Synthesis, crystal structure and thermal expansion of a novel borate, Ba3Bi2(BO3)4

Single crystals of novel metastable borate Ba3Bi2(BO3)(4), were obtained from melt with stoichiometric composition. The compound is structurally related to the A(3)Ln(2)(BO3)(4) family of luminescent borates (A = Ca, Sr, Ba; Ln - Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb). The crystal structure was solved by direct methods and refined to R = 0.041 (wR = 0.045), orthorhombic, Pnma, a = 7.9508(5), b = 17.399(1), c = 8.9791(5) angstrom, V = 1242.1(1) angstrom(3), Z = 4. The structure contains isolated BO3 triangles linked through Ba2+ and Bi3+ cations distributed among three cation sites, M1, M2 and M3. The mixture of the Ba3Bi2(BO3)(4) and BaBiBO4 phases in the similar to 1 : 1 ratio was investigated by means of high-temperature X-ray powder diffraction in air. Ba3Bi2(BO3)(4) demonstrates an anisotropic thermal expansion with the following coefficients: alpha(a) = 16, alpha(b) = 11, alpha(c) = 11, alpha(V) = 38 x 10(-6) K-1 at 25 degrees C. The anisotropy increases on heating: alpha(a) = 32, alpha(b) = 2, alpha(c) = 7, alpha(V) = 37 x 10(-6) K-1 at 700 degrees C. The reason for the strong anisotropy of thermal expansion is the preferable orientation of the BO3 triangles.