Pressure-Induced Martensitic Phase Transition and Low Lattice Thermal Conductivity of SrClF

In the present work, we report a pressure-induced martensitic phase transition for SrClF, which is analogous to BaClF and PbClF compounds. The predicted structural phase transition sequence for SrClF below 200 GPa is as follows: P4/nmm → Pmcn → P6₃/mmc with an increase in the coordination of a metal cation with structural motifs MCl₅F₄ [9] → MCl₆F₄ [10] → MCl₆F₅ [11], where M = Sr, Ba, and Pb, respectively. The martensitic phase transition is mainly driven by the cooperative displacive nature of M, Cl, and F atoms, which removes lattice distortion from the austenite (Pmcn) phase under pressure. Anharmonic lattice dynamics and thermal conductivity (k_l) are calculated using the temperature-dependent effective potential (TDEP) method. Dynamical stability of the predicted high-pressure phases is confirmed from the computed phonon dispersion curves and density of states at 300 K. Short phonon lifetimes and low group velocities of acoustic modes of CaClF and SrClF are attributed to their low k_l values along with their quasi-two-dimensional (2D) layered structures.