(1) As pointed out above, off-centre displacements have been reported for some, but not all, AlPO4 and GaPO4 zeotypes containing d4r cages, and diffraction methods can provide only limited insights into the preferred local environments. In order to study the dependence of the equilibrium position of fluoride anions, and of their dynamic behaviour, on the composition of the framework, AST-type zeotypes having four different compositions (SiO2, GeO2, AlPO4, GaPO4) are included.