Crystal structure of lanthanum oxyorthosilicate, La ₂ SiO ₅

Abstract

The crystal structure of La 2 SiO 5 was refined from laboratory X-ray powder diffraction data (CuK α 1 ) using the Rietveld method. The crystal structure is monoclinic (space group P 2 1 ∕ c , Z =4) with lattice dimensions a =0.93320(2) nm, b =0.75088(1) nm, c =0.70332(1) nm, β =108.679(1)°, 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 SiO 4 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 R 2 SiO 5 (R=Y and Gd). The volumes of RO 9 polyhedra steadily increase with increasing ionic radius of R, from Y 3+ to Gd 3+ to La 3+ , which causes substantial volumetric expansion of the crystals.