The inelastic neutron scattering study of low temperature lattice dynamics of K1−x (NH4) x H2PO4 mixed crystals

Abstract

The low-temperature lattice dynamics of K1−x(NH4)xH2PO4 mixed crystals have been studied using high-resolution inelastic-neutron-scattering; on powder samples for concentrationsx=0.0; 0.06; 0.1; 0.18; 0.3; 0.6 and 0.9; and on a single crystal forx=0.4. The modes have been assigned and are compared with optical spectroscopy results. It was found that the dynamics of acid protons in the hydrogen-bonds is decoupled from the heavy atom lattice. The displacement vectors for the bending modes are inclined by ca 30° with respect to the hydrogen-bond planes. The influence of ammonium ion concentration on the lattice dynamics has been analysed and the implications for the dipole-glass state are discussed. A vibrational mode at 16.8 meV cannot be distinguished in the glass composition spectra. The inhomogeneous piezoelectric coupling constants in these glasses broaden 16.8 meV band so much that it disappears into the noise. The glass-composition samples show line broadening for the NH4 whole body motions due to the random hydrogen-bond fields experienced by these ions.