Two-dimensional incommensurately modulated structure of (Sr0.13Ca0.87)(2)CoSi2O7 crystals

The incommensurate structure of (Sr0.13Ca0.87)(2)CoSi2O7 at room temperatur e has been determined from single-crystal X-ray diffraction data. The compo und has a non-centrosymmetric tetragonal basic cell of a = 7.8743 (4) and c = 5.0417 (2) Angstrom with the space group P (4) over bar 2(1)m. The refin ements of the basic structure converged to R = 0.038 for 757 main reflectio ns. The two-dimensional incommensurate structure is characterized by the wa vevectors q(1) = 0.286 (3)(a* +b*) and q(2) = 0.286 (3)( -a* +b*), where a* , b* are the reciprocal lattice vectors of the basic structure. With the (3 + 2)dimensional superspace group P-p4mg(P (4) over bar 21m), the refinemen ts converged to R = 0.071 for 1697 observed reflections (757 main and 940 s atellite reflections). The structure is described in terms of displacement of the atoms, rotation, distortion of CoO4 and SiO4 tetrahedra, and the par tial ordering of the Sr and Ca atoms accompanied with the modulation. Corre lated evolution of these features throughout the crystal gives rise to vari ous oxygen coordination around Ca/Sr. Comparison of the derived modulated s tructure to that of Ca2CoSi2O7 clarified that the partial substitution of C a by large alkaline-earth atoms such as Sr should decrease the distortion o f the polyhedra around the cations.