Order-disorder phase transition of NH4AlF4 through Raman scattering investigations

Abstract

The low-frequency (0-600 cm- 1) Raman spectra of NH4AlF4 single crystals (a perovskite-type layer compound) have been studied at different temperatures, and the D184h → D134h phase transition occurring around 150 K is evidenced. The Raman data show that it is purely an order-disorder transformation, connected to the reorientational dynamics of the NH +4 ions. A simple model of reorientable pseudo-spins in a rigid layered matrix of corner-sharing AlF6 octahedra describes the main spectral characteristics of the phase transition. The estimate of the critical exponent of the order-parameter is consistent with the essentially three-dimensional character of the transition in the vicinity of T c