Structural phase transition in AlF3: An EPR study of doped powder samples

Searching for a microscopic description of structural phase transitions in AlF3, these transitions were followed by electron paramagnetic resonance (E PR) in situ measurements in X-band in the temperature range 290 K less than or equal to T less than or equal to 800 K with Cr3+- and Fe3+-doped powder samples of alpha-AIF(3). For each temperature the zero-field splitting (zf s) parameters of the paramagnetic transition metal ions were determined by simulation. Starting from the axial zfs parameters b(2)(0)(T) and bearing i n mind the trigonal symmetry, the application of the superposition model al lowed the calculation of the angle theta(T) (theta: <(c-axis, AI-F)) with b (2)(0)(T) = 3b(2)(R)(3cos(2)theta(T)- 1) (c-axis // 3-fold axis). Direct re lations between theta and the rotational angle omega of the matrix in depen dence on temperature could be obtained by geometrical considerations. Local displacements of the anions were taken into account in terms of the parame ter x characterizing their lattice position in the hexagonal cell of alumin ium fluoride.