Precursor-order clusters, critical slowing down and 2D to 3D crossover above Tcand NH4AlF4:Fe3+through EPR

NH4AlF4 undergoes an order-disorder transition in the vicinity of 155K. The low-temperature phase is characterised by a parallel order of NH4+ in (001) layers and an antiparallel order along c. Information about local configurations and about dynamics of precursor-order clusters is obtained from EPR spectra of a Fe3+ probe. A model is set up postulating a certain set of local atomic configurations and the permissible jumps between them. The Kubo-Anderson theory is applied to this, and its parameters throughout the temperature range 300-150K are obtained by computer matching with the experimental curves. The parameters are compared with those calculated on the assumption of 2D-ordered domains, and give good agreement above 175K; below that temperature discrepancies begin to appear, which are interpreted as the onset of 3D order. The variation of domain lifetime with temperature fits Ising models: the changeover from a 2D to a 3D regime is a new and interesting feature.