Rotation-translation coupling in (NX4Y)x(KY)1-x for (X, Y) = {(H, I); (D, I); (H, Br)}?

Complete text of publication follows. Recently the mixed-crystal system (NH4I)x(KI)1-x has attracted considerable attention. Below a critical concentration xc these crystals are representative of a large family of orientational glasses [1]. Above xc a rich structural polymorphism with complex orientational order has been detected. The coupling between strain and polarization is investigated. Quasielastic neutron scattering studies have been performed on a powder sample (x = 0.07). Astonishingly, a rotational jump model was not able to describe the data. An additional component appears at about T ≅ 90 K. In addition, the temperature dependence of the transversal optical (TO) phonons have also been investigated using FT-IR technique. The phonons have been attributed to the vibration NH4+ ↔ I- and soften at about T ≅ 100 K. A minimum in the temperature dependence of the TA-phonon at the X-point has already been reported [2]. These results have been interpreted in terms of a rotation-translation coupling [3] in which the lattice vibrations soften due to the coupling to rotations of the NH4+-tetrahedra. Further results will be reported for (ND4I)x(KI)1-x and (NH4Br)x(KBr)1-x. (author)