Chromate aluminate sodalite, Ca₈[Al₁₂O ₂₄](CrO₄)₂: Phase transitions and high-temperature structural evolution of the cubic phase

The structural behavior of a chromate aluminate sodalite, Ca₈[Al₁₂O₂₄](CrO₄) ₂, at room pressure and on heating from 463 to 982° C was determined by using in situ synchrotron X-ray powder-diffraction data [λ = 0.91997(4) Å] and Rietveld refinements. The sample was heated at a rate of about 9.5°C/min., and X-ray traces were collected at intervals of about 17°C. The structure at 463° C is a modulated cubic phase, whereas above 512°C, the structure is cubic with space group I4̄3m. The a parameter increases linearly from 512 to 982°C, and the percent change in volume is 0.9(1)%. Between 463 and 982°C, the Al-O distance is nearly constant, the angle of rotation of the AlO₄ tetrahedron, φ _Al, decreases by 0.6° and simultaneously, the Al-O-Al bridging angle increases by 0.8(2)°. Between 463 to 982° C, the Ca-O distances are nearly constant. The Ca atom is located nearly in the plane of the three O' framework atoms, so movement of the Ca atom out of this plane will cause longer Ca-O' bonds instead of shorter bonds that occur when the Ca atom is migrating toward this plane. The highly charged [Ca₄·CrO₄]⁶⁺ clusters bond strongly to the framework oxygen atoms, which limits the rotation of the framework AlO₄ tetrahedra; as a result, the expansion of the structure is small. We evaluate the differences between the thermal behavior of chromate aluminate sodalite and that of other structurally related phases (e.g., sodalite and danalite).