ALTERNATE DISTORTION MODES AND STABILITY RANGES IN A2IIBIVO4 SOLID-SOLUTIONS POSSESSING THE K2NIF4 STRUCTURE TYPE - A NEW STRUCTURAL MODIFICATION OF THE T-PHASE

Six solid solutions of formula (A1-xA′x)2(B1-xB′x)O4 have been prepared where A and A′ = Ba, Sr; B and B′ = Ti, Sn, Pb in different combinations to investigate the distortion modes and stability ranges in the K2NiF4 (T) structure types. These series illustrate that distortions along the a-axis is dominated by the B site ion, whereas distortions along the c-axis are governed by the A site ion which contrasts the behavior of the RE2CuO4 phases possessing the T′ structure type. The (Sr1-xBax)2PbO4 and Sr2Sn1-xPbxO4 systems yielded T structure tolerance factors (tlim) of 0.91 at x = 0.5, and 0.89 at x = 0.75, respectively, before a transformation to the Sr2PbO4 structure type. A new lower symmetry ( P4 mmm) modification of the T phase was found for Sr2Sn1-xPbxO4 (0.5 < x ≤ 0.75) which has partially ordered Pb and Sn atoms on the B sites. A Reitveld refinement of Sr2Sn0.25Pb0.75O4 yielded the following parameters: 25°C, space group P4 mmm, a = 4.1769 (2) Å, c = 12.591 (1) Å, V = 219.67 (4) Å3, ρ{variant} = 6.415 (3) g/cm3, R = 9.32%, Rwp = 11.21%, and Rbragg = 8.4%. This structure represents an alternate distortion mode for stressed K2NiF4 phases with mixed B site atoms. © 1992.