Raman study of phonons in CaMn7O12 : effects of structural modulation and structural transition

Abstract

Polarized Raman spectra of oriented CaMn7O12 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 6Ag + 6Eg modes of the trigonal R¯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¯3 structure, arise well above the magnetic transitions near 90 and 50 K. Their appearance, however, correlateswith 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¯3 phase the spectrum transforms its shape to that expected for a structure with disordered Jahn-Teller distortions.