- journal_title：European Journal of Mineralogy
- Contributor：Kevin S. Knight ; C. Michael B. Henderson ; Simon M. Clark
- Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
- Date：2010-
- Format：text/html
- Language：en
- Identifier：10.1127/0935-1221/2010/0022-2025
- journal_abbrev：Eur J Mineral
- issn：0935-1221
- volume：22
- issue：3
- firstpage：411
- section：Articles
The crystal structures of eight tetragonal, gillespite-structured phases in the effenbergerite(BaCuSi<sub>4sub>O<sub>10sub>)–wesselsite (SrCuSi<sub>4sub>O<sub>10sub>) solid solution (Sr<sub>1−xsub>Ba<sub>xsub>CuSi<sub>4sub>O<sub>10sub>, where x is the mol fraction of the Ba end-member), have been refined from powder, neutron time-of-flight, diffraction data. The accommodation of the larger, more electropositive Ba<sup>2+sup> cation within the crystal structure of SrCuSi<sub>4sub>O<sub>10sub> is achieved by increasing the magnitude of the rotation of the square-planar CuO<sub>4sub> group about the c axis, coupled with an anti-phase rotation, and concomitant tilting, of the Si<sub>4sub>O<sub>10sub> 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 Sr<sup>2+sup> and Ba<sup>2+sup>. 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 SiO<sub>4sub> 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.