  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.
