Determination of structures of Ca2COSi2O7, Ca2MgSi2O7, and Ca-2(Mg0.55Fe0.45)Si2O7 in incommensurate and normal phases and observation of diffuse streaks at high temperature

The structures of Ca2CoSi2O7, Ca2MgSi2O7, and Ca-2(Mg0.55Fe0.45)Si2O7 have been determined in the temperature range between 297 and 773 K with arbitra ry intervals. The structures of the incommensurate phase of the three compo unds are characterized by the presence of the six-, seven-, and eight-coord inated Ca-O polyhedra and of the bundles along the c-axes consisting of fou r arrays of the six-coordinated Ca-O polyhedra and an array of (TO4)-O-1 (T -1: Co, Mg, or Mg-Fe) tetrahedra in the structures. The number of bundles i n each material decreases at elevated temperatures. The incommensurate phas e undergoes a phase transition into the normal phase at 493 K in Ca2CoSi2O7 , at 360 K in Ca2MgSi2O7, and at 510 K in Ca-2(ME0.55Fe0.45)Si2O7. The feat ures of the structures of the normal phase are almost the same as those fou nd in the basic structures (the averaged structures of the incommensurate s tructures), and this fact implies that the characteristics of the structure s, such as the six-coordinated Ca-O polyhedra or fragments of the bundles, should be partially preserved at higher temperatures both in the incommensu rate structures and also in the structures of the normal phase. Analyses of anisotropic displacement parameters clarified that disorder of the modulat ion waves is developed in the structures at higher temperatures. The evolut ion of a disorder in the structures was ascertained by observation of the c ircular diffuse streaks in the vicinity of the transition temperature betwe en the incommensurate and normal phases.