We present the Raman scattering spectra of the BaFe2X3 (X=S, Se) compounds in a temperature range between 20 K and 400 K. Although the crystal structures of these two compounds are both orthorhombic and very similar, they are not isostructural. The unit cell of BaFe2S3 (BaFe2Se3) is base-centered Cmcm (primitive P nma) giving totaly 18 (36) modes to be observed in the Raman scattering experiment. We have detected almost all Raman active modes, predicted by factor group analysis, which can be observed from the cleavage planes of these compounds. Assignment of the observed Raman modes of BaFe2S(Se)3 is supported by the lattice dynamics calculations. The antiferromagnetic long-range spin ordering in BaFe2Se3 below TN =255 K leaves a fingerprint both in the A1g and B3g phonon mode linewidth and energy.
Polarized Raman scattering spectra of superconducting K(x)Fe(2-y)Se2 and non-superconducting K0.8Fe1.8Co0.2Se2 single crystals were measured in the temperature range from 10 K up to 300 K. Two Raman active modes from the I4/mmm phase and seven from the I4/m phase are observed in the frequency range from 150 to 325 cm(-1) in both compounds, suggesting that the K0.8Fe1.8Co0.2Se2 single crystal also has a two-phase nature. The temperature dependence of the Raman mode energy is analyzed in terms of lattice thermal expansion and phonon-phonon interaction. The temperature dependence of the Raman mode linewidth is dominated by temperature-induced anharmonic effects. It is shown that the change in Raman mode energy with temperature is dominantly driven by thermal expansion of the crystal lattice. An abrupt change of the A1g mode energy near T(C) was observed in K(x)Fe(2-y) Se2, whereas it is absent in non-superconducting K0.8Fe1.8Co0.2Se2. Phonon energy hardening at low temperatures in the superconducting sample is a consequence of superconductivity-induced redistribution of the electronic states below the critical temperature.
Polarized Raman scattering measurements on IrTe2 single crystals carried out over 15 K -640 K temperature range, and across the structural phase transition, reveal new insights regarding the crystal symmetry. In the high temperature regime three Raman active modes are observed at all studied temperatures above the structural phase transition, rather than two as predicted by the factor group analysis for the assumed P3m1 symmetry. This indicates that the actual symmetry of the high temperature phase is lower than previously thought. Observation of an additional Eg mode at high temperature can be explained by doubling of the original trigonal unit cell along the c-axis and within the P3c1 symmetry. In the low temperature regime (below 245 K) the new Raman modes appear as a consequence of the symmetry lowering phase transition and corresponding increase of the primitive cell. All the modes observed below the phase transition temperature can be assigned within the monoclinic crystal symmetry. Temperature dependence of the Raman active phonons in both phases are mainly driven by anharmonicity effects. The results call for reconsideration of the crystallographic phases of IrTe2.
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