In situ electron diffraction study of synthetic (Ca1-xSrx)(2)MgSi2O7 akermanites

This paper deals with transmission electron microscope experiments of Ca, S r-akermanite solid solutions at temperatures between 100 K and 375 K. The a im of the investigations was to study the compositional and temperature dep endence of phase transitions from the normal to the incommensurately modula ted structure of (Ca1-xSrx)(2)MgSi2O7 Single crystals observed by electron diffraction. With increasing Sr-content the normal to incommensurate phase transition temperature decreases linearly from ca. 350 K (x = 0.04) to 295 K (x = 0.22). Depending on temperature and composition, the incommensurate phase is structurally not uniform, its stability field can be divided into two regions. The transition between the two regions can be recognized by th e additional appearance of higher order satellite reflections in the diffra ction pattern. Additionally, the transition is characterized by an abrupt c hange of the modulation wavelength. Within the high-temperature, low ordere d region, the modulation wavelength varies between 18.1 Angstrom and 20.5 A ngstrom, whereas no changes of the wavelength have been found in the low-te mperature, higher ordering region. With increasing Sr-content the high-temp erature, low ordered phase covers a steadily increasing temperature range.