Influence of d-electron distribution on phase transitions in A2MX6 (Hexahalometallates IV)

The influences of the various effects on the phase transitions in hexahalometallates(IV) is summarized in the following rules: Variation of metal ions of one d-series (cation and ligands constant) leads to a decrease of transition temperatures (structural and magnetic) with increasing LFSE (Ligand Field Stabilisation Energy) or an analogue electronic parameter. Variation of metal ions between the 4d- and 5d-series. (The number of 3d-systems studied is too small to draw any conclusion on their systematical behaviour at the moment.) Variation of ligands (cations and metal ions constant) reduces the transition temperatures in the sequence I > Br > Cl. Variation of cations (metal ions and ligands constant) leads to a decrease of transition temperatures with increasing size of the cations. No explanation can yet be made to the fact, that some systems show several, other ones only phase transition (e.g. rhenates-technetates). A first compilation of phase transition data of octahedrally coordinated systems such as perovskites and spinels, metal hexafluorides etc. shows a certain validity of the above mentioned rules. However, comparable data are rather rare in these systems. At least the Neel-temperatures in metal(II)oxides [36] seem to fit reasonably into the d-electron configuration-LFSE-scheme. (orig./HK)