RESUMEN
Raman spectra of the mixed crystalline oxides of the (1 - x)TeO2 + xTeO3 (x = 0, », ½, 1) series were recorded and simulated by using the DFT calculations. Good agreement between observed and calculated Raman spectra makes it possible to establish unambiguous assignment for all prominent Raman lines. This result gives an insight into relations between structural peculiarities and observed spectral features for the crystals promising as nonlinear optical materials. Highlights ⢠Several mixed TeO2-TeO3 crystals were synthesized by solid-state chemistry ⢠DFT calculations well describe structures and phonon spectra of TeO2-TeO3 oxides ⢠Raman lines can be assigned to internal vibrations of TeO4 and TeO6 polyhedrons ⢠Observed Raman bands can be used as fingerprints of different structural units.
RESUMEN
Crystal structure of LaF3 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 P63 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 2 in case of hexagonal structure P63 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 1g line at 214 cm-1 in Raman spectra of LaF3 related to the translation of F2 ions along c axis can be connected with F2 ionic conductivity.