Metal-insulator transition in the spinel-type (formula presented) system

Abstract

The normal thiospinel (formula presented) exhibits a temperature-induced metal-insulator (formula presented) transition around 230 K with structural transformation, showing hysteresis on heating and cooling. The symmetry changes from a high-temperature cubic phase in a metallic state to low-temperature tetragonal phase in an insulating state. A significant characteristic feature is the absence of localized magnetic moment below (formula presented) On the other hand, (formula presented) remains metallic down to 4.2 K without the structural transformation. We have systematically studied the structural transformation and electrical and magnetic properties of (formula presented) The variation of the metal-insulator transition with Ni concentration x is presented. A phase diagram between (formula presented) and x will be provided for the (formula presented) system. The (formula presented) varies drastically from 226 to 88 K with x from 0.00 to 0.13 and disappears around (formula presented) For (formula presented) the cubic and tetragonal phases coexist below (formula presented) For a high-temperature metallic phase, the value of the Pauli paramagnetic susceptibility increases monotonically with x, which shows (formula presented) at the Fermi energy (formula presented) through the decrease of the free-electron number density, where (formula presented) is the electronic density-of-state on the basis of a nearly free-electron model. By the introduction of a Ni ion to the A-site of (formula presented) in the spinel structure, whether the localized magnetic moment below (formula presented) arises or not will be discussed. © 2001 The American Physical Society.