The superconductor-semiconductor transition in the Li1+xTi2-xO4 spinel system

Abstract

Attempts to characterize the metal-semiconductor transition in the mixed system Li//1// plus //x Ti//2// minus //x O//4 are plagued by chemical instabilities associated with a deviation from Vegard's law that is a maximum near x approximately equals 0. 1. This paper reviews the structural chemistry to derive an energy-band model and the preparative chemistry to demonstrate surface disproportionation and chemical inhomogeneity between surface and bulk materials with the maximum instability occurring near x approximately equals 0. 09. The origin of this instability was identified; it is due to the coexistence of Ti**3** plus ions and Li** plus ions or cation vacancies on octahedral sites. The apparent critical composition x//c approximately equals 0. 1 previously reported for a metal-semiconductor transition is shown to arise from oxidation at grain boundaries. The bulk transition appears to be smooth and dependent on sample history. A proposed resolution of this problem is the existence of a spinodal decomposition into phases approaching Li left bracket Ti//2 right bracket O//4 and Li left bracket Li//1/////3Ti//5/////3 right bracket O//4 with a texture on the scale of 25 to about 500 A