X-ray diffraction, dynamical mechanical analysis and infrared reflectivity studies revealed an antiferrodistortive phase transition in EuTiO3 ceramics. Near 300K the perovskite structure changes from cubic Pm-3m to tetragonal I4/mcm due to antiphase tilting of oxygen octahedra along the c axis (a0a0c- in Glazer notation). The phase transition is analogous to SrTiO3. However, some ceramics as well as single crystals of EuTiO3 show different infrared reflectivity spectra bringing evidence of a different crystal structure. In such samples electron diffraction revealed an incommensurate tetragonal structure with modulation wavevector q ~ 0.38 a*. Extra phonons in samples with modulated structure are activated in the IR spectra due to folding of the Brillouin zone. We propose that defects like Eu3+ and oxygen vacancies strongly influence the temperature of the phase transition to antiferrodistortive phase as well as the tendency to incommensurate modulation in EuTiO3.Comment: PRB, in pres

Polycrystalline tungsten-substituted CaMn 1Àx W x O 3Àd (0.00 x 0.05) powders were synthesized from a polymeric precursor, pressed and sintered to high density. The impact of tungsten substitution on the crystal structure, thermal stability, phase transition, electronic and thermal transport properties is assessed. Tungsten acts as an electron donator and strongly affects high-temperature oxygen stoichiometry. Oxygen vacancies form in the high figure-of-merit (ZT)-region starting from about T ¼ 1000 K and dominate the carrier concentration and electronic transport far more than the tungsten substitution. The analysis of the transport properties yields that in the investigated regime the band filling is sufficiently high to overcome barriers of polaron transport. Therefore, the Cutler-Mott approach describes the electrical transport more accurately than the Mott approach for small polaron transport. The lattice thermal conductivity near room temperature is strongly suppressed with increasing tungsten concentration due to mass-difference impurity scattering. A ZT of 0.25 was found for x ¼ 0.04 at 1225 K. V

Here, we report thermoelectric study of crossroads material MnTe via iso-electronic doping S on the Te-site. MnTe 1-x S x samples with nominal S content of x ¼ 0.00, 0.05, and 0.10 were prepared using a melt-quench method followed by pulverization and spark plasma sintering. The X-ray powder diffraction, scanning electron microscopy, and ZAF-corrected compositional analysis confirmed that S uniformly substitutes Te up to slightly over 2%. A higher content of S in the starting materials led to the formation of secondary phases. The thermoelectric properties of MnTe 1-x S x samples were characterized by means of Seebeck coefficient, electrical conductivity, and thermal conductivity measurements from 300 K to 773 K. Furthermore, Hall coefficient measurements and a single parabolic band model were used to help gain insights on the effects of S-doping on the scattering mechanism and the carrier effective mass. As expected, S doping not only introduced hole charge carriers but also created short-range defects that effectively scatter heat-carrying phonons at elevated temperatures. On the other hand, we found that S doping degraded the effective mass. As a result, the ZT of MnTe 0.9 S 0.1 was substantially enhanced over the pristine sample near 400 K, while the improvement of ZT became marginal at elevated temperatures.

We combine neutron powder diffraction and x-ray single-crystal magnetic diffraction at the Eu L 2 edge to scrutinize the magnetic motif of the Eu ions in magnetoelectric EuTiO 3 . Our measurements are consistent with an antiferromagnetic G-type pattern with the Eu magnetic moments ordering along the a,b -plane diagonal. Recent reports of a novel transition at 2.75 K with a flop of magnetic moments upon poling the sample in an electric field cannot be confirmed for a nonpoled sample. Our neutron diffraction data do not show any significant change of the structure below the Néel temperature. Magnetoelastic coupling, if present, is therefore expected to be negligible.

Thermoelectric half-Heusler compounds with the nominal composition (Ti 0.33 Zr 0.33 Hf 0.33 )NiSn were synthesized and exposed to operating conditions, i.e., elevated temperatures and oxidizing atmosphere. As-prepared samples were found to be a stable multiphase system consisting of Ti-and (Zr,Hf)-rich phases with half-Heusler structure. The influence of an oxidizing atmosphere at elevated temperatures on the sample was studied in-and ex situ by optical and electron microscopy and thermal analysis. Exposure to air resulted in surface oxidation starting at T ¼ 545 K. During a defined heat treatment the Ti-rich phase decomposed while the (Zr,Hf)-rich phase was stable. In situ Atomic Force Microscopy and Scanning Kelvin Probe Force Microscopy experiments confirmed that interfaces, especially grain boundaries, are most prone to phase decomposition.