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.

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2. M. Riester, H. Böhm, "Phase transitions of modulated Co-Åkermanite, Ca₂CoSi₂O₇", Z. Kristallogr., 212, (1997), 506 - 509
3. C. van Heurck, G. van Tendeloo, S. Amelinckx, "The modulated structure in the melilite Ca₂ZnGe₂O₇" Physics and Chemistry of Minerals 18 (1992), 441 - 452