Competing electronic instabilities in the quadruple perovskite manganite PbMn$_{7}$O$_{12}$

Structural behaviour of PbMn$_{7}$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 PbMn$_{7}$O$_{12}$ onsets at 294 K with the incommensurate propagation vector $mathbf{k}_s=(0,0,sim2.08)$. At 110 K another structural transition takes place where the propagation vector suddenly drops down to a emph{quasi}-commensurate value $mathbf{k}_s=(0,0,2.0060(6))$. The emph{quasi}-commensurate phase is stable in the temperature range of 40K - 110 K, and below 40 K the propagation vector jumps back to the incommensurate value $mathbf{k}_s=(0,0,sim2.06)$. Both low temperature structural transitions are strongly first order with large thermal hysteresis. The orbital density wave in the emph{quasi}-commensurate phase has been found to be substantially suppressed in comparison with the incommensurate phases, which naturally explains unusual magnetic behaviour recently reported for this perovskite. Analysis of the refined structural parameters revealed that that the presence of the emph{quasi}-commensurate phase is likely to be associated with a competition between the Pb$^{2+}$ lone electron pair and Mn$^{3+}$ Jahn-Teller instabilities.