Tuning of p-n-p-Type Conduction in AgCuS through Cation Vacancy: Thermopower and Positron Annihilation Spectroscopy Investigations

Understanding the complex phenomenon behind the structural transformations is a key requisite to developing important solid-state materials with better efficacy such as transistors, resistive switches, thermoelectrics, etc. AgCuS, a superionic semiconductor, exhibits temperature-dependent p-n-p-type conduction switching and a colossal jump in thermopower during an orthorhombic to hexagonal superionic transition. Tuning of p-n-p-type conduction switching in superionic compounds is fundamentally important to realize the correlation between electronic/phonon dispersion modulation with changes in the crystal structure and bonding, which might contribute to the design of better thermoelectric materials. Herein, we have created extrinsic Ag/Cu nonstoichiometry in AgCuS, which resulted in the vanishing of p-n-p-type conduction switching and improved its thermoelectric properties. We have performed the selective removal of cations and measured their temperature-dependent thermopower and Hall coefficient, which demonstrates only p-type conduction in the Ag1-xCuS and AgCu1-xS samples. The removal of Cu is much more efficient in arresting conduction switching, whereas in the case of Ag vacancy, p-n-p-type conduction switching vanishes at higher vacant concentrations. Positron annihilation spectroscopy measurements have been done to shed further light on the mechanisms behind this structural transition-dependent conduction switching. Cation (Ag+/Cu+) nonstoichiometry in AgCuS significantly increases the vacancy concentration, hence, the p-type carriers, which is confirmed by positron annihilation spectroscopy and Hall measurement. The Ag1-xCuS and AgCu1-xS samples exhibit ultralow thermal conductivity (similar to 0.3-0.5 W/m-K) in the 290-623 K temperature range because of the low-energy cationic sublattice vibration that arises as a result of the movement of loosely bound Ag/Cu within the stiff S sublattice.