(69f) Synthesis and High Temperature Thermoelectric Characterization of YCo1-XRhxO3 (0 ? x ? 1.0)

Polycrystalline samples of YCo_1-xRh_xO₃ (0 ≤ x ≤ 1) were synthesized by solid state reaction, and found to be phase pure (with the exception of a minute amount of Y₂O₃) by x-ray diffraction.  Co containing compositions required reaction in O₂ at 925 or 1000 ^oC while YRhO­₃ required reaction in an evacuated quartz ampoule at 1100 ^oC. All compositions were shared the same distorted perovskite crystal structure, space group Pbnm.  A linear relationship between Rh content and unit cell volume (calculated via Lebail fit) was found, confirming direct substitution of Rh for Co.  Electrical resistivity, thermal diffusivity (converted to conductivity utilizing experimental density and published LaCoO­­₃ heat capacity), and Seebeck coefficient were measured from 450 – 750 K.  All compositions exhibited semiconductor-like behavior over the temperature range tested.  Conduction was facilitated either by mixed valence state of Co/Rh or by excitation of Co³⁺ from LS to HS.  The activation energy of polaron conduction was found to increase near 600 K in all compositions besides YRhO₃, corresponding with a distortion of the lattice induced by the Co³⁺ spin state transition.  In the high temperature limit, the Seebeck coefficient was shown to increase with Rh substitution, implying a stabilization of LS Co³⁺.  Thermal conductivity was found to be almost exclusively due to lattice vibrations, and the enhanced phonon scattering ability of air voids is thought to be the reason for decreased thermal conductivity compared to previous studies on YCoO₃ of less porosity.  A maximum ZT of 0.085 was achieved with x = 0.3 and x = 0.7 at 750 K, and is linearly increasing with temperature at that point.