THE PLASTICALLY CRYSTALLINE PHASE OF NAOH, NAOD, KOH AND KOD

The high-temperature cubic phases of sodium hydroxide and potassium hy droxide have been investigated using x-ray, elastic and inelastic neut ron scattering from single crystals. As a special feature of these exp eriments the crystals had to be kept at temperatures of around 550 K a s grown from melt, since they are destroyed when passing through the s tructural phase transition. In these compounds the OH or OD groups are known to undergo rapid reorientational motions. The x-ray diffraction results are characterized by a rapid decrease in the Bragg intensitie s with increasing diffraction angle and diffuse rods passing through t he Bragg reflections. By neutron diffraction, about 20 symmetrically n on-equivalent reflections have been observed. Different means of analy sis will be presented; one obtains the probability distribution of Hor D+ around oxygen, and the OH distance. This is of interest in the c ontext of possible H bonding and proton conductivity. The disorder of H or D gives rise to diffuse scattering of neutrons as an approximatel y spherical halo around the origin in Q-space. The orientations of OH groups at different sites being correlated cannot be excluded from the experimental findings; a 2D correlation model, however, does not pred ict a dramatic narrowing of this halo. Inelastic scattering yielded ra ther ill-defined phonon groups in constant-Q scans. In particular, the shear modes showed strong overdamping. The observed features can be e xplained by treating the OH groups as elastic dipoles coupled to the l attice distortions, with the dynamics of a relaxator, and by extending conventional soft-mode theory to allow for the mixing of several soft ened modes. The softened modes also explain the diffuse rods in x-ray photographs.