Raman study of transition-metal oxides with perovskite-like structure

Perovskite-like oxides exhibit a rich variety of properties of fundamental scientific interest and potential application value. The motivation for this work is to contribute to our knowledge of perovskite-like systems and strongly correlated systems in general. The polarized Raman spectra of single crystal and thin film CaCu₃Ti₄O₁₂, single crystal and thin film CaRuO₃, microcrystals of La_0.5Ca _0.5MnO₃, and ceramic and thin film CaMnO₃ have been investigated. In close comparison to results from lattice dynamics calculations most of the Raman lines in the CaCu₃Ti₄O₁₂, CaRuO₃, La_0.5Ca_0.5MnO₃ and CaMnO ₃ spectra, have been assigned to definite phonon modes. The validity of the model for twin orientation in the Pnma structure for CaRuO₃ and La_0.5Ca_0.5MnO₃ is confirmed. The analysis of the CaMnO₃ spectra contributed to the development of a model, based on four basic distortions of the (distorted) perovskite structure. The temperature behavior of the CaCu₃Ti₄O ₁₂ spectra shows that there is no evidence for structural phase transition in the temperature range 20–600 K, so such a transition cannot be responsible for the sharp drop in the dielectric constant below 100 K. The Raman spectra indirectly support the mechanism of formation of barrier layer capacitances in CaCu₃Ti₄O₁₂.The observation of additional Raman mode of nominal A_g symmetry is discussed in terms of coexistence of domains of different atomic arrangement, or alternatively of non-stoichiometry (Cu deficiency). In the case of the thin film, the tetragonal distortions could be responsible for the greater separation of the additional A_g line. No anomalies in the temperature behavior of the Raman lines of CaRuO₃ is observed, which is an indirect evidence for its lack of long-range magnetic ordering at low temperatures (depending on whether this ordering would be observable by Raman spectroscopy through spin-phonon coupling). In La_0.5Ca_0.5MnO ₃ the appearance of several Raman lines below T_N is analyzed in terms of ordering and freezing of the Jahn-Teller distortions in a superstructure. Polarized Raman spectra confirmed their usefulness in studying thin films and their properties.