Thermal behaviour of the rigid boron-oxygen groups in the α-Na2B8O13 crystal structure

The crystal structure of the alpha-Na2B8O13 stable modification has been refined by single crystal X-ray diffraction at 293, 573 and 773 K. The compound is monoclinic, P2(1)/a space group, unit-cell parameters at 273 K are: a = 6.507(3), b = 17.796(8), c = 8.377(5) Angstrom, beta = 96.60(4)degrees, Z = 4. The structure contains two interlocking boron-oxygen frameworks, each of them consisting of alternating single and double rings composed of two triangles and a tetrahedron so called triborate and pentaborate, groups. Two non-equivalent sodium atoms linking two frameworks are coordinated by 8 oxygens. They are connected into finite chains consisting of four NaO8 polyhedra by sharing oxygen edges. The alpha-Na2B8O13 thermal expansion investigated by high temperature powder X-ray diffraction has sharply anisotropic character including negative thermal expansion: the coefficients of a thermal expansion tensor in temperature region 273-IOOOK are all = 55-0.042 (.) (273 + t), alpha(22) = 11, alpha(33) = -15 + 0.032 (.) (273 + t) (x 10(-6) K-1), mu = (c<^> alpha(33)) = 42degrees. That high anisotropy is caused by anisotropic thermal vibrations of the Na atoms. Under heating a high stability of the B-O rigid groups and a weak contraction of some B-O bond lengths were revealed. In response to anisotropic thermal behaviour of Na atoms rigid B-O groups of the anion framework rotate relative to each other as a three-dimensional hinge.