Raman Spectra and Structural Phase Transitions of the Perovskite‐Type Layer Compounds RbAlF₄ and NH₄AlF₄

Abstract

The low frequency (0 -600 cm-') Raman spectra of RbAIF, and NH4A1F4 single crystals have been studied at different temperatures ranging from 20 K to 600 K, in their various structural modifications. It is concluded that the transition of NH,AlF, occuring at -150 K is purely an orderdisorder transition, governed by the reorientational dynamics of the NH: ions. For RbAIF,, the observed phase transitions at 553 and 282 K are both related to displacive mechanisms, as evidenced by the observation of soft-modes, but also to disordering processes probably connected with the AIF, octahedra, and which result in a breakdown of the K = 0 selection rules in the tetragonal Dih and Dih phases.

Crystal structure and phase transitions of a layered perovskite-like CsScF4crystal

“…37,38 Within this model, only pair-wise interactions and bondstretching force constants are considered. A simplified version of the Born-Karman model implies that the bondstretching force constant A depend on R only (R is the bond length), and the A(R) dependences are the same for all atom pairs: (10) where λ and ρ are the parameters describing the interaction of selected pairs. To find the parameters of the model, a special optimization software was written and tested for several compounds.…”

“…1 and 4-6). Earlier phase transitions in crystals of the MAlF 4 family (M = Tl, K, Rb, NH 4 ) were studied by various techniques (Brillouin and ultrasonic, 7,8 Raman, 9,10 Raman under high hydrostatic pres-sure, 11,12 neutron scattering, 13 group theory, 14 Monte Carlo simulation 15 ). Sc-containing layered perovskites have not been widely studied.…”