Structural variations in the wesselsite-effenbergerite (Sr _1-xBa_xCuSi₄O₁₀) solid solution

The crystal structures of eight tetragonal, gillespite-structured phases in the effenbergerite(BaCuSi₄O₁₀)-wesselsite (SrCuSi ₄O₁₀) solid solution (Sr_1-xBa _xCuSi₄O₁₀, where x is the mol fraction of the Ba end-member), have been refined from powder, netron time-of-flight, diffraction data. The accommodation of the larger, more electropositive Ba ²⁺ cation within the crystal structure of SrCuSi₄O ₁₀ is achieved by increasing the magnitude of the rotation of the square-planar CuO₄ group about the c axis, coupled with an anti-phase rotation, and concomitant tilting, of the Si₄O₁₀ polyhedral unit. To an excellent approximation, these structural changes are equivalent to a rigid sphere substitution, the radius of which is given by the compositionally averaged ionic radii of Sr²⁺ and Ba²⁺. The compositional-dependence of the lattice parameter c is significantly larger than that for a at low values of x, and is particularly well parameterised in terms of the variations of the calculated ionic radius of the alkaline-earth site and the observed tilt of the SiO₄ tetrahedron. The lattice parameter a exhibits a negative deviation from Vegard's rule resulting from the more complex, coupled structural response to the change in the effective ionic radius at the Sr/Ba site.