  Sodium cobaltate with high Na content is a promising thermoelectric material.
It has recently been reported that oxygen vacancies can alter the material
properties, reducing its figure of merit. However, experimental data concerning
the oxygen stoichiometry are contradictory. We therefore studied the formation
of oxygen vacancies in Na_xCoO_2 with first principles calculations, focusing
on x = 0.75. We show that a very low oxygen vacancy concentration is expected
at the temperatures and partial pressures relevant for applications.
