Structural aspects of the metal-insulator transitions in (Ti 0.9975V 0.0025) 4O 7

Abstract

The structure of (Ti 0.9975V 0.0025) 4O 7 has been refined at 298, 135, and 100°K from single-crystal X-ray diffraction data. This sample belongs to the region of the Ti 4O 7 V 4O 7 phase diagram where still two electrical transitions are observed. The three structures of the V-doped sample are almost identical to the corresponding structures of pure Ti 4O 7. The charges are disordered in the metallic phase, they order very slightly in the intermediate phase, and they are almost completely ordered along the 3113 and the 4224 chains in the low-temperature phase. There is no evidence of cation pair-bond formation in the intermediate phase, whereas the Ti 3+ cations are all paired in the low-temperature phase. The cationic Debye-Waller factors in the intermediate phase are anomalously large, which is indicative of a disorder. Thus, in order to explain the physical properties of the intermediate phase, as in the case of the pure sample, all the Ti 3+ cations are thought to form pair bonds with no long-range order. The TiTi distances along the pseudorutile c-axis are shorter in the V-doped sample than in the pure one. This indicates that the relative bond strengths increase with the incorporation of vanadium. The main difference between pure Ti 4O 7 and (Ti 0.9975V 0.0025) 4O 7 is in the lattice parameters and unit-cell volume variations with temperature. In the latter sample two discontinuities are observed, each corresponding to one transition, whereas in the former only one transition is observed, which corresponds to the higher transition. In the low-temperature phase the V cations are either 3+ or 4+ and are either on the 3113 chains or on the 4224 chains. The V doping gives rise to four different patterns. Each one is discussed in order to explain the differences between V-doped sample and pure Ti 4O 7.