Abstract
Searching for a microscopic description of structural phase transitions in AlF3, these transitions were followed by electron paramagnetic resonance (EPR) in situ measurements in X-band in the temperature range 290 K ≤T ≤ 800 K with Cr3+ — and Fe3+-doped powder samples of α-AlF3. For each temperature the zero-field splitting (zfs) parameters of the paramagnetic transition metal ions were determined by simulation. Starting from the axial zfs parametersb 02 (T) and bearing in mind the trigonal symmetry, the application of the superposition model allowed the calculation of the angle θ(T) (θ: <(c-axis, Al-F)) withb 02 (T) = 3b 2(R)(3cos2θ(T) − 1) (c-axis ∥ 3-fold axis). Direct relations between θ and the rotational angle ω of the matrix in dependence on temperature could be obtained by geometrical considerations. Local displacements of the anions were taken into account in terms of the parameterx characterizing their lattice position in the hexagonal cell of aluminium fluoride.