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Raman spectroscopy of the charge- and orbital-ordered state in La0.5Ca0.5MnO3

2001; American Physical Society; Volume: 64; Issue: 14 Linguagem: Inglês

10.1103/physrevb.64.144429

1095-3795

M. V. Abrashev, Joakim Bäckström, L. Börjesson, M. Pissas, Nikolay Ivanov Kolev, M. N. Iliev,

Advanced Condensed Matter Physics

Polarized Raman spectra from microcrystals of ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3}$ were studied as a function of temperature, excitation photon energy, and scattering configuration. At temperatures below the transition from ferromagnetic metallic phase to antiferromagnetic insulating charge- and orbital-ordered (COO) phase, several lines appear in Raman spectra excited with photon energies $\ensuremath{\Elzxh}{\ensuremath{\omega}}_{L}<2.5 \mathrm{eV}.$ We argue that their appearance signals ordering and freezing of the Jahn-Teller distortions at this transition temperature into a superstructure with a doubled elementary cell. We suggest a simplified structural model based on an analysis of the normal phonon modes in the COO phase, neglecting the octahedral tilts and containing ordered ${\mathrm{Mn}}^{4+}{\mathrm{O}}_{6}$ (undistorted) and ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}$ (Jahn-Teller distorted) octahedra. The symmetries of the experimentally observed Raman lines are determined by comparing their relative intensities to those predicted by the polarization selection rules for a finely twinned quasicubic crystal. The most intensive Raman lines are assigned to definite normal modes in close comparison with corresponding modes in COO layered manganites and undoped $R{\mathrm{MnO}}_{3}$ $(R=\mathrm{La},$ Y).