Effect of Hydrostatic Pressure on Lone Pair Activity and Phonon Transport in Bi$_2$O$_2$S

Dibismuth dioxychalcogenides, Bi$_2$O$_2$Ch (Ch = S, Se, Te) are emerging class of materials for next generation electronics and thermoelectrics with an ultrahigh carrier mobility and excellent air stability. Among these, Bi$_2$O$_2$S is fascinating because of stereochemically active 6$s^2$ lone pair of Bi$^{3+}$ cation, heterogeneous bonding and high mass contrast of constituent elements. In this work, we systematically investigate the effect of hydrostatic pressure and its implications on lattice dynamics and phonon transport properties of Bi$_2$O$_2$S by employing first principles calculations along with the Boltzmann transport theory. The ambient $Pnmn$ phase exhibits a low average lattice thermal conductivity ($\kappa_l$) of 1.71 W-m/K at 300 K. We also find that Bi$_2$O$_2$S undergoes a structural phase transition from low symmetry ($Pnmn$) to a high symmetry ($I4/mmm$) structure around 4 GPa due to the Bi$^{3+}$ cation centering. Upon compression the lone pair activity of Bi$^{3+}$ cation is suppressed, which increases $\kappa_l$ by nearly 3 times to 4.92 W-m/K at 5 GPa for $I4/mmm$ phase. The calculated phonon lifetimes and Gr\"uneisen parameters show that anharmonicity reduces with increasing pressure due to further suppression of lone pair, strengthening of intra and inter molecular interactions, which raises the average room temperature $\kappa_l$ to 12.82 W-m/K at 20 GPa. Overall, the present study provides a comprehensive understanding of hydrostatic pressure effects on stereochemical activity of the Bi$^{3+}$ cation lone pair and its consequences on phonon transport properties of Bi$_2$O$_2$S.