Raman spectroscopy of CaSnO3 at high temperature: a highly quasi-harmonic perovskite.

Calcium stannate perovskite (CaSnO(3)) has been studied by Raman spectroscopy at two excitation wavelengths (514.5 and 632.8 nm). No phase transition was observed. Rather, the thermal evolution of the Raman lines showed a high degree of harmonicity with small Grüneisen parameters and thermal line broadening following Γ=Acothθ/T, where the quantum temperature θ is determined by the phonon branch without further coupling with other degrees of freedom. The geometrical nature of phonon lines has been identified. High-temperature powder x-ray diffraction measurements provide thermal expansion coefficients of α(x)=13.9 × 10(-6) K(-1), α(y)=2.7 × 10(-6) K(-1), α(z)=14.3 × 10(-6) K(-1). The strongly quasi-harmonic behaviour observed and the lack of any indication of instability with respect to the post-perovskite structure points to the strongly first-order character of the reported perovskite to post-perovskite phase transition in this material, which appears to behave as a very good analogue to MgSiO(3) in the Earth's interior.

A phase transition close to room temperature in BiFeO3 thin films.

BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be mapped by using appropriately substrate-induced strain in epitaxial films. By using Raman spectroscopy, we conclusively show that films of the so-called supertetragonal T-BFO phase, stabilized under compressive strain, display a reversible temperature-induced phase transition at about 100 °C, and thus close to room temperature.

Temperature-dependent Raman scattering of DyScO3 and GdScO3 single crystals.

We report a temperature-dependent Raman scattering investigation of DyScO(3) and GdScO(3) single crystals from room temperature up to 1200 °C. With increasing temperature, all modes decrease monotonically in wavenumber without anomaly, which attests to the absence of a structural phase transition. The high temperature spectral signature and extrapolation of band positions to higher temperatures suggest a decreasing orthorhombic distortion towards the ideal cubic structure. Our study indicates that this orthorhombic-to-cubic phase transition is close to or higher than the melting point of both rare-earth scandates ([Formula: see text]), which might exclude the possibility of the experimental observation of such a phase transition before melting. The temperature-dependent shift of Raman phonons is also discussed in the context of thermal expansion.

Raman study of CeO2 texture as a buffer layer in the CeO2/La2Zr2O7/Ni architecture for coated conductors.

The CeO(2)/La(2)Zr(2)O(7)/Ni piled-up structure is a very promising architecture for YBa(2)Cu(3)O(7) (YBCO) coated conductors. We have grown YBCO/CeO(2)/LZO/Ni epitaxial structures by metalorganic decomposition (MOD) and metalorganic chemical vapor deposition (MOCVD) methods. The crystallographic quality of the CeO(2) layer is not well determined by conventional X-ray diffraction (XRD) due to the superposition of LZO and CeO(2) reflections. An alternative simple Raman spectroscopy analysis of the crystalline quality of the CeO(2) films is proposed. The F(2g) Raman mode of CeO(2) can be quantified either by using two polarization configurations (crossed or parallel) or at two different rotation angles around the normal axis (0 degrees and 45 degrees ) to obtain information about the sample texture. The sample texture can be determined via a quality factor (referred to as the Raman intensity ratio, RIR) consisting of calculating the ratio of the integrated intensity of the CeO(2) F(2g) mode at 0 degrees and 45 degrees in parallel polarization. This factor correlates with superconducting performance and the technique can be used as an on-line nondestructive characterization method.

Raman scattering of perovskite DyScO(3) and GdScO(3) single crystals.

We report an investigation of DyScO(3) and GdScO(3) single crystals by Raman scattering in various scattering configurations and at various wavelengths. The Raman spectra are well defined and the reported spectral signature together with the mode assignment set the basis for the use of Raman scattering for the investigation of rare earth scandates. The observed positions of Raman modes for DyScO(3) are for most bands in reasonable agreement with recent theoretical ab initio predictions of the vibrational spectrum for the same material. Further to the phonon signature, a luminescence signal is observed for both scandates. While the luminescence is weak for DyScO(3), it is very intense for GdScO(3) when using a 488 or 514 nm excitation line, which in turn inhibits full analysis of the phonon spectrum. We show that a meaningful phonon Raman analysis of GdScO(3) samples can be done by using a 633 nm excitation.