Micron and Microscopica Ada, Vol. 22, No. 3. p. 281. 1991. Printed in Great Britain.
0739-6260/91 $3.00 +0.00 Pergamon Press plc
ELECTRON MICROSCOPIC AND ELECTRON DIFFRACTION STUDY OF THE MELIL1TE
Ca2ZnGe2O7. C. Van Heurck, G. Van Tendeloo and S. Amelinckx University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
Abstract. The Ca2ZnGe2O7 member of the family of “melilite” compounds, which are all crystallographically related and exhibit two-dimensional incommensurately modulated structures, is the subject of a detailed electron microscopic and electron diffraction study. This specific compound was chosen since its modulation period is only weakly temperature dependent and close to a lock-in value of 3.5 at. The structure consists of layers of corner linked Ge04 and Zn04 tetrahedra having different possible orientations, thus giving rise to an orientation modulated structure. The structure is redescribed in terms of sub-units, allowing the introduction of a simple notation for the complicated tilt and rotation patterns that occur. The electron diffraction patterns are explained by assuming that the modulated structure consists of a “tartan’ like pattern of microdomains (consisting of subunits) having alternatingly different tilt patterns. Image simulations allow the interpretation of high resolution images made in thin crystal parts and direct evidence for the modulation is found using intermediate resolution in slightly thicker parts of the crystal. Based on these observations a model is proposed in which we assume that the structure is fragmented into microdomains which differ only by their tilt patterns. In the most frequently occurring case the interfaces between microdomains form a “tartan-like pattern consisting of 4 x 4, 4 x 3, 3 x 4 and 3 x 3 sub-units and in the ideai~sedcase the qve~torsin the two mutually perpendicular  and directions are = ~ a*  and Ei~ = ~a* leading to an apparent 7-fold period. Incommensurate structures may occur for more complicated and non-periodic uniform mixtures of 4 and 3 blocks. Moreover some crystal areas exhibit a one-dimensional modulation, still consistent with the lattice of subunits. This 1-q structure occurs in narrow bands and videotape recordings of “in-situ’ beam heating of the specimen shows that this apparently “frozen-in” metastable phase is replaced by the 2-q structure.