LOCAL-STRUCTURE OF (CA, SR)(2) (MG, CO, ZN) SI2O7 MELILITE SOLID-SOLUTION WITH MODULATED STRUCTURE

The local structure around Co, Zn and Sr atoms in incommensurately mod ulated, melilite-type X(2)T(1) (T2O7)-O-2 (X = Ca and Sr, T-1 = Mg, Co and Zn, T-2 = Si) solid-solutions has been investigated by EXAFS anal yses. The modulated structure was confirmed in Ca2-xSrxCoSi2O7 solid-s olutions with X = 0.0 to 0.6 and for both Ca2Mg1-yCoySi2O7 and Ca2Mg1- yZnySi2O7 solid-solutions over the whole compositional range at room t emperature. The actual bond-distances determined by the EXAFS method f or the T-1 site (Co-, Zn-O) in the modulated structure are longer than the mean bond-distances obtained from the X-ray diffraction method. T his is attributable to the libration of the T1 tetrahedra. In the Ca1- xSrxCoSi2O7 solid-solution both the Sr-O and Co-O distances by the EXA FS method for the X-site increase from Ca end-member to Sr end-member. These increases are respectively 0.8% and 0.6%. This means the local expansions of the tetrahedral sheets and of the XO polyhedra are well matched. In the modulated Ca2Co1-YMgYSi2O7 and Ca2Zn1-YMgYSi2O7 solid- solutions, the actual Co-O and Zn-O distances for the T-1-sites are ne arly constant in the whole compositional range. The compositional vari ations of the local structure around the cations in the solid-solution are different for the X and T-1 sites. It is concluded that the local geometric restriction for the size of substituted cation in X site is larger than that in T-1 site. The dimension of the tetrahedral sheet puts restriction on the size of the cations situated at the interlayer X sites. In other words, the different behavior of the local geometri c restriction between the X and T-1 sites is an important feature of t he melilite structure and is also related to the modulated structure.