Structural, electronic and vibrational properties of LaF 3 according to density functional theory and Raman spectroscopy

Crystal structure of LaF single crystal is refined in tysonite-type trigonal unit cell P [Formula: see text] c1 using density functional theory calculations and Raman spectroscopy. It is shown that trigonal structure with P [Formula: see text] c1 space group is more energy-efficient than hexagonal structure with space group P6 cm. Simulated Raman spectra obtained using LDA approximation is in much better agreement with experimental data than that obtained with PBE and PBEsol functionals of GGA. The calculated frequency value of silent mode B in case of hexagonal structure P6 cm was found to be imaginary (unstable mode), thus the energy surface obtains negative curvature with respect to the corresponding normal coordinates of the mode which leads to instability of the hexagonal structure in harmonic approximation. The A line at 214 cm in Raman spectra of LaF related to the translation of F2 ions along c axis can be connected with F2 ionic conductivity.