Enhancing Thermoelectric Properties of Some Defect Pyrochlores by Lowering Thermal Conductivity

Lowering thermal conductivity (κ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>κ) is vital in designing thermoelectrics as it can significantly increase their figure of merit. This study aims to lower κ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>κ by changing site occupancies in metal-like defect pyrochlores (A₂B₂O₆O´_x, 0≤x<1), specifically Pb₂Ru₂O₆O´_0.5, and derivatives with electron lone pairs on the A site and vacancies on the O´ site. Partial B site substitutions (Pb²⁺₂(Ru⁴⁺_2-yPb⁴⁺_y)O_6.5-x, y = 0, 0.3, 0.5, 0.7, 0.9) increased the lattice parameter (^VIPb⁴⁺ > ^VIRu⁴⁺), affected electrical conductivity (σ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>σ) and replaced Pb–O–Ru by Pb–O–Pb bonds. The number of lone pair electrons (^VIIIPb²⁺ on A site) remained constant. Removing some Pb²⁺ from the A site (Pb_1.8Ru₂O₆O´_x, 0.5≤x<1) reduces lone pair electrons and introduces O´ vacancies. ‘x’ indicates potential changes in the number of O´ vacancies (not measured). In all cases, lattice defects and resulting anharmonicity increase phonon scattering κ<!--[if gte msEquation 12]>κ and σ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>σ were measured and the underlying scattering processes were explained by existing quantum-physical models. Relative to Pb₂Ru₂O_6.5, derivatives showed up to 2.3× increase of σ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>σ and significant decreases in κ_L, the lattice component of κ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>κ, (κ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>κ = κ_e + κ​​​​​​​_L). Lowering Pb²⁺ on the A site yielded the electronic component κ​​​​​​​_e to be greater than σ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>σ and the lattice component κ​​​​​​​_L to be negative, suggesting failure of the Wiedemann-Franz law when using the Sommerfeld value for the Lorenz number (L) to calculate κ​​​​​​​_e. This phenomenon will be discussed along with an attempt to calculate a correct L using Seebeck coefficient.