Cause, Consequence, and Control of Ag Vacancies in BaAg_2-As₂

The impact of transition metal (Ag) deficiencies on the structural and transport properties of ThCr₂Si₂-type arsenides are investigated. We experimentally confirm a partial occupancy of Ag in BaAg_2-As₂, which can be predictably controlled within 0.053(5) ≤ x ≤ 0.19(1) by varying the quenching temperature during the crystal growth. Density functional theory calculations reveal that substoichiometric concentrations of Ag lower the density of states at the Fermi level, providing an electronic cause for the tendency of Ag to form vacancies. This vacancy concentration is linked to a characteristic kink in electrical resistivity that can be substantially shifted from 22 to 125 K and is established as the metric of a low-temperature structural phase transition (SPT) in BaAg_2-As₂. Along with electrical resistivity, the Seebeck coefficient and heat capacity for selected BaAg_2-As₂ samples are presented, which also exhibit anomalies due to the SPT.