Defects and polaronic electron transport in Fe₂WO₆

We report the synthesis of phase pure Fe₂WO₆ and its structural characterization by high quality synchrotron X-ray powder diffraction, followed by studies of electric and thermoelectric properties as a function of temperature (200-950 °C) and pO₂ (1-10^-3 bar). The results are shown to be in accordance with a defect chemical model comprising formation of oxygen vacancies and charge compensating electrons at high temperatures. The standard enthalpy and entropy of formation of an oxygen vacancy and two electrons in Fe₂WO₆ are found to be 113(5) kJ mol^-1 and 41(5) J mol^-1 K^-1, respectively. Electrons residing as Fe²⁺ in the Fe³⁺ host structure act as charge carriers in a small polaron conducting manner. A freezing-in of oxygen vacancies below approximately 650 °C results in a region of constant charge carrier concentration, corresponding to an iron site fraction of X_Fe≅ 0.03. By decoupling of mobility from conductivity, we find a polaron hopping activation energy of 0.34(1) eV and a charge mobility pre-exponential u₀ = 400(50) cm² kV^-1 s^-1. We report thermal conductivity for the first time for Fe₂WO₆. The relatively high conductivity, large negative Seebeck coefficient and low thermal conductivity make Fe₂WO₆ an interesting candidate as an n-type thermoelectric in air, for which we report a maximum zT of 0.027 at 900 °C.