Structure and Lattice Dynamics of MF2 (M = Ca, Sr, Ba, Pb) in Cubic and Orthorhombic Phases: The ab initio Calculations

The MF2 (M = Ca, Sr, Ba, Pb) crystals undergo a structural phase transition from the cubic to the orthorhombic phase in a range of 1-10 GPa. Although the cubic fluorites are well studied experimentally, there has been almost no information on their orthorhombic phase to date. Despite a few data on the Raman spectra available in the literature, there is still a lack of IR spectra of these structures. In this respect, this work aims to study the crystal structure and the phonon spectrum of MF2 crystals (M = Ca, Sr, Ba, Pb) in both the cubic and orthorhombic phases, using the MO LKAO approach with hybrid DFT functionals that take the contribution of nonlocal exchange in the HartreeFock formalism into account. The frequencies and types of fundamental IR and Raman vibrations, as well as the frequencies of silent modes, are found. The calculations of elastic constants for the orthorombic phase have been performed at the first time. The program CRYSTAL14 intended for simulation of periodic structures within the MO LKAO approach was used for the calculations.