High-Resolution X-Ray Diffraction Study of Commensurate-Incommensurate Phase Transition in [N(CH₃)₄]₂MnCl₄ under Pressure

Abstract

Using synchrotron radiation, we have performed a high-resolution X-ray diffraction study of a phase transition between the incommensurate phase and the commensurate phase characterized by the wave vector q=3c^*/7 in [N(CH₃)₄]₂MnCl₄ under pressure. The pressure dependence of the wave vector q exhibits an ``incomplete devil's staircase.'' The high-resolution capability of the synchrotron radiation reveals that the incommensurate satellite reflection shows an asymmetric and broad line shape along the modulation direction, while the q=3c^*/7 commensurate superlattice reflection is symmetric and its width is equal to the instrumental resolution. Such a profile of the incommensurate satellite reflection is found to depend on the history of the crystal.