The nature of the incommensurate structure in åkermanite, Ca2MgSi2O7, and the character of its transformation from the normal structure

Abstract

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.