SYNTHESIS AND CRYSTAL STRUCTURE OF NEW OXIDE MATERIALS IN CaO-Y₂O₃-SiO₂ SYSTEM

Toru Nagasawa, Hisanori Yamane and Masahiko Shimada

Institute for Advanced Materials Processing, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan, E-mail : shimada@ibis.iamp.tohoku.ac.jp

Keywords : rare-earth calcium silicates, cuspidine-type structure, silico-carnotite-type,structure, apatite-type structure

Oxides including rare-earth elements have possibility to become phosphor and laser materials because of doping activated rare-earth ions. Actually, some of these oxides are used practically as laser and phosphor materials. Finding a new compounds including rare-earth elements is needed to make more efficient materials.

In CaO-Y₂O₃-SiO₂ ternary system, apatite-type structure compounds have been reported at the compositions of Ca₂Y₈Si₆O₂₆ and Ca₃Y₆Si₆O₂₄ [l, 2]. It was pointed out that the real chemical composition of apatite-type compounds had not been established. The preparation of Ca₂Y₂Si₂O₉ having a cuspidine-type structure was carried out [3], but the crystallographic data has not been reported yet. Generally, silicates have many isomorphs and form many complicated structures. For this reason, new compounds having new structures would be expected in this multicomponent system. The present study attempted to prepare the compounds in CaO- Y₂O₃-SiO₂ ternary system and make clear their crystal structures.

CaCO₃, SiO₂ and Y₈O₃ powders were used as starting materials. These powders were weighed and mixed with ethanol. After drying and pelletizing, the samples were sintered on a Pt-Rh plate at 1700 K for 72 h in air. This sintering process was repeated until the X-ray diffraction pattern was unchanged. The sintered pellets were ground into powder and characterized by X-ray diffraction. The crystal structures of these compounds were refined by Rietveld analysis. Single crystals of new compounds, Ca₃Y₂Si₃O₁₂ and Ca₃Y₂Si₆O₁₈, were prepared from melts by slow cooling. The crystal structures were analyzed by single crystal X-ray diffraction. In CaO-SiO₂, SiO₂-Y₂O₃ and CaO-Y₂O₃ binary systems, the following compounds were prepared at 1673- 1700 K; CaSiO₃, Ca₃Si₂O₇, Ca₂SiO₄, Ca₃SiO₅, Y₂Si₂O₇ and Y₂SiO₅. These results agreed with previous reports. In CaO-Y₂O₃-SiO₂ ternary system, the compounds prepared by reaction sintering at 1700 K were Ca₂Y₂Si₆O₂₆ (apatite-type structure), Ca₂Y₂Si₂O₉ (cuspidine-type structure), Ca₃Y₂Si₃O₁₂ (silico-carnotite-type structure) and Ca₃Y₂Si₆O₁₈(new structure). These results are shown in Fig.1.

A single-phase having an apatite-type structure was prepared at Ca₂Y₈Si₆O_26. This phase is hexagonal, a = b = 0.9341(1), c = 0.6788(1) nm. The sample with the composition of Ca₃Y₆Si₆O₂₄ which was reported as apatite- type compound was two phase, Ca₂Y₈Si₆O₂₆ and CaSiO₃. The sample prepared at the composition of Ca₄Y₆Si₆O₂₅ consisted of Ca₂Y₈Si₆O₂₆ and Ca₃Y₂Si₃O₁₂. It was confirmed that Ca₂Y₂Si₂O₉ had a cuspidine-type structure (monoclinic P 21/c, a = 0.74468(3), b =1.049240(3), c =1.09C61(4) nm, b =110.170(3)^o).

The new compound, Ca₃Y₂Si₃O₁₂ is isostructual with silico-carnotite (Ca₅(PO₄)₂SiO₄). Ca₃Y₂Si₃O₁₂ is orthorhombic with space group Pnma, a = 0.65574(2), b =1.56066(3), c =1.00399(2) nm, and has nine -eight- and seven-coordinate sites for Ca²⁺ and Y³⁺ cations. Ca₃Y₂Si₆O₁₈ (monoclinic C 2/c, a =1.33422(5), b = 0.77320(3), c =1.47959(6) nm, b = 90.263(3)^o) is composed of ternary SiO₄ rings. Ca and Y atoms are in eight-, seven- and sixfold coordination sites between the layers of ternary SiO₄ rings(Si₃O₉).

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