Effect of hydrostatic pressure on stereochemically active lone-pair of Bi$_2$O$_2$S

Abstract

Dibismuth dioxychalcogenides, Bi 2 O 2 Ch (Ch = S, Se, Te) are emerging class of materials for the next generation electronics and thermoelectrics with an ultrahigh carrier mobility and excellent air stability. These are layered materials with weak electrostatic interlayer interactions which makes them unique compared to layered materials with weak interlayer van der Waals bonding. In particular, Bi 2 O 2 S is fascinating because of stereo-chemically active 6s 2 lone-pair of Bi 3+ cation, heterogeneous bonding and high mass contrast. Understanding the effect of hydrostatic pressure on lone-pair and its implications on phonon transport have attracted enormous research interest recently. In the present work, we systematically investigated the high pressure behavior of Bi 2 O 2 S, it undergoes a structural phase transition from low symmetry (P nmn) to a high symmetry (I4/mmm) structure around 4 GPa. Pressure dependent static enthalpy, lattice, bond parameters and elastic constants strongly suggest that the phase transition is continuous. Enhanced Born effective charges (BECs) are predicted for both the phases due to significant cross-band-gap hybridization from their electronic structure. This brings the lattice in the vicinity of ferroelectric instability, which is favorable to achieve low κ l in Bi 2 O 2 S. The obtained low κ l value 1.47 W-m/K for P nmn phase is more than double 3.63 W-m/K for I4/mmm phase, which is due to suppression of lone-pair at Bi 3+ cation, consequently, anharmonicity diminishes with pressure as shown from electron localization function, potential energy surfaces and mean square atomic displacements. Overall, the current study provides an in depth understanding on how hydrostatic pressure effects the phonon transport in stereo-chemically active 6s 2 lone-pair containing Bi 2 O 2 S through a continuous structural phase transition.