The novel ternary phase CaAl2±xCr2±xO7 in the Al2O3–CaO–Cr2O3 system: What is the chromium solubility?

Abstract

Recently, in the Al2O3–CaO–Cr2O3 system, a Cr3+ ternary phase named wuboraite (CaAl2Cr2O7) was synthesized for the first time in an air atmosphere, which later showed an undetermined hint of solid solubility between Cr3+ and Al3+ in different atmosphere. In the present investigation, the solubility limits (or x values) of the CaAl2±xCr2±xO7 were determined by conducting high-temperature equilibrium experiments in both air and N2 atmospheres with the various proportionate mixtures of CaO, Al2O3, and Cr2O3, and subsequently using powder X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis. In the air atmosphere, the Cr richest end-member of the Wuboraite solid solution phase is CaAl1.77Cr2.23O7 (or x = 0.23), while that of Al richest end-member is CaAl2.18Cr1.82O7 (or x = 0.18). Whereas, in the N2 atmosphere, the Cr richest end-member of the wuboraite solid solution phase is CaAl1.77Cr2.23O7 (or x = 0.23), while that of Al richest end-member is CaAl2.15Cr1.85O7 (or x = 0.15).