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 1 and d 2 3d transition-metal-halides and oxyhalides. Their electronic structure 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-transfer p-d excitations. Possible origins are discussed. A strong temperature dependence for this anomalous absorption is seen to accompany structural phase distortions in alpha -TiCl 3 , alpha -TiBr 3 and TiI 2 , and these transitions are examined further with magnetic, far-infrared absorption and structural studies. The well known transition for the above alpha -trihalides appears to be explained satisfactorily by a weak Ti-Ti pairing towards the MoCl 3 structure. The previously unknown transition for TiI 2 seems 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 beta -TiX 3 and TiOX materials (X=Cl, 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.