The α form has monoclinic crystal structure, with the P21/a space group. The unit cell parameters at 273 K are: a = 650.7 pm, b = 1779 pm, c = 837.7 pm, β = 96.6 °, 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, the so called triborate and pentaborate groups. The two frameworks are connected by two (non-equivalent) sodium atoms, each surrounded by 8 oxygens, comprising finite chains of four NaO8 polyhedra with shared edges. The thermal expansion is sharply anisotropic, including negative thermal expansion. The thermal expansion tensor in 273–1000 K in function of absolute temperature T has α11 = 55–0.042T, α22 = 11, α33 = -15 + 0.032T (×10–6) K−1, μ = (c^α33) = 42°.[10] The β form has monoclinic crystal structure, with the P21/c space group. The unit cell parameters are a = 1173.1 pm, b = 788.0 pm, c = 1041.0 pm, β = 99.883 °, Z = 4. The structure consists of two infinite, independent, and interleaved boron–oxygen networks containing a complex borate anion [B8O13]2− formed by six BO3 triangles (Δ) and two BO4 tetrahedra (T), which can be viewed as a B5O10 group linked to a B3O7 group. This fundamental building block is identical to that of the α form and of silver octaborate Ag2B8O13, with some subtle differences