ATOMIC DISPLACEMENTS IN THE NORMAL-INCOMMENSURATE PHASE-TRANSITION IN CO-AKERMANITE (CA2COSI2O7)

New in-situ high-temperature X-ray powder diffraction data on the normal-incommensurate phase transition in Co-akermanite (Ca2CoSi2O7) are presented. Evidence for the phase transition is found in the abrupt change in the thermal expansivity of the c lattice parameter at 220 degrees C. In addition, the c lattice parameter exhibits premonitory effects of the phase transition through the leveling out of the thermal expansivity at temperatures from 87 to 220 degrees C. The c/a ratio for X(2)ZSi(2)O(7) incommensurate melilites is shown to be constant for a wide variety of compositions. Correlation of the trends in mean atomic positions with c/a ratio and the sigmoidal variation in the c/a ratio for Ca2CoSi2O7 provide insight into the atomic displacements occurring in the melilite structure as a function of temperature. Three temperature regimes are discussed in detail (i.e. temperatures well above Tc, temperatures just above Tc, and temperatures below Tc). The atomic displacements occurring in each temperature regime are correlated with the changes in anisotropy and orientation of the atomic displacement ellipsoids for each site, as well as spectroscopic evidence for the increasing number of Z- and T-site environments.