THE LOCK-IN PHASE OF TWO-DIMENSIONALLY MODULATED Co-ÅKERMANITE Ca2CoSi2O7
M. Riester 1, H. Böhm 1 and V. Petricek 2
1 Institut für Geowissenschaften /
Mineralogie, Univ. Mainz, Germany
2 Institute of Physics./ Acad. of Sciences,
Prague, Czech Rep.
Keywords: Modulated
Structures, Akermanite, Lock-In Phase
Like other selected melilite compounds, Ca2CoSi2O7 exhibits a two-dimensional displacive structure modulation. A model for the incommensurate structure at room temperature was presented by Hagiya et al [1]. X-ray diffraction studies on the low temperature behaviour of melilite compounds revealed a low temperature lock-in phase transition for Ca2CoSi2O7 [2]. The experimentally determined magnitude of the modulation wave vector is a = 0.324(6). This value is commensurate within 2s. The model of the lock-in structure was refined in physical space based on a (3´3´1) superstructure. Strong distortions within the tetrahedral layer are accompanied by six- and seven-fold co-ordinated calcium sites. The strongly distorted regions of the structure can be described as units consisting of an deformed CoO4 tetrahedron and Si2O7 groups bent by an angle of 28°, in these units calcium has a co-ordination lower than 8. These structural units are located in the centre and on the edges of the supercell forming an octagonal arrangement, which results in low co-ordinations for 2/3 of the calcium sites. An analysis of literature data ( e.g. TEM investigations of the incommensurate structure of Ca2ZnGe2O7 [3]) show that similar structural units can be found in the incommensurate structure. Their wider octagonal arrangement (diameter 4 unit cells) leads to low co-ordinations for 2/7 of the calcium atoms. Thus, the tendency to form Ca-sites of lower co-ordination is the driving force for the modulation.
The structural changes occurring at the phase
transition into the lock-in phase becomes apparent by the
displacement modes of the cobalt atoms. These displacements
follow an in-phase rotational mode around the origin and an
anti-phase breathing mode around the centre.
Strongly distorted regions of the lock-in
structure. The circles mark the strongly distorted structural
units where six- and seven- fold oxygen co-ordination of calcium
is realised.