High-pressure rare earth disilicates REE2Si2O7 (REE = Nd, Sm, Eu, Gd):: Type K

A new structure type (K) is reported for the disilicates of Nd, Sm, Eu, and Gd made at high pressure. Crystals of type K were synthesized at 10 GPa, 1600-1700 degreesC in an MA6/8 superpress and used for single-crystal X-ray structure study by Kappa CCD diffractometry at room temperature. Crystal data are: monoclinic, space group P2(1)/n, Z = 4; Nd2Si2O7- a = 6.6658(2), b = 6.7234(3), c = 12.3975(6) Angstrom, beta = 102.147(3)degrees, V = 543.2 Angstrom (3), R = 0.029, and D-x = 5.584 g/cm(3); Sm2Si2O7- a = 6.6039(3), b = 6.6849(3), c = 12.3069(5) Angstrom, beta = 102.489(3)degrees, V = 530.4 Angstrom (3), R = 0.038, and D-x = 5.871 g/cm(3); Eu2Si2O7- a = 6.5777(3): b = 6.6652(4), c = 12.2668(8) Angstrom, beta = 102.671(4)degrees, V = 524.7 Angstrom (3), R = 0.030, and D-x = 5.976 g/cm(3); Gd2Si2O7- a = 6.5558(4), b = 6.6469(4), c = 12.2394(6)Angstrom, beta =102.844(3)degrees, V=520.0 Angstrom (3), R = 0.026, and D-x = 6.166 g/cm(3). The type K structure is built from a diorthosilicate group [Si2O7] interconnected by REE3+ cations in eightfold coordination with oxygen. The bridging oxygen (Si-O-Si) bond angle of the diorthosilicate group of rare earth disilicates stable at 1 bar (types A to G) is related to spatial accommodation of the REE3+ cation and ranges from 130 to 135 degrees in the light rare earth disilicates (types A, F, G) to 180 degrees in Lu2Si2O7 (type C). Volume reduction in the high-pressure type K structure is achieved largely by closure of the Si-O-Si bond angle to 122.7-124.4 degrees, through rigid body rotation of the two SiO4 tetrahedra. This also permits a marginal increase in the average coordination of the REE3+ cation. (C) 2001 Academic Press.