Anomalous behaviour in the layer halides and oxyhalides of titanium and vanadium: a study of materials close to delocalisation

We have examined the MX2,MX, and MOX families (X = CI, Br, I) of titanium and vanadium compounds magnetically, optically and structurally, as representatives of materials known to be close to d-electron delocalisation. We are interested in ascertaining the actual proximity of the various compounds to delocalisation, the effect of temperature upon the effective magnetic moments, the coexistence of single-electron with many-electron optical excitations, and antiferromagnetism versus structural instability under metal-metal site interactions. In the MX, hexagonal layer family we also encounter the question of geometric frustration of antiferromagnetic order. Throughout, we question just how close our current one-electron band structure calculations are able to come in describing the electronic condition of these materials.