ESR studies of Gd3+centres to investigate the Oh1to D4h18structural transition of the perovskite fluorides RbCaF3and RbCdF3

Abstract
The influence of the rotational order parameter phi on the ESR line position for Gd3+ centres in AMF3 crystals is considered. First the rotation of spin-Hamiltonian axes closely linked to metal-ligand bonds permits easy and accurate measurements of phi , without any determination of the spin-Hamiltonian parameters. Second, the axial field parameter b2 degrees , for (GdF6)3- below Tc, depends on phi and (c/a)-1 while the precursive behaviour of the cubic parameter b4 could evidence rotational fluctuations above Tc.