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Single crystals of the composition 6 Ca 2 Si0 4 · 1 Ca 3 (P0 4 ) 2 have been grown by the Verneuil-method. The crystals represent a structure derivative of a'-Ca 2 Si0 4 . Due to twinning of the crystals a structure determination was possible only on the basis of the weak superlattice reflections [Primly a = 9.40(8), 6 = 21.71(2), c = 6.83(3) Ä, Ζ = 2], Important features are: (1) All tetrahedra are tilted away from the ideal positions, (2) there is no strong ordering nor statistic distribution of Si, P, Ca, (3) there exists a relation between the tetrahedra positions enriched in Ρ and the occupancies of the neighbouring Ca positions. (4) In reality the superlattice reflections proved to be satellites suggesting a modulated structure.

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