Structural studies of Li 0.7VO 2 in the temperature range 20–300°C

Abstract

The ordered rock-salt type compound Li 0.7VO 2 has recently been found to exhibit some unusually large hysteresis effects in its magnetic and thermal properties between 150 and 280°C, related to the magnetic transition known to occur at about 190°C in LiVO 2. X-ray and neutron powder diffraction measurements have been made on samples of Li 0.7VO 2 in an effort to classify structural changes believed to accompany this transition but not hitherto reported in any detail. The unit cell parameters were found to change from a = 2.837, c = 14.775 Å at 24°C to a = 2.913, c = 14.640 Å at 263°C, with a large discontinuity in the transition region. Rietveld refinement was carried out on X-ray data collected at these temperatures and on neutron data collected at 35°C, the main change being an increase in the VO and VV distances from 1.98 and 2.84 Å at 24°C to 2.02 and 2.91 Å at 263°C, respectively. The only indication of any departure from R 3m symmetry in the low-temperature phase was the presence of two extremely weak superlattice peaks in the 24°C X-ray data which can be indexed in terms of an enlarged cell with a′ = a√3, c′ = c. Above 250°C, the gradual development of a second phase was noted, identified as the cubic spinel LiV 2O 4 resulting from the disproportionation of Li 0.7VO 2 into LiVO 2 and the spinel. Hysteresis effects were observed in the X-ray data which closely parallel those previously reported for the magnetic and thermal properties and which can be attributed to the disproportionation reaction.