STRUCTURE INVESTIGATIONS OF THE HIGH-TEMPERATURE PHASES OF LA2SI2O7, GD2SI2O7 AND SM2SI2O7

Synchrotron X-ray powder diffraction data have been applied to study t he pressure dependence of the type G La2Si2O7 structure, to test the p recision of the type E Gd2Si2O7 structure and to investigate the unkno wn type F Sm2Si2O7 structure. Type G La2Si2O7 shows a phase transition above 150 kbar at 300 K. The precision of the atomic coordinates obta ined from a profile refinement of the synchrotron powder diffraction d ata of type E Gd2Si2O7 is one order of magnitude lower than that known from a single crystal X-ray diffraction analysis. The positions in th e type F Sm2Si2O7 structure of the Sm and Si atoms were located from p acking considerations using the similarity between the type F Sm2Si2O7 structure and the type G Ce2Si2O7 structure. The crystal structure of type F Sm2Si2O7 was solved and refined from single crystal X-ray diff raction data. The structure is triclinic, a = 8.553(5), b = 12.849(5), c = 5.397(2) Angstrom, alpha = 91.08(2), beta = 88.61(4), gamma = 89. 68(4)degrees. Space group P1, No.2, Z = 4, for the composition Sm2Si2O 7. The structure has SmO7 and SmO8 coordination polyhedra and Si2O76- ions. The two independent Si2O76- ions have the angles Si-O-Si of 132( 1)degrees.