Competing electronic instabilities in the quadruple perovskite manganite PbMn7O12

Structural behavior of $\mathrm{Pb}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ has been studied by high resolution synchrotron x-ray powder diffraction. This material belongs to a family of quadruple perovskite manganites that exhibit an incommensurate structural modulation associated with an orbital density wave. It has been found that the structural modulation in $\mathrm{Pb}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ onsets at 294 K with the incommensurate propagation vector ${\mathbf{k}}_{s}=(0,0,\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}2.08)$. At 110 K another structural transition takes place where the propagation vector suddenly drops down to a quasicommensurate value ${\mathbf{k}}_{s}=(0,0,2.0060(6))$. The quasicommensurate phase is stable in the temperature range of 40--110 K, and below 40 K the propagation vector jumps back to the incommensurate value ${\mathbf{k}}_{s}=(0,0,\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}2.06)$. Both low temperature structural transitions are strongly first order with large thermal hysteresis. The orbital density wave in the quasicommensurate phase has been found to be substantially suppressed in comparison with the incommensurate phases, which naturally explains unusual magnetic behavior recently reported for this perovskite. Analysis of the refined structural parameters revealed that that the presence of the quasicommensurate phase is likely to be associated with a competition between the ${\mathrm{Pb}}^{2+}$ lone electron pair and ${\mathrm{Mn}}^{3+}$ Jahn-Teller instabilities.