Modulated structure of (Sr0.13Ca0.87)2CoSi2O7

What is it about?

The incommensurate structure of (Sr0.13Ca0.87)2CoSi2O7 at room temperature has been determined from single-crystal X-ray diffraction data. The structure is described in terms of displacement of the atoms, rotation, distortion of CoO4 and SiO4 tetrahedra, and the partial ordering of the Sr and Ca atoms accompanied with the modulation. Correlated evolution of these features throughout the crystal gives rise to various oxygen coordination around Ca/Sr. Comparison of the derived modulated structure to that of Ca2CoSi2O7 clarified that the partial substitution of Ca by large alkaline-earth atoms such as Sr should decrease the distortion of the polyhedra around the cations.

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Why is it important?

(Sr0.13Ca0.87)2CoSi2O7 belongs to the family of melilite-type compounds of general stoichiometry A2T1T22O7, where A is a large eight-coordinated cation (A = Ca, Sr, Na, Ba, La), and T1 and T2 are small cations in a tetrahedral coordination (T1 = Co, Mg, Fe, Cr, Zn, Al,...; T2 = Si, Ge, Ga, Be,...). These compounds have been extensively studied for their two- dimensional incommensurate structures at ambient tempera- ture and for their phase transition to normal phase at heating and to commensurate structure at cooling.

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