On synthesis, phase purity, and oxygen stoichiometry of sodium and calcium cobaltate thermoelectrics

Relatively high concentrations of oxygen vacancies and effects on thermoelectric properties have been reported in the literature on state-of-the-art thermoelectric sodium cobaltates. However, first principle calculations suggest high energies and low concentrations of such vacancies. Experiments in our laboratory indicate that apparent oxygen non-stoichiometry may have stemmed from secondary phases present after decomposition in two-phase regions during synthesis and measurements. In a similar manner, experimental evaluation and defect-chemical interpretation of non-stoichiometry in calcium cobaltate thermoelectrics are troubled by non-trivial synthesis and sample fabrication. Recent results on these matters are presented and discussed.