Crystal structure of lanthanum oxyorthosilicate, La2SiO5

The crystal structure of La2SiO5 was refined from laboratory X-ray powder diffraction data (CuK alpha(1)) using the Rietveld method. The crystal structure is monoclinic (space group P2(1)/c,Z =4) with lattice dimensions a=0.93320(2) nm, b=0.75088(1) nm, c=0.70332(1) nm, beta =108.679(1)degrees, and V=0.46687(1) nm(3). The final reliability indices were R-wp=7.14%, R-p=5.52%, and R-B=3.83%. There are two La sites in the structural model, La1 and La2. La1 is ninefold coordinated to oxygen, forming a tricapped trigonal prism with a mean La1-O distance of 0.263 nm. The La2O(7) coordination polyhedron is a distorted capped octahedron with a mean La2-O distance of 0.251 nm. The La1O(9) polyhedra share faces and the La2O(7) polyhedra share edges, forming two sets of sheets that alternate parallel to the (100) plane. These sheets are linked through SiO4 tetrahedra and non-silicon-bonded oxygen atoms to form a three-dimensional structure. This compound is isomorphous with the low-temperature (X-1) phases of R2SiO5 (R=Y and Gd). The volumes of RO9 polyhedra steadily increase with increasing ionic radius of R, from Y3+, to Gd3+ to La3+, which causes substantial volumetric expansion of the crystals. (C) 2006 International Centre for Diffraction Data.