ALUMINATE SODALITES - SR8(AL12O24)(MOO4)2(SAM) AT 293-K, 423-K, 523-K, 623-K AND 723-K AND SR8(AL12O24)(WO4)2(SAW) AT 293-K

The crystal structure of the aluminate sodalite Sr8-[Al12O24](MoO4)2 (SAM) has been determined at 293 K from single-crystal X-ray data. The structure refinement was also performed employing the Rietveld method with neutron powder-diffraction data collected at nominally 293, 423, 523, 623 and 723 K. SAM is tetragonal below, but cubic above, 571 K. Crystal data: Sr8[Al12O24](MoO4)2, M(r) = 1728.6, F(000) = 800; at 293 K, I4(1)/acd, a = 18.8751 (6), c = 18.7839 (9) angstrom, V = 6692.1 (7) angstrom-3, Z = 8, D(x) = 3.43 Mg m-3; at 423 K, I4(1)/acd, a = 18.9056 (8), c = 18.822 (1) angstrom, V = 6727 (1) angstrom-3, Z = 8, D(x) = 3.41 Mg m-3; at 523 K, I4(1)/acd, a = 18.9218 (8), c = 18.850 (1) angstrom, V = 6749 (1) angstrom-3, Z = 8, D(x) = 3.40 Mg m-3; at 623 K, Im3BARm, a = 9.4643 (3) angstrom, V = 847.7 (1) angstrom-3, Z = 1, D(x) = 3.39 Mg m-3; at 723 K, Im3BARm, a = 9.4725 (3) angstrom, V = 850.0 (1) angstrom--3, Z = 1, D(x) = 3.38 Mg m-3. The tetragonal phase is pyroelastic with a spontaneous-strain coefficient of about 16 x 10(-4). Structurally it is characterized by ordered arrangements of the cage anions MoO4, which adopt the 'tetragonal orientation'. This results in conformational distortion of the sodalite framework and in displacive modulation of the Sr atoms. The cubic phase exhibits reorientational disorder of the MoO4 tetrahedra. The temperature dependencies of lattice parameters, spontaneous-strain and atomic displacement parameters have been determined. The mean linear thermal-expansion coefficient is 12 x 10(-6) K-1. Structural changes, within a given phase, are smaller than the e.s.d.'s of this powder experiment. At 293 K, SAW corresponds almost perfectly to its Mo analogue: Sr8[Al12O24](WO4)2, M(r) = 1904.4, F(000) = 864, I4(1)/acd, a = 18.8771 (6), c = 18.7819 (9) angstrom, V = 6692.8 (7) angstrom-3, Z = 8, D(x) = 3.78 Mg m-3.