Syntheses, Thermal Stability, and Structure Determination of the Novel Isostructural RBa₃B₉O₁₈ (R = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb)

A novel borate compound YBa₃B₉O₁₈ has crystallized in a melt of BaYB₉O₁₆. Single-crystal X-ray diffraction measurements reveal that YBa₃B₉O₁₈ adopts a hexagonal space group P6₃/m with cell parameters of a = 7.1761(6) Å and c = 16.9657(6) Å. The structure is made up of the planar B₃O₆ groups parallel to each other along the (001) direction, regular YO₆ octahedra, and irregular BaO₆ and BaO₉ polyhedra to form an analogue structure of β-BaB₂O₄. A series of isostructural borate compounds RBa₃B₉O₁₈ (R = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) were prepared by powder solid-state reactions. The DTA and TGA curves of YBaB₉O₁₆ show an obvious weight loss at about 955 °C associated with a decomposition into YBO₃, B₂O₃, and YBa₃B₉O₁₈ due to its incongruent melting behavior. The DTA and TGA curves of YBa₃B₉O₁₈ show that it is chemically stable and a congruent melting compound. A comparison of the structures of YBa₃B₉O₁₈ and β-BaB₂O₄ is presented.