An optical investigation into the 3d1and 3d2transition- metal halides and oxyhalides, compounds near to delocalisation

Examination is made of the low d-electron count d’ and d23d transition-metal- halides and oxyhalides. Their electronicstructure is considered with particular emphasis on the approach to delocalisation in these materials. Optical absorption measurements reveal strong unexpected bands overlapping with the standard localised d-d ‘ligand field’, and charge-transferp dexcitations.Possibleoriginsarediscussed.Astrongtemperaturedepen- dence for this anomalous absorption is seen to accompany structural phase distortions in (Y- TiCI,, a-TiBr, and Ti12,and these transitions are examined further with magnetic, far- infrared absorption and structural studies. The well known transition for the above a- trihalides appears to be explained satisfactorily by a weak Ti-Ti pairing towards the MoCI, structure. The previously unknown transition for Ti12seems most reasonably interpreted as a change from magnetic Mott insulator to delocalised symmetry-broken semiconductor, with the dibromide and dichloride seen as being already in the distorted, semiconducting state at room temperature. The significant positive trigonal distortion of these dihalides below the transition is seen through LMTO band-structure calculations to be important for achieving semiconductivity, although a further symmetry reduction is required for complete, filled-band separation. The magnetic properties of the materials considered to be nearest to delocalisation are characterised by a low, temperature-independent, paramagnetic condition. For the B-TiX, and TiOX materials (X = CI, Br, I) this seems inconsistent with their non-metallic condition, and with an undistorted crystal structure. Further high- resolution crystallographic work is called for here.