Polarized Raman spectra of oriented $\mathrm{Ca}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ single crystals have been studied in a broad temperature range (5–500 K) covering the crystallographic, ferroelectric, and magnetic transitions. From the spectra obtained at room temperature (RT) 10 out of the total 12 Raman active $6A_g+6E_g$ modes of the trigonal $R\overline{3}$ phase have been identified and assigned to definite atomic motions in close comparison to results of lattice-dynamics modeling. With the decrease in temperature below 250 K some Raman bands split, and additional spectral features, not expected for the $R\overline{3}$ structure, arise well above the magnetic transitions near 90 and 50 K. Their appearance, however, correlates with the transformation due to incommensurate structural modulation revealed by recent x-ray and neutron-diffraction studies. As temperature increases above RT the intensity of the Raman lines strongly decreases, and entering the high-temperature $Im\overline{3}$ phase the spectrum transforms its shape to that expected for a structure with disordered Jahn-Teller distortions.

• Received 28 March 2014
• Revised 21 May 2014

DOI:https://doi.org/10.1103/PhysRevB.89.214302