Cause, Consequence,\nand Control of Ag Vacancies in\nBaAg<sub>2–<i>x</i></sub>As<sub>2</sub>

The impact of transition metal (Ag) deficiencies on the\nstructural\nand transport properties of ThCr<sub>2</sub>Si<sub>2</sub>-type arsenides\nare investigated. We experimentally confirm a partial occupancy of\nAg in BaAg<sub>2–<i>x</i></sub>As<sub>2</sub>, which\ncan be predictably controlled within 0.053(5) ≤ <i>x</i> ≤ 0.19(1) by varying the quenching temperature during the\ncrystal growth. Density functional theory calculations reveal that\nsubstoichiometric concentrations of Ag lower the density of states\nat the Fermi level, providing an electronic cause for the tendency\nof Ag to form vacancies. This vacancy concentration is linked to a\ncharacteristic kink in electrical resistivity that can be substantially\nshifted from 22 to 125 K and is established as the metric of a low-temperature\nstructural phase transition (SPT) in BaAg<sub>2–<i>x</i></sub>As<sub>2</sub>. Along with electrical resistivity, the Seebeck\ncoefficient and heat capacity for selected BaAg<sub>2–<i>x</i></sub>As<sub>2</sub> samples are presented, which also\nexhibit anomalies due to the SPT.