The Phase Diagram CaO–Al2O3–Ta2O5and the Crystal Structures of Ca2AlTaO6and CaAlTaO5

The subsolidus phase diagram CaO–Al2O3–Ta2O5has been determined at 1300–1350°C. Two ternary phases are present, Ca2AlTaO6and the new phase, CaAlTaO5. From Rietveld refinement of powder X-ray diffraction data, CaAlTaO5has the sphene structure, monoclinic 6.6760(3), 8.9546(3), 7.3494(3) ?,=114.098 (3)°, space groupC2/c, and Ca2AlTaO6has a partially ordered double perovskite structure, monoclinic 5.3915(3), 5.4321(4), 7.6508(4) ?,=90.076(5)°, space groupP21/c. The Al and Ta cations form a rock-salt-type arrangement with 3% cross substitution of Al on Ta sites and vice versa.