Pyroelectric response and temperature-induced α-β phase transitions in α-In2Se3 and other α-III2VI3 (III = Al, Ga, In; VI = S, Se) monolayers

The pyroelectric response of materials is a promising phenomenon that is used in many applications, but two-dimensional (2D) pyroelectricity has not received due attention. Here we report ab initio molecular dynamics simulations to investigate the pyroelectric effect in α-III₂VI₃ (III = Al, Ga, In; VI = S, Se) monolayers, an emerging class of 2D pyroelectric materials. The in-plane and out-of-plane spontaneous polarizations are found to be robust at room temperature. We demonstrate large pyroelectric response in these materials that is associated with temperature-induced ferroelectric-to-paraelectric phase transitions. Identifying the high-temperature nonpolar phase as the β-phase, we demonstrate strong displacive character in the α-β phase transition, which is achieved through relative displacements of the top three and bottom two atomic sublayers in opposite directions. The present work unravels and highlights the primary pyroelectric effect and the α-β phase transitions that are responsible for the promising pyroelectricity in α-III₂VI₃ monolayers.