Phase transition and abnormal electrical properties of AgInSe2 driven by high-pressure sintering

Phases of I-III-VI2 chalcopyrite compounds are attracting ever-increasing attention, particularly in the current era of the rise of functional materials. In this work we found that the phase transition of AgInSe2 can be triggered by high-pressure sintering from a tetragonal to a rhombohedral structure. The crystal structure, phase transition conditions, and stability of the high-pressure phase are thoroughly investigated, revealing the structural information, drawing the phase diagram, and improving the structural stability. Given the potential high thermoelectric performance in AgInSe2, we carry out the study of the electrical transport properties for the high-pressure phase. Compared to the intrinsic phase, the electrical conductivity and power factor increase by six orders of magnitude and 1000 times, respectively. The electronic structure by density functional theory calculations indicates that the abnormal electrical properties originate from the variation in cationic coordination number caused by the phase transition, which leads to the increased contribution of 5s orbitals of Ag and In atoms to the conduction-band minimum, largely reducing the band gap and motivating the carrier concentration. These findings provide a strategy for structural design and performance optimization of AgInSe2 manipulated by the phase transition.