Crossover from itinerant-electron to localized-electron behavior in Sr_1−<i>x</i>Ca_<i>x</i>CrO₃perovskite solid solution

Abstract

Polycrystalline samples of the perovskite family Sr(1-x)Ca(x)CrO(3) have been prepared at high pressure and temperature in steps of 1/6 over the range 0 ≤ x ≤ 1. Rietveld analysis shows a series of structural phase transitions from cubic to tetragonal to orthorhombic with increasing x. The cubic samples have no long-range magnetic order; the other samples become antiferromagnetically ordered below a T(N) that increases with x. At ambient pressure, the electric transport properties of the cubic and tetragonal phases are semiconducting with a small (meV range) activation energy that increases with x; the orthorhombic phase exhibits variable-range hopping rather than the small-polaron behavior typically found for mixed-valent, localized-electron configurations. Above a pressure P=P(C), a smooth insulator-metal transition is found at a T(IM) that decreases with increasing P for a fixed x; P(C) increases with x. These phenomena are rationalized qualitatively with a π(∗)-band model having a width W(π) that approaches crossover from itinerant-electron to localized-electron behavior as W(π) decreases with increasing x. The smaller size of the Ca(2+) ion induces the structural changes and the greater acidity of the Ca(2+) ion is primarily responsible for narrowing W(π) as x increases.