Polarized IR and Raman spectra of Ca2MgSi2O7, Ca2ZnSi2O7 and Sr2MgSi2O7 single crystals: Temperature-dependent studies of commensurate to incommensurate and incommensurate to normal phase transitions

IR and Raman spectra of Ca2MgSi2O7, Ca2ZnSi2O7 and Sr2MgSi2O7 oriented single crystals and powders have been measured. The temperature dependence of phonons has been established in the 4–650 K range. Discussion of the results has been performed on the basis of factor group approach for the tetragonal P4̄21m (D2d3) space group with Z=2. The assignment of the bands observed to the internal and external modes has been made on their polarization behaviour, phonon energy calculations and literature data. The evidences of the LO–TO splitting and commensurate (C) to incommensurate (IC) as well as incommensurate (IC) to normal (N) phase transitions have been found. As a representative example, the results obtained for the spontaneous Raman scattering have also been used in the discussion of the stimulated Raman spectra of Ca2ZnSi2O7.