High-pressure rare earth silicates: Lanthanum silicate with barium phosphate structure, holmium silicate apatite, and lutetium disilicate type X

摘要

The phase relations of a wide selection of rare earth disilicates have been investigated up to 10 GPa and 1700 degrees C using piston cylinder and multi-anvil equipment. Single-crystal X-ray structures have been obtained for the following high-pressure phases: (1) La-2.67(SiO4)(2): monoclinic, space group C2/m, Z = 2, a = 9.419(2), b = 5.445(1), c = 7.214(1) angstrom, beta = 115.71(3)degrees, R = 0.042; disordered Ba-3(PO4)(2) structure type, with 3 x b and 7 x b superstructures identified. (2) Ho-8.67(SiO4)(6)(OH)(2): hexagonal, P6(3)/M, Z = 1, a = 9.3221(4), c = 6.7347(2)A, R = 0.026; silicate hydroxyapatite. (3) Lu2Si2O7: tetragonal, P4(1)2(1)2, Z = 4, a = 6.5620(2), c = 11.9535(4) angstrom, R = 0.023; type X diorthosilicate structure, and the silicate analogue of tetragonal Er2Ge2O7. (c) 2005 Elsevier Inc. All rights reserved.