Effect of vacancies on thermoelectric properties of β-CuAgSe studied by positron annihilation

CuAgSe is a promising thermoelectric material due to its superionicity. In this work, beta-Cu1-xAg1-ySe (x = 0, 0.02, and 0.04; y = 0, 0.02, and 0.04) samples are synthesized by solid-state reaction method. The vacancies in samples are characterized by positron annihilation spectroscopy. Thereafter, the effects of vacancies on thermoelectric properties are investigated. The positron annihilation results reveal that Ag vacancies exist in the Ag-deficient samples (beta-CuAg0.98Se and beta-Cu0.98Ag0.98Se) but also in the Cu-deficient samples (beta-Cu0.96AgSe and beta-Cu0.98AgSe). For the Cu-deficient samples, the existence of Ag vacancies is attributed to the formation of impurity phases. For the nonstoichiometric samples, the vacancies are responsible for the decrease in the Seebeck coefficient in the temperature range from 300 to 400 K. However, for beta-CuAgSe, no decrease in the Seebeck coefficient is observed due to the lack of extra holes, and electrons are still the majority carriers. For CuAgSe, the ZT value is mainly determined by the Seebeck coefficient. Therefore, for the nonstoichiometric samples, the ZT value reduces drastically with increasing temperature and drops to nearly zero at 400 K. In contrast, with the temperature increasing from 300 to 450 K, the ZT value of beta-CuAgSe goes up from 0.4 to 0.5.