Pressure-induced charge ordering transition in CaMn7O12

Abstract

We use high-pressure resistivity and single crystal x-ray diffraction at ambient and low temperature to investigate the charge ordering phase transition of ${\mathrm{CaMn}}_{7}{\mathrm{O}}_{12}$. We have found that at ambient temperature the Jahn-Teller distortion of the ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}$ octahedra rapidly decreases above 20 GPa, and vanishes at 28 GPa, when two Mn octahedral sites initially occupied by ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ become regular and equivalent as the result of a charge delocalization. Such a change correlates with a two orders of magnitude drop in the resistivity and a symmetry increase from the low-pressure rhombohedral R$\overline{3}$ phase to the cubic Im$\overline{3}$ structure, the same as one found at ambient pressure above 440 K. This yields the slope of the charge ordering phase boundary of $d{T}_{c}/dp\ensuremath{\approx}\ensuremath{-}6$ K/GPa. This result is further supported by the lack of a structural phase transition up to the maximum measured pressure of 30 GPa when the experiment is performed at 70 K. The satellite reflections of the structural modulation of the multiferroic phase of ${\mathrm{CaMn}}_{7}{\mathrm{O}}_{12}$ observed at 70 K were found to hold up to 25 GPa with the structure keeping a constant modulation vector $k=(0\phantom{\rule{0.28em}{0ex}}0\phantom{\rule{0.28em}{0ex}}0.925)$ with pressure. The average structure at 70 K does not show other indications of further phase transition.