Synthesis and structure of a new polymorph Ba₂CuSi₂O₇

A new phase of dibarium copper disilicate, β-Ba₂CuSi₂O₇, was obtained by the solid-state reaction. Crystal data: M_r = 506.4, tetragonal, P 4̄ 2₁m, a = 8.284(2), c = 5.313(3) Å, V = 364.6(4) ų, Z = 2, D_c = 4.61 g/cm³, λ(MoKα) = 0.71703 Å, μ = 13.91 mm^-1, F(000) = 450 and T = 293(2) K; the final R = 0.0343, wR = 0.0382 and S = 1.02 for 512 observed reflections (I > 3σ(I)) refined by 34 variables. The compound was characterized by a layered structure composed of CuO₄ tetrahedra and Si₂O₇ anions with the Ba cations filling the cavities between the layers. The building blocks of the present structure are similar to those in the α-Ba₂CuSi₂O₇, but the linkage of CuO₄ tetrahedra and Si₂O₇ anions is significantly different. In particular, the double-tetrahedral anions of Si₂O₇^6- form an orthogonal arrangement in the present structure, in contrast to the parallel arrangement reported in the α-Ba₂CuSi₂O₇.