THE MODULATED STRUCTURE OF SYNTHETIC CO-AKERMANITE, CA2COSI2O7

Diffraction patterns of synthetic Co-akermanite, Ca2CoSi2O7, reveal distinct satellite reflections at room temperature. An incommensurate modulation in this material has been determined by five-dimensional refinement of the structure. The basic structure is tetragonal P42(1)mBAR, unit-cell dimensions a = 7.8417 (6), c = 5.0249 (3) angstrom, V = 308.99 angstrom3, Z = 2, M(r) = 307.26, D(m) = 3.307 (4) g cm -3 at 293 K, D(x) = 3.302 g cm-3, Mo Kalpha with graphite monochromator, mu = 47.7 cm -1, F(000) = 302, R = 0.049 for 734 unique reflections. The modulated structure is also tetragonal P(P4mg)P42(1)m, k1 = 0.2913 x (a* + b*), k2 = 0.2913 x ( - a* + b*) where k 1, k2 are wavevectors and a*, b* are reciprocal-lattice vectors of the basic structure; R = 0.098 for 3915 unique reflections. The modulation is caused by the shifts of the constituent atoms and no evidence of occupancy anomaly was found at metal sites. The transition temperature from the incommensurate to the normal phase was determined to be 489 (5) K by in situ observation of the satellite reflections at high temperature.