Disorder of BeF₄ in the Normal Phase of (NH₄)₂BeF₄

Abstract

The crystal structure of (NH₄)₂BeF₄ has been investigated at several temperatures by X-ray diffraction with particular interest to the temperature dependence of temperature factors. It is revealed that the mean-square thermal amplitudes U₃₃’s for F(2) and F(3) and U₂₂ for F(1) are not proportional to absolute temperature in the normal (N) phase. This is understood by assuming the disorder of F atoms, or of BeF₄ groups. The phase transition to the ferroelectric (F) phase is characterized by the order of BeF₄ groups.