The rhombohedral structure of tricalcium silicate at 1200°C

Crystals of tricalcium silicate having a monoclinic superstructure transformed into a rhombohedral structure when they were heated at 1200 °C with gas flame: a=7.135(6) A, c=25.586(15) A, space group R3m, and contents of 9 x Ca2,9sSio,9sAlo,o40s. The rhombohedral structure has been determined based on a set of 371 diffraction intensities measured with a single-crystal diffractometer giving a final value of R = 10% for 159 reflections used. The structure obtained is similar to the reported rhombohedral average structure of the monoclinic phase, but it has the following significant differences: (i) of the three independent silicate tetrahedra in the threefold axis, the orientation of the one about Si(1) is rotated by 60°, about the threefold axis, compared to that of the corresponding tetrahedron in the latter. (ii) The tetrahedron about Si(2) statistically points into opposite c directions, the fraction of the tetrahedron that points into one c direction being 70 %, and (iii) all tetrahedra are tilted in such a way that their apical bonds, otherwise on the threefold axes, slightly swing away from the threefold axes with Si's being kept in the threefold axes. The structural relationship is discussed between the rhombohedral and the triclinic phases.