Temperature dependence of the modulation wavelength in Co-åkermanite

Two-dimensional modulation in synthetic Co-åkermanite, Ca2CoSi2O7, has been precisely studied using synchrotron X-ray diffraction methods in the temperature range between 180 and 523 K to determine the temperature dependence of q (the magnitude of the primary wavevectors of modulation waves). Three kinds of plateau-like regions are observed for q ∼ 2/7, 3/10 and 4/13 in addition to the low-temperature commensurate phase (q = 1/3). Using a simulation method we were able to generate nearly commensurate structures. The most probable structure of incommensurate Co-åkermanite is octagonal arrangements of the bundles along the c-axis, which are composed of four CaO6 arrays and a central array of CoO4 tetrahedra.