Modulations in incommensurate (Ca_1–xSr_x)₂MgSi₂O₇ single crystals

Abstract 

Single crystals of (Ca_1–xSr_x)₂MgSi₂O₇ slightly doped with 1000 ppm Mn²⁺ and with x ranging from 0.04 to 0.32 were grown from the melt in a mirror furnace applying the Czochalski technique. Transmission electron microscopy (TEM) revealed incommensurately modulated structures at room-temperature for all compositions in accordance with earlier studies by electron paramagnetic resonance (EPR). Electron diffraction patterns clearly show satellite reflections typical for two-dimensional modulation, and their successive destabilization with increasing Sr content. The modulation is of tartan-like appearance. Beyond a Sr/(Sr+Ca) ratio of about 0.32 the synthesis of stable solid solution åkermanite type crystals was proved not to be feasible, indicating the existence of a miscibility gap in the Sr åkermanite system. As presumed from the diffuse scattering around the satellite reflections, and suggested more conclusively by crystallographic processing of high resolution EM images the Sr ions incorporated into the incommensurate crystal phase are distributed in an ordered fashion and are partly adapted to the displacive modulation of the pure åkermanite. This means, occupational modulation even makes a contribution to the overall modulation characteristics in (Ca_1–xSr_x)₂ MgSi₂O₇.