Temperature dependence of the modulation wavelength in Co-åkermanite

Abstract

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.

Acknowledgements

The authors would like to thank Professor Kazuaki Iishi (Yamaguchi University) for syntheses of the samples, Dr. Yusuke Wakabayashi and Professor Hiroshi Sawa (KEK) for the assistance at the X-ray measurements at beamline BL-4C. The synchrotron radiation experiments were performed at the Photon Factory, KEK, under the approval of PF-program Advisory Committee (Proposal No. 2002G228).