Order–Disorder Transition of NH4AlF4 through EPR Investigations

Abstract

An EPR investigation of NH4AlF4:Fe3+ evidences an order—disorder transition in this compound. The full ordering occurs below 145 K. The P42/mbc space group is proposed for the low temperature phase, and we discuss the fluorine octahedra distortion coupled to the NH4+ ordering. The lineshape evolution from 300 to 145 K is explained by a gradual slowing down of the reorientation rates.

Order-disorder phase transition of NH4AlF4 through Raman scattering investigations

“…Below the phase transition temperature ( ~ 150 K), the AlF6 octahedron layers do not undergo noticeable distortion, but the NH' tetrahedra are now ordered, with a « ferro » type ordering for adjacent tetrahedra in the (001) plane and « anti-ferro » type ordering for tetrahedra in consecutive layers. The space-group determined for this ordered phase is P 42/mbc-D ", with Z = 4 [5,6]. All these facts tend to indicate that the phase change of NH4AIF, is Fig.…”

“…to Zl jA1u (Table IV). Hence, there is clearly an order- to a « ferro » type ordering of the NH' in the layer planes, and, because of cell doubling involving the point Z, to an « anti-ferro » ordering between planes as expected in D 13 [5,6]. Both transitions Dlh -+ D 18 and D184h -> D 13 fulfil all the Landau and Lifshitz symmetry requirements allowing second-order transformations between commensurate phases.…”

“…In this series of compounds, NH4AlF4 is of special interest because of the occurrence of an order-disorder phase transition in the vicinity of 150 K [5,6]. This transition was first observed by means of EPR investigations, but could not be evidenced by X-ray dif- Fig.…”

“…Ammonium tetrafluoroaluminate, NH4AlF4, belongs to the family of perovskite-type layer compounds MAlF4 (M = K, Rb, Tl, NH4) whose structures are closely related to that of the TlAlF4 aristotype (P 4jmmm-Dlh, with Z = 1) [1][2][3][4][5][6]. They consist of infinite layers of AlF6 octahedra sharing four atoms, Feq, in the (001) plane.…”

“…out of phase from one layer to the next one. This is the description of the soft-mode which condenses in a static deformation of the lattice in D184h [5,6], and which corresponds in this phase to a totally symmetric vibration ( Table I). The order-disorder processes occurring in NH4AlF4 can be described by a pseudospin variable u associated with the orientation of the NH4+ group.…”