Quantum Physical Interpretation of Thermoelectric Properties of Ruthenate Pyrochlores

Abstract

Lead- and lead-yttrium ruthenate pyrochlores were synthesized and investigated for Seebeck coefficients, electrical- and thermal conductivity. Compounds A2B2O6.5+z with 0 ≤ z < 0.5 were defect pyrochlores and p-type conductors. The thermoelectric data were analyzed using quantum physical models to identify scattering mechanisms underlying electrical (σ) and thermal conductivity (κ) and to understand the temperature dependence of the Seebeck effect (S). In the metal-like lead ruthenates with different Pb:Ru ratios, σ (T) and the electronic thermal conductivity κe (T) were governed by ‘electron impurity scattering’, the lattice thermal conductivity κL (T) by the 3-phonon resistive process (Umklapp scattering). In the lead-yttrium ruthenate solid solutions (Pb(2-x)YxRu2O(6.5±z)), a metal–insulator transition occurred at 0.2 moles of yttrium. On the metallic side (<0.2 moles Y) ‘electron impurity scattering’ prevailed. On the semiconductor/insulator side between x = 0.2 and x = 1.0 several mechanisms were equally likely. At x > 1.5 the Mott Variable Range Hopping mechanism was active. S (T) was discussed for Pb-Y-Ru pyrochlores in terms of the effect of minority carrier excitation at lower- and a broadening of the Fermi distribution at higher temperatures. The figures of merit of all of these pyrochlores were still small (≤7.3 × 10−3).