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

Abstract

The crystal structure of the α-Na₂B₈O₁₃ stable modification has been refined by single crystal X-ray diffraction at 293, 573 and 773 K. The compound is monoclinic, P2₁/a space group, unit-cell parameters at 273 K are: a=6.507(3), b=17.796(8), c=8.377(5) Å, β=96.60(4)°, 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 α-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 273–1000 K are α₁₁ = 55–0.042·(273+t), α₂₂ = 11, α₃₃ = -15 + 0.032·(273+t) (×10^–6 K^–1), μ = (c^α₃₃) = 42°. 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.