Atomic displacements in the normal-incommensurate phase transition in Co-åkermanite (Ca2CoSi2O7)

Abstract

Abstract. New in-situ high-temperature X-ray powder diffraction data on the normal-incommensurate phase transition in Co-fikermanite (Ca2CoSi207) are presented. Evidence for the phase transition is found in the abrupt change in the thermal expansivity of the c lattice parameter at 220 ~ 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 ~ C.The c/a ratio for XzZSi207 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 CazCoSi20 7 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 To, temperatures just above Tc, and temperatures below T~). 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.

Single crystal growth and electron microscopy studies of Co/Zn-melilites with modulated structure

“…Changing the structural misfit by substitution of other cations or by temperature variation affects the amplitude of the modulation and the length of the modulation vector. X-ray refinement in the (3+2)-dimensional space suggests that the modulation is mainly caused by a displacive shift of the constituent atoms resulting in a rotation and deformation of the T 1 and T 2 tetrahedra [5,6]. These changes are accompanied by changes of the interlayer X-cation environment in a way that reduces the coordination number of X from eight to seven or even six [7][8][9][10].…”

“…In the past, the majority of studies of the modulation phenomena in melilites have been performed in the a˚kermanite (Ca 2 MgSi 2 O 7 ) [3,11], Co-a˚kermanite (Ca 2 CoSi 2 O 7 ) [6,11,12,19,22] and Fe-a˚kermanite (Ca 2 FeSi 2 O 7 ) [4,11] systems, eventually including substitutions on the X site (Ca-Sr) [8,19,21,23]. The present work deals with the growth and investigation of single crystals of Ca 2 Co 1Àx Zn x Si 2 O 7 solid solutions.…”

Melilite-like modulation and temperature-dependent evolution in the framework structure of K2Sc[Si2O6]F

“…The origin of the observed modulation has been explained by the structural misfit between the sizes of the tetrahedral T cations with respect to the interlayer X cations and explains the different magnitudes of modulation as a function of chemistry (Bindi et al, 2006, and references therein). On decreasing temperature the incommensurately (IC) modulated structure transforms to a commensurate lock-in structure, whereas on increasing temperature it transforms to a 3d-translational structure labelled a normal (N) structure, whereby the temperatures of the transitions are dependent on the chemical composition (Bagautdinov et al, 2002;Bindi & Bonazzi, 2003Brown et al, 1994;Hagiya et al, 1993Hagiya et al, , 2001Jia et al, 2006;Mą czkaa et al, 2015;McConnell et al, 2000;Merlini et al, 2005;Seifert et al, 1987;Seifert & Rö thlisberger, 1993;Riester & Bö hm, 1997;Riester et al, 2000;Rö thlisberger et al, 1990;Schaper et al, 2001). For Ca 2 MgSi 2 O 7 and Ca 2 Al 2 SiO 7 high-pressure studies have shown a phase transition from the IC to an N tetragonal phase comparable to that observed with increasing temperature (Ardit et al, 2011(Ardit et al, , 2012Merlini et al, 2009;Yang et al, 1997).…”

Structure and phase transitions in Ca2CoSi2O7–Ca2ZnSi2O7 solid-solution crystals

“…Much experimental and theoretical work has been carried out in order to clarify the deformation of the basic crystal lattice, the displacement patterns and the corresponding symmetry changes resulting from the misfit between the layers formed by the [T 1 O 4 ] and [T 2 2 O 7 ] groups, and the interlayer cations that lead to complex incommensurate modulation regimes (Hemingway et al, 1986;Seifert et al, 1987;Rö thlisberger et al, 1990;Van Heurck et al, 1992;Tamura et al, 1996). Particular insight into the fundamental mechanisms of the modulation formation was gained by studying the phase transformational processes toward the non-modulated normal phase (Iishi et al, 1990;Brown et al, 1994;Jiang et al, 1998;McConnell et al, 2000;Schosnig et al, 2000;Kusaka et al, 2001;Kusz & Bö hm, 2001;Bindi & Bonazzi, 2005;Merlini et al, 2005) and more specifically into the low-temperature commensurate lock-in phase (Riester & Bö hm, 1997;Schosnig et al, 2000;Riester et al, 2000;Bagautdinov et al, 2002; via the variation of temperature and/or chemical composition.…”