Experimental Results Related to the Normal-Incommensurate Phase Transition in A₂MX₄ Compounds

Abstract

Inelastic neutron scattering results taken in the incommensurate phase of K₂SeO₄ show that the phase mode response is over-damped at low q wave-vector. A rather complete set of data obtained at 120 K, 110.5 K, 100 K, 96 K tends to demonstrate that the phase mode dispersion curve exhibits a gap at q = 0 and is relatively insensitive to temperature. The present results can be related to available Raman data. A single crystal neutron diffraction experiment has been carried out at 588 K in the normal phase of K₂ZnCl₄. In refining the data we tested two models: a two-position, split-atom harmonic model (R = .026) and an ordered anharmonic model (R = .019). For Cl atoms anharmonic temperature factors based on the Gram-Charlier expansion have been refined up to fourth order.