Short-range ordering inβ−YH2+δandβ−YD2+δthin films studied by Raman spectroscopy

Abstract

The metal-insulator transition of Y films grown on (111) ${\mathrm{CaF}}_{2}$ substrates and capped with Pd is monitored using Raman spectroscopy. At room temperature the spectra display one Raman active line for cubic ${\mathrm{YH}}_{2.0}$ and nine Raman active lines for hexagonal ${\mathrm{YH}}_{3}.$ For an intermediate hydrogen concentration in ${\mathrm{YH}}_{2+\ensuremath{\delta}}$ with $\ensuremath{\delta}l~0.1$ a third spectrum is observed, which cannot be explained by a mixture of the border cubic and hexagonal phases. The new spectrum is also observed for an in situ grown ${\mathrm{YH}}_{2+\ensuremath{\delta}}$ film, i.e., a sample which was hydrogenated during the growth process of the film. A factor group analysis indicates that this intermediate phase has the $I4/mmm$ structure. Temperature-dependent measurements for these samples point to a short-range ordering of the hydrogen sublattice above 150 K.