Modulations in incommensurate (Ca1-xSrx)(2)MgSi2O7 single crystals

Single crystals of (Ca1-xSrx)(2)MgSi2O7 slightly doped with 1000 ppm Mn2+ a nd with x ranging from 0.04 to 0.32 were grown from the melt in a mirror fu rnace 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 paramagnet ic resonance (EPR). Electron diffraction patterns clearly show satellite re flections typical for two-dimensional modulation, and their successive dest abilization with increasing Sr content. The modulation is of tartan-like ap pearance. Beyond a Sr/(Sr+Ca) ratio of about 0.32 the synthesis of stable s olid solution akermanite type crystals was proved not to be feasible, indic ating the existence of a miscibility gap in the Sr akermanite system. As pr esumed from the diffuse scattering around the satellite reflections, and su ggested more conclusively by crystallographic processing of high resolution EM images the Sr ions incorporated into the incommensurate crystal phase a re distributed in an ordered fashion and are partly adapted to the displaci ve modulation of the pure akermanite. This means, occupational modulation e ven makes a contribution to the overall modulation characteristics in (Ca1- xSrx)(2) MgSi2O7.