Raman effect and structure ofYH3andYD3thin epitaxial films

The first-order Raman spectra of ${\mathrm{YH}}_3$ and ${\mathrm{YD}}_3$ thin epitaxial films deposited on ${\mathrm{CaF}}_2$ substrates have been studied in the temperature range from 4 to 300 K. Several rather broad Raman lines from ${\mathrm{YH}}_3$ have been found. The linewidth becomes smaller with decreasing temperature and the lines are very distinct and sharp at 4 K. No indications for phase transitions are found. Isotopically exchanged samples give a clear indication that ${\mathrm{YH}}_3$ is at the origin of the Raman lines. Polarization resolved measurements together with a factor group analysis for all suggested crystal structures of ${\mathrm{YH}}_3$ reveals the number and the symmetry of the phonon modes. The number of observed $A_1$ modes is not compatible with a $P3\mathrm{}¯c1\mathrm{}$ structure. Calculations of the angular dependence at different inclination angles result in predictions for characteristic intensity patterns for the suggested crystal structures. The measured angular intensity pattern show no indications for a $P3\mathrm{}¯c1\mathrm{}$ structure. In measurements at different inclination angles new Raman lines appear, which points towards a ${P6\mathrm{}}_3\mathrm{cm}$ or ${P6\mathrm{}}_3$ structure. Also the comparison of the energies of the lines in the Raman and in IR spectra promotes the picture of a noncentrosymmetric structure such as ${P6\mathrm{}}_3\mathrm{cm}$ or ${P6\mathrm{}}_3.$