Neutron spectroscopy of H impurities in PdD: Covibrations of the H and D atoms

Three powder samples of solid isotopic solutions PdD1-x Hx with x=0.050, 0.072, and 0.091 are prepared under high pressure, and the spectra of their optical vibrations are studied by inelastic neutron scattering at ambient pressure and 5 K using the high-luminosity IN1-BeF spectrometer at ILL, Grenoble. These spectra are shown to be well represented by a linear combination of a contribution from the undisturbed matrix of stoichiometric PdD and a contribution due to the H impurity. The optical spectrum of PdD consists of a strong peak at 37 meV with a shoulder extending to 65 meV. The spectrum due to the H impurity is composed of a broad peak of defect H modes centered at 68 meV and superimposed on a broad N-shaped feature with a range of negative intensity near 36 meV and a shallow maximum at 45 meV. Simulations using the Born-von Kármán model show that this unusual feature is a result of the formation of a band of optical vibrations of the D and H atoms with equal frequencies (covibrations) caused by H-D interactions.