Observations of multiferroicity and giant improper ferroelectricity in ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$ invigorated the research on this material. Delicate structural changes have been reported to cause ferroelectricity in the magnetically ordered state of ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$ as well as high-temperature crystallographic transformations. Through high-resolution synchrotron x-ray diffraction experiments we show for the first time the large negative thermal expansion (NTE) behavior of the cubic phase of ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$. A large NTE coefficient ${\alpha }_L$ for a temperature window of around 60 K is inferred for the cubic phase in the phase coexistence region during the high-temperature structural transition. It is explained using quasirigid unit modes. We also observe a large phase coexistence region of about 100 K across the high-temperature first-order structural transition, which is different from previous reports.

• Received 28 September 2016
• Revised 7 March 2017

DOI:https://doi.org/10.1103/PhysRevB.95.144112