Study of the temperature dependence of the localized vibrations of H and D in niobium

The second harmonic of the local deuterium vibration in Nb${\mathrm{D}}_{0.85}$ has been observed for the first time. Its energy $\hslash \omega =170\mathrm{}$ meV is 7 meV below the harmonic value of twice the lower local-mode energy, $2\hslash {\omega }_1$. The measured isotope effect for the lowest local-mode energy is $\frac{{\omega }_1^{\mathrm{H}}}{{\omega }_1^{\mathrm{D}}}=1.365\mathrm{}\pm 0.005$ which differs from the harmonic value of ${\left(\frac{M_{\mathrm{D}}}{M_{\mathrm{H}}}\right)}^{\frac{1}{2}}=\sqrt{2}$. The temperature dependence of the local modes for both the deuteride and hydride system exhibit pronounced precursor effects prior to the $\beta \to {\alpha }^{\prime }$ order-disorder phase transition.