Crossover from itinerant-electron to localized-electron behavior in Sr1−xCaxCrO3 perovskite solid solution

Abstract

Polycrystalline samples of the perovskite family Sr1−xCaxCrO3 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 TN 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 = PC, a smooth insulator–metal transition is found at a TIM that decreases with increasing P for a fixed x; PC increases with x. These phenomena are rationalized qualitatively with a π∗-band model having a width Wπ that approaches crossover from itinerant-electron to localized-electron behavior as Wπ decreases with increasing x. The smaller size of the Ca2+ ion induces the structural changes and the greater acidity of the Ca2+ ion is primarily responsible for narrowing Wπ as x increases.