Structural Modulation and Phase Transitions in Melilites
Crystals can often be seen in our daily life and a variety of high-tech aspects. The ideal crystal has three-dimensional lattice periodicity, which can be described by the developed space group. The existence of a lattice periodicity is apparent in the diffraction pattern which consists of sharp spo...
Mineralogie, Petrologie und Kristallographie
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|Zusammenfassung:||Crystals can often be seen in our daily life and a variety of high-tech aspects. The ideal crystal has three-dimensional lattice periodicity, which can be described by the developed space group. The existence of a lattice periodicity is apparent in the diffraction pattern which consists of sharp spots located on points of a reciprocal lattice. Half century ago, the crystal with a diffraction pattern involving sharp spots not belonging to a reciprocal lattice was founded. These sharp spots, so-called satellite reflections, are not simple fractions with respect to the lattice of main reflections. Those fractional indices are interpreted as due to a long-period ordering. The observation of satellite peaks showing a continuous temperature dependency leads to the concept of an incommensurate crystal structure. Since then, many more incommensurate crystal phases have been observed, including quasi-one-dimensional conductors (Comes et al., 1976), ferroelectrics (Dmitriev et al., 1998), alloys (Portier and Gratias, 1980), minerals (Hemingway et al., 1986; Seifert et al., 1987), composite crystal (Heimann et al., 1979), and so on. According to the origin of the incommensurability, the incommensurate crystals have been divided into different types. An important class is that of the modulated structures. These crystals can be described in terms of a basic structure with three-dimensional space group symmetry and a periodic derivation (the modulation) which in the incommensurate case has a period that does not belong to the lattice of the basic space group. The melilite families, occurring as solid solution minerals in natural rock assemblages, are observed independently with the incommensurately modulated features by Hemingway et al. (Hemingway et al., 1986) and Seifert et al. (Seifert et al., 1987) in synthetic end-member of Ca2MgSi2O7 åkermanite. The reason for the formation of the modulated structure is the misfit between the large X cations and the sheet-like tetrahedral framework. Changing the structural misfit by substitution of other cations or by temperature variation affects the amplitude of the modulation and the length of the modulation vector (Böhm, 1983; Iishi et al., 1990; Jiang et al., 1998; Röthlishberger et al., 1990; Schosnig et al., 2000). X-ray refinement in the (3+2)-dimensional space suggests that the modulation is mainly caused by a displacive shift of the constituent atoms resulting in a rotation and deformation of the T1 and T2 tetrahedra (Brown et al., 1994; Hagiya and Ohmasa, 1993). These changes are accompanied by changes of the interlayer X-cation environment in a way that reduces the coordination number of X from eight to seven or even six (Bagautdinov et al., 2000; Bindi et al., 2001; Riester et al., 2000; Tamura et al., 1996). It is further assumed that the flattened T1-tetrahedra surrounded by low-coordinated X cations show the tendency to form octagonal clusters, and that the arrangement of these clusters determines the strength of the overall modulation (Kusaka et al., 2001; Kusaka et al., 1998; Riester et al., 2000). The investigations of their composition and properties play an important role to understand the formation of rocks and furthermore, the evolution of the earth. It Synthetic melilite solid solutions extended this family and made it possible to study their structure features and other properties. Special end-members of melilite like Ca2Al2SiO7 and Ca2MgSi2O7 have potential applications as laser active materials (Le Boulanger et al., 2000) and (Foing et al., 1998) and long-lasting phosphorescent materials (Kodama et al., 1999), (Yamaga et al., 2002) and (Jiang et al., 2003). On the other hand, an interesting feature is that many melilites exhibit at room temperature a two-dimensional modulated structure. The modulation depends on temperature and composition. Although it is now clear that the general reason causing the modulation is due to the misfit between the intermediate large X cations and the sheet-like tetrahedral framework, the detailed nature of the modulation is not clear completely. Single crystal X-ray diffraction and the developing (3+2)-dimensional refinement provide evidence that the modulation attributes to the displacement of the constituent atoms as well as to the formation of the octagonal clusters arranging by four low coordinated Ca polyhedra and one T1-tetrahedron. Further, the modulation in melilites varies with composition. For example, an occupational modulation is observed in Sr/Ca-melilites. The transition from the incommensurate to the commensurate lock-in phase accompanying the formation of domains in melilites is not clear until now. Also, the studies of the structural modulated features of Co/Zn-melilite series are still lack till now. In this work, the first aim is to grown a series of high-quality single crystals including Ca2Mg1-xZnxSi2O7, Ca2Co1-xZnxSi2O7 and (Ca1-xSrx)2CoSi2O7 by the floating zone melting technique. The second aim is to investigate the variations of the modulation with composition and temperature, and to find out the transition features from the incommensurate phase to the low-temperature commensurate phase and the details of microdomains formed during the phase transition. Transmission electron microscopy and electron diffraction as well as XRD and DSC as compensatory way are used to study the structural modulation in selected melilites.|