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

TL;DR

This study investigates the temperature-dependent structural modulations in PbMn$_{7}$O$_{12}$, revealing incommensurate and quasi-commensurate phases linked to orbital density waves and magnetic behavior, using high-resolution synchrotron X-ray diffraction.

Contribution

It provides detailed insights into the structural phase transitions and modulation vectors in PbMn$_{7}$O$_{12}$, highlighting the competition between electronic instabilities and structural distortions.

Findings

Structural modulation onsets at 294 K with incommensurate vector

Transition to quasi-commensurate phase at 110 K with vector $ extbf{k}_s=(0,0,2.0060(6))$

Low temperature transitions are first order with thermal hysteresis

Abstract

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.