Evidence for Ionic Bonding in YH(3-delta).

The infrared transmission spectra of YH(3-delta) and YD(3-delta) thin films have been measured in the energy range from 300 to 6000 cm(-1), and the optical constants have been calculated by Kramers-Kronig analysis. From a fit of the frequency-dependent conductivity and the dielectric loss functions with one Drude term and five Lorentz oscillators, the longitudinal and transverse optical-phonon frequencies have been determined. The Born and Szigeti effective charges have been calculated and the ionicities of the ions in YH(3-delta) have been derived. We find that hydrogen does not enter as a proton in the compound, but that it is negatively charged, with a Szigeti effective charge of nearly 0.5e.