The lattice dynamics of and were studied by inelastic neutron scattering (INS). The hydrogen optic modes in the INS spectra were shown to form a broad structured band which exhibited an harmonic isotopic behaviour. In contrast, the low-energy part of the phonon spectra, formed mainly by the vibrations of the metal atoms, showed pronounced anharmonicity. The temperature dependence of the heat capacity derived from the INS data agrees with the available experimental dependence at 50 - 150 K.