthorhombic framework Therefore, domains with specific orientations of the orthorhombic unit cell can coherently grow on each other, yielding a twinning phenomenon However, the coexistence of twinned domains is directly correlated to the magnitude of the octahedra rotation angle u around ½1 1Šc , by which an upper limit ulim exists where twins are not observed [41] In cobaltate perovskite-type phases LnCoO3 , the DyCo0:95 Ni0:05 O3 phase does not reveal twins and presents a u ¼ 18:78 while the PrCo0:95 Ni0:05 O3 compound exhibits a twinned microstructure with a u ¼ 10:14 [16] Likewise, in rare-earth orthoferrites LnFeO3 , the proportion of Fig (a) HRTEM, (b) ED pattern of the CaMn0:98 Nb0:02 O3 phase and (c) magnified view of the nano-sized twinned domains where the b b axis is oriented along two perpendicular directions in the plane The inset figures in (a) present the FFT of the domains A1 and B1 A schematic interpretation of the experimental ED pattern is shown at the top right L Bocher et al / Acta Materialia 57 (2009) 5667–5680 5673 Fig (a–c) HRTEM images of the CaMn0:98 Nb0:02 O3 phase along a twinned domain boundary Highlights (a) the A2 domain, and the boundaries separating (b) the domains A2 and B2 and (c) the domains B2 and D2 The inset figures present the FFT of the domains A2 and B2 Dotted arrows indicate domain boundaries and dashed arrows point out the planar faults twinned domains is related to the octahedra rotation angle, resulting in an upper limit of ulim ¼ 17:2 [42] In the present study, the octahedra rotation angle u can be determined from the structural parameters [26] as follows For the Oð2Þ located at the Wyckoff position 8(d), the atomic position y is defined by the Eq (1) [43]: pffiffiffiffiffi y ¼ Àðtan uÞ= 48 ð1Þ resulting in u ¼ 13:14 for the CaMn0:98 Nb0:02 O3 phase This result corroborates the presence of twinned domains evidenced in Fig 4a The combination of similar lattice parameters, i.e ao ’ co , and a low octahedra rotation angle u is a prerequisite for the formation of twins in orthorhombic crystal structure [17] Various microstructural features, such as twinned domains, are frequently generated during phase transitions that involve a change in space group symmetry In perovskite-type phases, the low-temperature phases often reveal lower symmetry than the high-temperature phases [44] If the low symmetry phase belongs to a subgroup of the high symmetry one, a symmetry-breaking transition induces domain microstructures [14] For example, orthorhombic perovskite-type phases, e.g CaTiO3 (Pbnm S.G.), undergo transitions to the cubic phase (Pm3m S.G.) upon heating The presence of 90° rotation twins about ½0 1Šo and reflections twins on f1 0go and f1 2go characterized the CaTiO3 phase at room temperature [45] In the present study, the high-temperature structural transformation from cubic to orthorhombic crystal structure of the CaMn0:98 Nb0:02 O3 phase upon cooling induces 90° rotation twins, favored by the octahedral tilting characteristic of the distorted perovskite structure A second example of specific microstructure observed in the phase is illustrated in Fig 5a, where the presence of several domain boundaries, assigned as A2 À B2 ; B2 À D2 and A2 À C2 , are indicated by dotted arrows The A2 À B2 domain boundary is highlighted in Fig 5b and their FFTs are given in the inset Fig 5a They allow to confirm the presence of twinned domains where the ^b axis is either oriented out of the plane ðA2 Þ or in the plane ðB2 Þ As previously determined, the ^b axis rotates by 90° across ½1 1Šo Between the B2 and D2 regions, characteristic features of twinned domains are not observed, as presented in Fig 5c In both domains, the ^b axis is oriented in the same direction and the lattice fringes present similar values (b $ 0.74 nm) Contrast differences, which should emerge between twinned domains, are not observed between both regions However, the B2 –D2 defect boundary appears clearly with an unusual interdistance equivalent to twice the ^b axis length This domain boundary between the B2 and D2 regions might be due to an intergrowth Ruddlesden–Popper planar defect involving anti-phase boundaries [46,47] Ruddlesden–Popper series are well known as layered perovskite-type structures Anþ1 Bn O3nþ1 with n the number of AO layers Thus, perovskite-type manganate phases such as CaO½ðCaMnO3 Þn Š could be a planar defect structure localized between the B2 and D2 regions This assumption indicates that the main phase is a slightly Carich phase The counterpart Mn-rich phase might therefore be the secondary phase, i.e CaMn2 O4 is revealed as being present in a small quantity by Rietveld refinements The A2 –C2 domain boundary certainly results from similar planar defects 5674 L Bocher et al / Acta Materialia 57 (2009) 5667–5680 In addition to the high-temperature XRPD measurements, in situ high-temperature ED of the CaMn0:98 Nb0:02 O3 phase was performed focusing on a zone axis which comprises information about the long ^ b axis, characteristic of the orthorhombic structure with Pnma S.G In Fig 6a the experimental ED pattern, recorded at room temperature, is indexed in the orthorhombic phase with the ½1 À 1Šo zone axis The additional set of diffraction spots, indicated with red circles, confirms the ^b axis orientation Examining high-order reflections, e.g ðÀ2 À À 2Þo or ðÀ1 À À 1Þo , indicated by arrows, spots splitting by 2U % 0:295 can be distinguished Spot splitting is caused by a slight discrepancy in the lattice parameters values between the ao and co axes in the present case The spot splitting of the two superposed domains can be calculated by the following Eq (2) [48]:   p co ð2Þ U ¼ À arctg ao where ao and co are the lattice parameters of the orthorhombic cell The high-temperature ED pattern, shown in Fig 6b, can be indexed in the cubic perovskite structure, i.e with ½0 1Šc zone axis, since any additional set of diffraction spots is observed The diffraction patterns were carefully recorded on the same region of the particle at room temperature and at T ¼ 770 K This result confirms the evidence of a structural transformation from an orthorhombic to a cubic structure with increasing temperature, as concluded from the structural Rietveld refinements Since the [0 0] 90° rotation twins are a characteristic microstructure of the orthorhombic structure, the detwinning phenomenon occurs by reduction of the fourfold rotation axis during the ðo ! cÞ structural transition upon heating [15] 3.2 Thermal cycles – Influence on the electrical and thermal transport properties The potential thermoelectric manganate phases, i.e CaMn1Àx Nbx O3 ðx 0:10Þ, were investigated by thermogravimetric analyses regarding their thermal stability at high temperatures Successive in situ thermal cycles were performed on the CaMn0:98 Nb0:02 O3 phase under synthetic air atmosphere until T ¼ 1253 K Fig 7a reveals a weight loss upon heating and a gain of weight upon cooling, recovering the starting sample weight The loss/gain of weight is related to a reversible oxygen release/uptake, respectively, since the cationic composition remains constant during the thermal experiments The subsequent and reproducible thermal reduction and reoxidation processes occur above T HT ¼ 1090 K (Hereafter, the onset Fig In situ high-temperature ED of the CaMn0:98 Nb0:02 O3 phase recorded at (a) 298 K and (b) 770 K Schematic interpretations of the ED patterns are represented below the figures L Bocher et al / Acta Materialia 57 (2009) 5667–5680 5675 102.5 1400 (a) 102.0 3.15 1200 101.5 3.10 3.00 800 0.045 < δ < 0.049 2.95 2.90 600 100.5 100.0 99.5 99.0 Weight loss (%) CaMn0.98Nb0.02O2.98 Oxygen content Temperature (K) 101.0 3.05 1000 98.5 CaMn0.98Nb0.02O2.98-δ 2.85 98.0 400 200 100 200 Weight loss (%) 101.0 400 500 600 97.5 2.75 97.0 Time (min) 101.5 (b) 300 2.80 x = 0.02 x = 0.05 x = 0.08 100.5 100.0 99.5 THT = 1090 K THT = 1145 K THT = 1180 K 99.0 98.5 98.0 400 600 800 1000 1200 Temperature (K) Fig In situ thermogravimetric analyses of (a) CaMn0:98 Nb0:02 O2:98 and (b) CaMn0:98 Nb0:02 O3 (with x ¼ 0:02; 0:05 and 0.08) phases upon heating and cooling cycles under synthetic air atmosphere Two successive thermal cycles including a h isothermal step at T ¼ 1253 K are presented in (a) temperature relating to the thermal reduction will be termed T HT ) From the thermogravimetric data, the oxygen deficiency d is determined to be equal to d ¼ 0:470 Æ 0:002 at 1253 K No further oxygen release is observed while keeping the compound at 1253 K for a h isothermal step Similar investigations on the CaMn1Àx Nbx O3 phases present a shift of T HT to higher temperatures with increasing the Nb content, i.e for x ¼ 0:05 : T HT ’ 1145 K and for x ¼ 0:08 : T HT ’ 1180 K, as reported in Fig 7b Previous studies on the CaMnO3 phase reveal a thermal instability above 930 K under air and oxygen atmospheres, reflecting the formation of oxygen vacancies [19] Inserting cations having higher electronegativity, i.e vMn ¼ 1:55 and vNb ¼ 1:60, reduces the tendency to form oxygen vacancies Hence, the thermal stability of the CaMn1Àx Nbx O3 phases can be tuned depending on the Nb substitution levels Furthermore, the onset of the thermal reduction temperature T HT apparently corresponds to the structural transition temperature T S For instance, the XRPD results of the CaMn0:98 Nb0:02 O3 phase agree with the corresponding thermogravimetric analyses Dabrowski et al [19] previously argued that the oxygen vacancy formation is directly related to the structural transition from orthorhombic to cubic symmetry in the Sr1Àx Cax MnO3 phases Based on this hypothesis, a possible mechanism relating the structural transition and the thermal reduction is proposed On the one hand, detwinning phenomena cause structural reconstructions where domain boundaries disappear to accommodate the cubic structure On the other hand, a small amount of oxygen might be released at the twinned domain 5676 L Bocher et al / Acta Materialia 57 (2009) 5667–5680 boundaries Hence, the oxygen vacancy formation and the detwinning of the orthorhombic structure might be interdependent phenomena The in situ high-temperature TEM investigations were performed under vacuum, therefore a possible thermal reduction could already occur at 500 K < T < 700 K, inducing a structural transformation of the phase at a significantly lower temperature than under ambient conditions Thus, the ED pattern recorded at 770 K could be indexed as a cubic perovskite structure under vacuum conditions, while the structural transition temperature has been defined at 1073 K < T c < 1173 K from the high-temperature XRPD studies under ambient conditions It should also be pointed out that the present study focused mainly on the CaMn0:98 Nb0:02 O3 phase since the available equipments can only be used up to 1273 K For instance, the 2% Nb-containing CaMnO3 phase presents the lowest thermal reduction temperature among the studied phases The occurrence of both, a structural transition and a reversible thermal reduction/reoxidation at a defined temperature, can influence the physical properties of the studied phases, especially at operating temperatures characteristic of the thermoelectric oxide modules In the present study, the electrical and thermal transport properties were investigated with regard to the role of (i) the structural transition and (ii) the thermal reduction/reoxidation, or (iii) both phenomena simultaneously Potential thermoelectric materials have to present a large Seebeck coefficient S, also called thermopower The Seebeck coefficient depends strictly on the electronic struc- ture of the material S is not affected by the morphology or the microstructure Its sign indicates the character of charge carriers in a solid Fig presents the temperature dependence of the Seebeck coefficient for the CaMn1Àx Nbx O3 series (for x ¼ 0:02; 0:05; 0:08 and 0.10) Equal thermopower values are obtained upon heating and cooling under air atmosphere This finding indicates that no gradual deterioration of the compounds and their related TE properties occurs after several thermal cycles up to 1273 K Two different S(T) behaviors can be distinguished in the studied temperature range for all compositions The low-temperature region presents a nearly linear temperature dependent thermopower, characteristic of a polaronic conduction [26] A drop of the absolute value of the Seebeck coefficient is observed above a certain temperature, as indicated by the dotted line in the figure The comparison of the thermopower curves for all the studied compositions suggests that an increase in the Nb concentration yields a shift of the S(T) drop to higher temperatures, e.g from T HT ¼ 1090 K for x ¼ 0:02 to T HT ¼ 1200 K for x ¼ 0:10 In analogy to the thermogravimetric results, this transition temperature can be assigned to the onset temperature of the thermal reduction process T HT The sudden change in S(T) slope at T ’ T HT indicates a modification of the electronic conduction in the hightemperature region The Nb5þ insertion in the CaMnO3 insulating phase corresponds to an electron doping in the eg band [9] Thus, the decrease in jSj, above the transition temperature, can be interpreted either as a modification of the band structure due to the structural transition or -80 -1 Seebeck coefficient (μV K ) -100 x = 0.10 THT= 1200 K -120 -140 x = 0.08 * THT= 1200 K THT= 1180 K -160 -180 x = 0.05 * THT= 1145 K -200 -220 x = 0.02 * -240 -260 -280 600 THT= 1090 K CaMn1-xNbxO3 700 800 CaMn1-xNbxO3-δ 900 1000 1100 1200 Temperature (K) Fig Temperature dependences of the Seebeck coefficient for CaMn1Àx Nbx O3 (for x ¼ 0:02; 0:05; 0:08 and 0.10) while heating (closed symbols) and cooling (open symbols) under air atmosphere The dotted line is just intended to guide the eye with respect to the transition temperature evolution * Results for T < 1000 K are reported from a previous work [26] L Bocher et al / Acta Materialia 57 (2009) 5667–5680 as an increase in the Mn3þ concentration in the Mn4þ matrix related to the thermal reduction On the one hand, the structural transition from orthorhombic to cubic symmetry leads to a different overlap between the Mn 3d and the O 2p orbitals, which therefore influences the transport properties, as previously reported for cobaltate phases [49] On the other hand, the thermal reduction induces oxygen-deficient compounds in which the concentration of Mn3þ =Mn4þ varies to satisfy the electroneutrality The thermogravimetric studies previously indicated an oxygen deficiency of d ¼ 0:0470 Æ 0:002 for the high-temperature phase, i.e CaMn0:98 Nb0:02 O3Àd The Heikes formalism states that the thermopower should tend to in the high-temperature limit (where k B S Heikes ¼ À kjejB ln 1Àx x is the Boltzmann constant and x is the charge carrier concentration) [50], and can be applied at high temperatures for nearly constant S Assuming that, at high temperatures, the 2% Nb-containing manganate presents a Seebeck coefficient of S 1060K ¼ À231 lV KÀ1 , this corresponds to 6% of Mn3+ in the manganese sublattice This reduced compound has a S 1273K ¼ À152 lV KÀ1 , corresponding to 14.5% of Mn3þ in the Mn4þ matrix With respect to the electroneutrality, the and 14:5%Mn3þ -containing phases and correspond to the CaMn0:98 Nb0:02 O2:980 CaMn0:98 Nb0:02 O2:935 phases, respectively The difference in oxygen deficiency determined from the Seebeck coefficient data is equal to d ¼ 0:045, which fits well with the thermogravimetric results This finding confirms the influence of the thermal reduction process on the Mn3þ concentration and therefore on the thermopower temperature dependence As a result, upon cooling, the thermal reoxidation allows the starting Mn3þ concentration at room temperature to be recovered, inducing identical S(T) values after several thermal cycles Fig 9a presents the temperature dependence of the electrical resistivity for the CaMn1Àx Nbx O3 phases with x ¼ 0:02; 0:05, and 0.08 The qðT Þ curve of the 2% Nb-con- (a) taining CaMnO3 first presents a slope change at T ’ 800 K, leading to nearly constant resistivity values This could be considered as the onset of the structural transition At T > 1050 K, a decrease in qðT Þ is clearly observed and corroborates the thermopower drop previously noticed at this temperature, as indicated by the dotted line in the figure This last finding confirms the variation in charge carrier concentration in terms of an increase in the Mn3þ concentration above T HT A similar change in qðT Þ slopes is observed for the 5% Nb- and 8% Nb-containing CaMnO3 phases The electrical resistivity values converge to a metallic resistivity value of 16 mX cm for all studied phases, which can be related to the thermopower evolution in the high-temperature limit The increase in the charge carrier concentration due to thermal reduction of the CaMn1Àx Nbx O3 phases enhances their electrical conduction above T HT The influence of the oxygen deficiency on the electrical resistivity in the CaMnO3Àd phase has been previously reported by Taguchi [22] A shift of the resistivity drop to higher temperatures is observed with increasing Nb concentration, a common feature with the thermopower data, and further with the onset temperature of the thermal reduction The high-temperature TE properties of the CaMn1Àx Nbx O3 phases are clearly dependent on their thermal stability The heat capacity study performed on the 2% Nb-containing CaMnO3 phase is reported in Fig 9b The manganate phase undergoes two successive endothermic thermal events, at 830 K < T < 980 K and at T > 1085 K, upon heating to 1273 K in air atmosphere The second peak is energetically more significant than the first one Previous work on the CaMnO3Àd phase reported two endothermic peaks fitting with weight loss observed by thermogravimetric analysis [11] DSC methods allow enthalpy variations to be monitored and therefore structural transitions can be observed [51] However, DSC measurements are also sensitive to thermal reduction events since this process induces (b) 260 CaMn1-xNbxO3-δ CaMn1-xNbxO3 -2 4.0x10 x = 0.02 * -1 -1 -2 3.0x10 -2 2.0x10 x = 0.05 * 220 200 180 x = 0.08 * 16 mΩ cm at 1200 K 830 K < T < 980 K 140 100 80 0.0 600 THT = 1085 K 160 120 -2 1.0x10 240 Heat capacity (J mol K ) Electrical resistivity (Ohm cm) 5677 800 1000 Temperature (K) 1200 CaMn0.98Nb0.02O3 400 600 800 CaMn0.98Nb0.02O3-δ 1000 1200 Temperature (K) Fig (a) Temperature dependences of electrical resistivity for CaMn1Àx Nbx O3 (for x ¼ 0:02; 0:05 and 0.08) while heating (closed symbols) and cooling (open symbols) under air atmosphere and (b) specific heat capacity temperature dependence of CaMn0:98 Nb0:02 O3 The dotted line is just intended to guide the eye with respect to the transition temperature evolution *Results for T < 1000 K are reported from a previous work [26] 5678 L Bocher et al / Acta Materialia 57 (2009) 5667–5680 changes in the anionic composition In analogy with the resistivity data, the first peak ðT Þ can be assigned to the onset temperature of the structural transition and the second one ðT Þ corresponds to the onset temperature of the thermal reduction process This corroborates our previous hypothesis that, upon heating, the manganate phases first undergo an ðo ! cÞ structural transition which might favor the formation of oxygen vacancies via detwinning phenomena Since different heating rates were used depending on the characterization methods, these assumptions should be considered carefully The influence of the oxygen content variation on the thermopower is studied in detail by investigating (i) stoichiometric and (ii) oxygen-deficient CaMn0:98 Nb0:02 O3Àd phases, i.e (i) CaMn0:98 Nb0:02 O2:988 and (ii) CaMn0:98 Nb0:02 O2:965 , also referred to as d ¼ 0:020 and d ¼ 0:035, respectively The oxygen-deficient manganate phase was obtained by quenching the sample at T > T HT , i.e T ¼ 1273 K Both samples were investigated during heating/cooling cycles under a synthetic air atmosphere to evaluate their oxygen release/uptake capabilities and the direct influence on the thermopower Fig 10a presents S(T) of the oxidized and reduced compounds At low temperatures, e.g at T ¼ 500 K, both phases present different thermopower values Higher absolute Seebeck coefficient values are obtained for the CaMn0:98 Nb0:02 O2:988 phase compared to the CaMn0:98 Nb0:02 O2:965 compound, e.g at T ¼ 500K : S ðd¼0:020Þ ¼ À193 lV KÀ1 and S ðd¼0:035Þ ¼ À172 lV KÀ1 The CaMn0:98 Nb0:02 O2:988 phase presents a lower ½Mn3þ Š=½Mn4þ Š ratio than the CaMn0:98 Nb0:02 O2:965 compound, therefore involving higher absolute thermopower values according to the Heikes formula Upon heating (step to 2), the non-stoichiometric CaMn0:98 Nb0:02 O2:965 phase first reveals a linear temperature dependence of S(T) until T ’ 840 K (zone A) Above this temperature (zones B and C), the characteristic thermopower evolution, i.e SðT Þ $ T , is no longer observed, indicating a variation in the Mn3þ concentration and therefore the oxygen content From T HT ¼ 1090 K, the non-stoichiometric phase undergoes a thermal reduction corresponding to a sudden drop of S(T) to lower absolute thermopower values, i.e higher Mn3þ concentration Upon cooling (step to 3), the reoxidation process takes place, resulting in identical thermopower values as reported for the stoichiometric phase This finding indicates that a complete reoxidation of the non-stoichiometric manganate phases allows to recover the starting ½Mn3þ Š=½Mn4þ Š ratio of the most stable phase, i.e the stoichiometric one, leading to the initial thermoelectric properties of the CaMn0:98 Nb0:02 O2:988 phase The complete reoxidation of the non-stoichiometric phase is confirmed by a second heating/cooling cycle (step to 5), for which the reoxidized phase exhibits similar thermopower values to the CaMn0:98 Nb0:02 O2:988 phase Conclusion The present study reveals that the CaMn1Àx Nbx O3 ðx 0:10Þ phases undergo a thermal reduction and a structural transition from orthorhombic -120 A C B CaMn0.98Nb0.02O2.935 -1 Seebeck coefficient (μV K ) -140 -160 CaMn0.98Nb0.02O2.965 -180 -200 -220 -240 CaMn0.98Nb0.02O2.980 -260 -280 500 600 700 800 900 1000 1100 1200 1300 Temperature (K) Fig 10 Seebeck coefficient temperature dependence of stoichiometric and oxygen-deficient CaMn0:98 Nb0:02 O3Àd phases upon successive heating and cooling under air atmosphere Prior to the measurement, the non-stoichiometric compound was prepared by quenching in liquid nitrogen after a 60 h isothermal step at T ¼ 1273 K The numbers indicate the different steps performed during the measurement (first heating: step to 2; second heating: step to 4) L Bocher et al / Acta Materialia 57 (2009) 5667–5680 to cubic symmetry at high temperatures, both phenomena being reversible upon cooling Increasing the Nb content in the CaMnO3 structure results in a shift of the thermal reduction onset temperature to higher temperatures Thus, the CaMn1Àx Nbx O3 phases are thermally stable up to a high-temperature limit, i.e T HT ¼ 1090 K for x ¼ 0:02 to T HT ¼ 1200 K for x ¼ 0:10 The microstructure of the CaMn0:98 Nb0:02 O3 phase is characterized at room temperature by different features, i.e the presence of twinned domains and rock-salt planar defects (i) Twinned domains present in the orthorhombic crystal structure correspond to 90° rotation twins across f1 1go The rotation twins are often observed in orthorhombic phases that undergo structural transition from higher to lower symmetry upon cooling The structural transition induces a detwinning of the crystal structure at high temperature that might be related to a small amount of oxygen being released at the domain boundaries (ii) The microstructure of the CaMn0:98 Nb0:02 O3 phase also reveals the intergrowth of Ruddlesden–Popper planar defects involving anti-phase boundaries It consists of a single rock-salt layer where the Ca and Mn columns are shifted by half a unit cell from both sides of the boundary Furthermore, the electrical and thermal transport properties of the CaMn1Àx Nbx O3 ðx 0:10Þ phases undergo serious changes at high temperatures A polaronic-type conduction characterizes the low-temperature range ðT < T HT Þ Lower absolute Seebeck coefficient and electrical resistivity values are observed in the high-temperature range (above ’ 1000 K), indicating an enhancement of the electrical conduction Two distinct effects can induce such changes of the electrical transport properties: the modification of the overlap between the Mn 3d and the O 2p orbitals during the structural transition from orthorhombic to cubic symmetry, and the increase in the Mn3þ concentration in the Mn4þ sublattice caused by the thermal reduction For instance, the thermopower evolution in the high-temperature region can be explained by the oxygen vacancy formation Moreover, the changes in S(T) and qðT Þ are observed at higher temperatures with increasing Nb concentration since the thermal stability of the CaMn1Àx Nbx O3 phases increases with the Nb content Hence, the thermoelectric properties of the manganate phases can be further tuned considering their temperatures of application In addition, the manganate phases not reveal any degradation of their thermoelectric properties even after several heating/cooling cycles up to 1273 K since they reveal complete oxygen release/uptake capabilities Acknowledgements Financial support from the Swiss Federal Office of Energy (BfE) and the Ministry of Education of the Czech Republic (project LC 523) is gratefully acknowledged The authors thank Dr L Castaldi and Prof C Baerlocher for their invaluable help and the use of the high-temperature diffractometer at the Laboratory of Crystallography, 5679 Swiss Federal Institute of Technology, ETHZ, Switzerland, as well as Dr S He´bert for the fruitful discussions References [1] Rao CNR, Raveau B Transition metal oxides: structure, properties, and synthesis of ceramic oxides 2nd ed New York: John Wiley & Sons; 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1961 [51] Aguirre MH, Logvinovich D, Bocher L, Robert R, Ebbinghaus SG, Weidenkaff A Acta Mater 2008;57:108 [...]... Appl Phys 2006;100:084911 [32] Weidenkaff A Adv Eng Mater 2004;9:709 [33] Robert R, Bocher L, Sipos B, Do¨beli M, Weidenkaff A Prog Solid State Chem 2007;35:447 [34] Bocher L, Aguirre MH, Robert R, Trottmann M, Logvinovich D, Hug P, et al Thermochim Acta 2007;457:11 5680 [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] L Bocher et al / Acta Materialia 57 (2009) 5667–5680 Rietveld HM J Appl Cryst 1969;2:65... dependence of CaMn0:98 Nb0:02 O3 The dotted line is just intended to guide the eye with respect to the transition temperature evolution *Results for T < 1000 K are reported from a previous work [26] 5678 L Bocher et al / Acta Materialia 57 (2009) 5667–5680 changes in the anionic composition In analogy with the resistivity data, the first peak ðT 1 Þ can be assigned to the onset temperature of the structural... quenching in liquid nitrogen after a 60 h isothermal step at T ¼ 1273 K The numbers indicate the different steps performed during the measurement (first heating: step 1 to 2; second heating: step 3 to 4) L Bocher et al / Acta Materialia 57 (2009) 5667–5680 to cubic symmetry at high temperatures, both phenomena being reversible upon cooling Increasing the Nb content in the CaMnO3 structure results in a shift... von Molnar S Adv Phys 1999;48:167 [8] Ohtaki M, Koga H, Tokunaga T, Eguchi K, Arai H J Solid State Chem 1995;120:105 [9] Maignan A, Martin C, Damay F, Hejtmanek J, Raveau B Phys Rev B 1998;58:2758 [10] Bocher L, Robert R, Aguirre MH, Malo S, He´bert S, Maignan A, et al Solid State Sci 2008;10:496 [11] Taguchi H, Nagao M, Sato T, Shimada M J Solid State Chem 1989;78:312 [12] Mitchell RH Perovskites –... 2002;12:1806 [24] Miclau M, He´bert S, Retoux R, Martin C J Solid State Chem 2005;178:1104 [25] Rowe DM Thermoelectrics handbook – macro to nano Boca Raton (FL): CRC Press/Taylor & Francis; 2006 [26] Bocher L, Aguirre MH, Logvinovich D, Shkabko A, Robert R, Trottmann M, et al Inorg Chem 2008;47:8077 [27] Kobayashi T, Takizawa H, Endo T, Sato T, Shimada M J Solid State Chem 1991;92:116 [28] Pi L, Martin...L Bocher et al / Acta Materialia 57 (2009) 5667–5680 as an increase in the Mn3þ concentration in the Mn4þ matrix related to the thermal reduction On the one hand, the structural transition from orthorhombic... Robert R, Aguirre MH, Hug P, Reller P, Weidenkaff A Acta Mater 2007;55:4965 [50] Heikes RR, Ure RW Thermoelectricity: science and engineering New York: Interscience; 1961 [51] Aguirre MH, Logvinovich D, Bocher L, Robert R, Ebbinghaus SG, Weidenkaff A Acta Mater 2008;57:108