Unraveling the Hidden Martensitic Phase Transition in BaClF and PbClF under High Pressure Using an Ab Initio Evolutionary Approach.

We predict crystal structures of MClF (M = Ba and Pb) compounds by performing an ab initio evolutionary simulation at ambient as well as high pressure. We propose a structural transition sequence in MClF compounds as follows: P4/ nmm → Pmcn → P63/ mmc below 100 GPa. The predicted ambient and intermediate phases are consistent with X-ray and Raman spectroscopic measurements, while the newly proposed high pressure P63/ mmc phase is thermodynamically more favorable than the previously proposed monoclinic ( P21/ m) phase. It is found that the P4/ nmm → Pmcn transition is first order in nature, while the Pmcn → P63/ mmc transition is a martensitic phase transition, which is accompanied by a slight volume change and is of a displacive nature. The austenite and martensite phases coexist in a wide pressure range, especially for PbClF. The martensite phase transition is mainly driven by (1) tilting and transformation of distorted heptahedron to pentahedron environment of MCl6, which leads to negative area compressibility, and (2) cooperative displacive movement of F- ions to form a trigonal bypyramidal (MF5) structure around a metal cation. Overall, the metal cation coordination increases from 9 (MF4Cl5- P4/ nmm) to 10 (MF4Cl6- Pmcn) and, further, to 11 (MF5Cl6- P63/ mmc) under high pressure. The predicted ambient and high pressure phases are mechanically and dynamically stable under the studied pressure range. Electronic structure, bonding, and optical properties are calculated and discussed using new parametrization of Tran Blaha modified Becke Johnson potential. We find nearly isotropic optical properties (except for the ambient phase of PbClF), even though all the predicted ambient and high pressure phases are structurally anisotropic.