RESUMO
Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO(2) reveal that the antiferromagnetic transition observed at T(N1) = 14 K might be induced by an R3m --> pseudoproper ferroelastic transition. In that case, the group theory states that the order parameter associated with the structural transition must belong to a two-dimensional irreducible representation E(g) (x(2) - y(2), xy). Since this type of transition can be driven by a Raman E(g) mode, we performed Raman scattering measurements on CuFeO(2) between 5 and 290 K. Considering that the isostructural multiferroic compound CuCrO(2) might show similar structural deformations at the antiferromagnetic transition T(N1) = 24.3 K, Raman measurements have also been performed for comparison. At ambient temperature, the Raman modes in CuFeO(2) are observed at ω(E(g)) = 352 cm(-1) and ω(A(1g)) = 692 cm(-1), while these modes are detected at ω(E(g)) = 457 cm(-1) and ω(A(1g)) = 709 cm(-1) in CuCrO(2). The analysis of the temperature dependence of the modes in both compounds shows that the frequencies of all modes increase with decreasing temperature. This typical behavior is attributed to anharmonic phonon-phonon interactions. These results clearly indicate that none of the Raman active modes observed in CuFeO(2) and CuCrO(2) drive the pseudoproper ferroelastic transitions observed at the Néel temperature T(N1). Finally, a broad band at about 550 cm(-1) observed in the magnetoelectric phase of CuCrO(2) below T(N2) could be associated with magnons.
RESUMO
Raman scattering measurements were performed on the ferroelastic compound Rb(4)LiH(3)(SO(4))(4) to determine the nature of the structural transition observed at T(c) = 134 K. A double-grating spectrometer was used to obtain the Raman active B modes of Rb(4)LiH(3)(SO(4))(4). Back-scattering measurements, as a function of temperature, reveal that the 31 cm(-1) mode shows softening consistent with that of an order parameter. As a result, our investigation indicates that the structural transition in Rb(4)LiH(3)(SO(4))(4) should be cataloged as a pseudo-proper ferroelastic transition rather than a proper ferroelastic one.
RESUMO
Using sound velocity measurements, we report a detailed investigation of the elastic properties of Rb(4)LiH(3)(SO(4))(4) realized as a function of temperature and pressure. Results are compared to predictions of a phenomenological Landau model. Supported by recent Raman scattering measurements, we assume that the [Formula: see text] structural transformation observed at T(c) = 134 K corresponds to a pseudo-proper ferroelastic transition. For the numerical analysis, all coupling parameters are determined using the temperature dependence of the frequency of the soft optical B mode, the temperature dependence of spontaneous strains, and the pressure dependence dT(c)/dP = 191 ± 2 K GPa(-1) also determined in this work. Our comparison indicates that the [Formula: see text] structural transition in Rb(4)LiH(3)(SO(4))(4) is fully consistent with predictions derived using our pseudo-proper ferroelastic model. Thus, all data presented in this paper corroborate that the mechanism leading to the structural transition at T(c) = 134 K results from the softening of the B optical mode observed at 31 cm(-1). This detailed analysis also refutes the idea that Rb(4)LiH(3)(SO(4))(4) shows incomplete softening of the soft acoustic mode also associated with that structural transition.
RESUMO
The refractive index of liquid carbon monoxide at 78.3 +/- 0.5 K was measured by means of a relatively simple interferometric technique. The values obtained were n = 1.2122 for lambda = 632.8 nm and n = 1.2213 for lambda = 514.5 nm with 0.2% uncertainty.
RESUMO
A data acquisition system for high-resolution Fabry-Perot interferometry is described. Used in conjunction with a piezoelectrically scanned Fabry-Perot, this apparatus compensates for all frequency drifts, maintains interferometer alignment, and allows extended data acquisition times in selected regions of a spectrum.
