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

New in-situ high-temperature X-ray powder diffraction data on the norm al-incommensurate phase transition in Co-akermanite (Ca2CoSi2O7) are p resented. Evidence for the phase transition is found in the abrupt cha nge in the thermal expansivity of the c lattice parameter at 220 degre es C. In addition, the c lattice parameter exhibits premonitory effect s of the phase transition through the leveling out of the thermal expa nsivity 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 wi de variety of compositions. Correlation of the trends in mean atomic p ositions with c/a ratio and the sigmoidal variation in the c/a ratio f or Ca2CoSi2O7 provide insight into the atomic displacements occurring in the melilite structure as a function of temperature. Three temperat ure regimes are discussed in detail (i.e. temperatures well above Tc, temperatures just above Tc, and temperatures below Tc). The atomic dis placements occurring in each temperature regime are correlated with th e changes in anisotropy and orientation of the atomic displacement ell ipsoids for each site, as well as spectroscopic evidence for the incre asing number of Z- and T-site environments.