The nature of the incommensurate structure in åkermanite, Ca2MgSi2O7, and the character of its transformation from the normal structure
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J.D.C. McConnell
Study of the Mössbauer effect over a range of both temperature and pressure on polycrystalline Fe-doped åkermanite, and X-ray measurements of spontaneous strain on single crystals of åkermanite under pressure, have been used to study the nature of the order parameters, and the characteristics for the transition of the normal to incommensurate structure in this compound. A recent study implies that the known atomic displacements in this phase must satisfy the symmetry of a space group irreducible representation induced from the representations A2 and B1 of the group associated with the vector 000 of the space group P¯421m, the high temperature space group of melilite. The Mössbauer effect deals with local order, and may be explained directly in terms of the existence of two types of 57Fe2+ containing Z tetrahedra, one of which is simply rotated on the basis of the A2 representation (η) and the other distorted on the basis of the diad symmetry of B1,(Ξ). In contrast the single crystal strain effects observed by single crystal X-ray diffraction must be explained in terms of both local strains associated with the A2 and B1 distortions, and a macroscopic strain field which couples the local distortions under the full symmetry of the tetragonal phase.
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Articles in the same Issue
- Cocrystallization of chiral cobalt complexes via formation of quasiracemates
- An X-ray profile analysis on the growth imperfections and internal strains in vapour-deposited lead films
- Solving crystal structures without Fourier mapping. II. Non-centrosymmetric case
- On the question of commensurability – The Nowotny chimney-ladder structures revisited
- Analysis of occupational and displacive disorder using the atomic pair distribution function: a systematic investigation
- The nature of the incommensurate structure in åkermanite, Ca2MgSi2O7, and the character of its transformation from the normal structure
- Thermal annealing of metamict titanite: A synchrotron radiation and optical birefringence study
- Thermal motion of the univalent metal ions in KCr5S8-type chalcogenides, ternary chromium selenides MxCr5Se8 (M = Rb, Cs)
- An electron diffraction study of Cu ordering in Cu2.33-xV4O11
- Variable temperature study of the crystal structure of paracetamol (p-hydroxyacetanilide), by single crystal neutron diffraction
- A comparison of the molecular and crystal structures of the dimethylacetamide complexes from zinc chloride, bromide, and iodide
