Cluster chemistry in the solid state: Structured diffuse scattering, oxide/fluoride ordering and polar behaviour in transition metal oxyfluorides

Oxide/fluoride ordering in three families of 'disordered' transition metal oxyfluoride compounds is carefully investigated and used to obtain insight into why the constituent inherently acentric octahedral units do not usually order in a long range ordered polar sense. Observed highly structured diffuse intensity distributions are used in each case to analyze the local crystal chemistry and orientational ordering rules governing the way in which the dipole moments from the individual polar pseudo-octahedral units add together. The common local oxide/fluoride ordering rules, the consequences of these rules for polar behaviour on the local scale as well as the role of configurational entropy and energy minimization in suppressing long range polar order are highlighted.