Lattice dynamics and spin-phonon coupling in CaMn sub(2)O sub(4): A Raman study

We present polarized Raman spectra of CaMn sub(2)O sub(4) (marokite) measured in several exact scattering configurations from oriented single crystals. Most of the 11A sub(g) + 12B sub(1g) + 10B sub(2g) + 9B sub(3g) Raman-allowed modes are identified and assigned to definite atomic motions by comparison to the predictions of ab initio lattice dynamical calculations. The Raman modes at 474 cm super(-1) (A sub(g)), 478 cm super(-1) (B sub(1g)), and 614 cm super(-1) (B sub(1g)) exhibit anomalous softening at temperatures below T sub(N) approximately 220K due to a spin-phonon coupling which depends strongly on the phonon eigenvectors. There are no indications for a structural transition in the temperature range between 80 and 850 K. We argue quantitatively that the mechanism of the phonon softening below T sub(N) is similar to that in the undoped manganese perovskites.