Formation and Transformation of δ‐Ta 2 O 5 Solid Solution in the System Ta 2 O 5 ‐Al 2 O 3

In the system Ta 2 O 3 ‐Al 2 O 5 solid solutions of metastable δ‐Ta 2 O 5 (hexagonal) are formed up to 50 mol% Al 2 O 3 from amorphous materials prepared by the simultaneous hydrolysis of tantalum and aluminum alkoxides. The values of the lattice parameters decrease linearly with increasing Al 2 O 3 , content. The to β‐Ta 2 O 5 (orthorhombic, low‐temperature form) transformation occurs at ∼950°C. The solid solution containing 50 mol% Al 2 O 3 transforms at 1040° to 1100°C to orthorhombic TaAlO 4 . Orthorhombic TaAlO 4 contains octahedral TaO 6 groups in the structure.