Potassium rich rare earth (RE) borates K3RE(BO3)2

A series of new compounds in the K3RE(BO3)2 (RE = Y, Nd, Sm, Gd, Tb, Er and Lu) system were synthesized. Powder X-ray diffraction indicates that structures of the K3RE(BO3)2 series can be separated into two different types with boundary between Gd and Tb. Single crystals of two representative compounds K3Sm(BO3)2 and K3Y(BO3)2 were obtained from a K2O–B2O3 melt. The structure of K3Y(BO3)2, determined from single crystal X-ray diffraction data, belongs to Pnnm space group, with lattice constants of a = 9.3377(9) Å, b = 6.7701(6) Å and c = 5.5058(4) Å. With a larger rare earth element, e.g. Sm3+, K3Sm(BO3)2 crystallizes in space group Pnma, with cell parameters of a = 9.046(3) Å, b = 7.100(2) Å and c = 11.186(3) Å. The structure of K3Y(BO3)2 can be described as a three-dimensional framework formed by isolated YO6 octahedra jointed by BO3 triangles by sharing their apical oxygen atoms. The structure of K3Sm(BO3)2 contains infinite [SmO4BO3]∞ chains formed by corner sharing SmO7 pentagonal dipyramid and BO3 group, and those chains are interconnected by the other BO3 groups.

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