## Abstract

The crystal structures of eight tetragonal, gillespite-structured phases in the effenbergerite(BaCuSi_{4}O_{10})-wesselsite (SrCuSi _{4}O_{10}) solid solution (Sr_{1-x}Ba _{x}CuSi_{4}O_{10}, 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 SrCuSi_{4}O _{10} is achieved by increasing the magnitude of the rotation of the square-planar CuO_{4} group about the c axis, coupled with an anti-phase rotation, and concomitant tilting, of the Si_{4}O_{10} 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^{2+} and Ba^{2+}. 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_{4} 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.

Original language | English |
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Pages (from-to) | 411-423 |

Number of pages | 13 |

Journal | European Journal of Mineralogy |

Volume | 22 |

Issue number | 3 |

DOIs | |

Publication status | Published - May 2010 |

Externally published | Yes |