Large negative thermal expansion in the cubic phase of CaMn7O12

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 ${\ensuremath{\alpha}}_L$ 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.