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The crystal structure of La2 Si O5 was refined from laboratory X-ray powder diffraction data (CuK α1) using the Rietveld method. The crystal structure is monoclinic (space group P 21 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) nm3. The final reliability indices were Rwp =7.14%, RP =5.52%, and RB =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 La2 O7 coordination polyhedron is a distorted capped octahedron with a mean La2-O distance of 0.251 nm. The La1 O9 polyhedra share faces and the La2 O7 polyhedra share edges, forming two sets of sheets that alternate parallel to the (100) plane. These sheets are linked through Si O4 tetrahedra and non-silicon-bonded oxygen atoms to form a three-dimensional structure. This compound is isomorphous with the low-temperature (X1) phases of R2 Si O5 (R=Y and Gd). The volumes of R O9 polyhedra steadily increase with increasing ionic radius of R, from Y3+ to Gd3+ to La3+, which causes substantial volumetric expansion of the crystals.
Crystal structure of lanthanum oxyorthosilicate, La2 Si O 5
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