High-resolution Brillouin Research Articles

The glass transition beyond the time trap: Opto-acoustic dispersion

High-resolution Brillouin spectroscopy has been used as a crucial experimental technique to investigate the nature of the thermal glass transition in an epoxy. In order to study the coupling between the α-relaxation process and the thermal glass transition around the quasi-static glass transition temperature T g we introduce the opto-acoustic dispersion function as a very sensitive measure of the α-relaxations and show that close to but above T g the related relaxation spectrum still contains components within the microwave frequency region which finally is truncated by the thermal glass transition.

Microstructural and Mechanical Properties of Sono-Ormosils

Monolithic pieces and films of ormosils were obtained from ultrasound-assisted polycondensation of tetraethoxysilane (TEOS) and polydimethylsiloxane (PDMS). These sono-Ormosils were studied by means of high resolution Brillouin spectroscopy and their textures analyzed from adsorption isotherms. Evidence is given for the occurrence of organic and inorganic micro-phase separation in samples with high polymer concentrations. Nitrogen isotherms show that the texture of the samples is highly dependent on the composition.

Anomalous elastic behaviour of Pb0.76Ca0.24TiO3 (PTC) at the transition from the amorphous to the nanocrystalline state compared to Pb0.88La0.08(Zr0.65Ti0.35)O3 (PLZT)

AbstractThin amorphous films of PTC and PLZT of 1‐3 μm thickness were produced by rf‐sputtering at ambient temperature. Different grain sizes between the subnanometer and the micrometer region could be prepared by a subsequent annealing process. The increasing size of the crystallites was characterised by profile analysis of x‐ray powder diffraction spectra. It is expected that the small grain size of compact nanocrystalline materials will have drastic influences on their elastic properties. High resolution Brillouin spectroscopy was used to probe the hypersonic properties of the two materials at ambient temperature. For the PLZT samples the elastic modulus c11 shows the expected increase at the transition from the amorphous to the nanocrystalline state. On the contrary the PTC samples exhibit a surprising elastic instability in the same region.

Influence of the microstructure on the macroscopic elastic and optical properties of dried sonogels: A Brillouin spectroscopic study

The elastic and optical properties of organically modified silicates prepared by ultrasonics aided polycondensation of tetraethoxysilane and polidimethylsiloxane are studied by means of high resolution Brillouin spectroscopy. Nuclear magnetic resonance data evidence the microseparation of the organic and inorganic phases for the systems with high content of polymer formation. The elastic and optical properties are clearly influenced by the change in microstructure. We propose a three component mechanical model that qualitatively explains the observed variation of the elastic constant c11 versus molar fraction of dimethylsiloxane in these vitreous materials and develop a structural model that accounts for the observed dynamical behavior.

High-resolution Brillouin light scattering and angle-dependent 9.4-GHz ferromagnetic resonance in MBE-grown Fe/Cr/Fe on GaAs

Exploiting the high sensitivity of a recently assembled Brillouin-light-scattering (BLS) spectrometer, we investigate the magnetic excitations with nonvanishing in-plane wave vector k in antiferromagnetically coupled Fe/Cr/Fe (001), grown by molecular-beam epitaxy. The BLS spectra reveal surprisingly rich structures not previously observed in this very sample. In addition, we use angle-dependent 9.4-GHz ferromagnetic resonance to probe the uniform (k=0) collective excitation, previously investigated at 35 GHz. The observation of the acoustic and optical modes by both techniques allowed the determination of an accurate set of material parameters from the fit of theoretical curves to all data. While the results do not invalidate earlier conclusions, we offer an updated model which provides an alternative explanation for the observed spectra without the assumption of an extra effective surface anisotropy field.

Brillouin scattering study of gallium nitride: elastic stiffness constants

High-resolution Brillouin scattering measurements on a high-quality wurtzite gallium nitride (GaN) single crystal were carried out and elastic stiffness constants were determined. A comparison is given with the results of a recently reported model for calculation of the elastic constants of III - V semiconductors based on the modified version of Keyes's relations. A good agreement is found between the experimental and theoretical elastic constants for GaN.

Brillouin spectroscopy on dried sonogels

The elastic properties of organically modified silicates prepared by ultrasonics aided polycondensation of Tetraethoxysilane and Polidimethylsiloxane is studied by means of high resolution Brillouin spectroscopy. Our results evidence the microseparation of the organic and inorganic phases for the systems with high content of polymer. We propose a mechanical model that qualitatively explains the observed variation of the elastic constant c11 vs molar fraction of dimethylsiloxane in these vitreous materials and develop a structural model that accounts for the observed dynamical behavior.

The elastic behaviour of polycarbonate in the glassy state determined by Brillouin scattering

Measurements of the temperature and hydrostatic pressure dependence of the elastic constants of amorphous polycarbonate in the glassy state using high-resolution Brillouin spectroscopy are reported. It was found that the temperature and the pressure dependence of the longitudinal and the transverse sound velocity are strictly linear, in contrast to the behaviour found in polymethylmethacrylate (PMMA). A simple equation for the behaviour of the elastic constants c11 and c44, and the shear, Young's and bulk modulus, can be given, which fits the experimental data well.

Pressure dependence of the elastic constants of poly(methyl methacrylate)

We report measurements of the hydrostatic pressure dependence of the elastic constants of poly(methyl methacrylate) using high resolution Brillouin spectroscopy. We have measured the density ρ( p) 0 up to 500 MPa, and the longitudinal and transverse sound velocities up to 300 MPa. We have calculated the bulk modulus, Youngs modulus, shear modulus and Poisson's ratio. Initially there is a strong increase in the intensity of the shear phonons which levels off at p > 100 MPa. At this pressure a dispersion-like step in the shear modulus as well as a decrease in the bulk modulus is found, which is tentatively discussed as a ‘weak glass transition’ within the glass.

Premelting features and acoustic mode softening in the rotator phases of linear telomers: C17H36

The polymer induced alignment (PIA) technique is presented as a new technique to grow single-crystalline films of linear telomers. Combining the PIA technique with special high-resolution Brillouin techniques, it is possible to determine the coefficients of the elastic stiffness tensor of the crystalline state of linear telomers. Especially in their rotator phases. As model systems we have chosen the perfluoroalkane C20F42 and the linear n-alkane C17H36. A comparison of elastic data obtained from classically and PIA grown samples proves their equivalence as well as their complementarity. Precursors of critical melting discovered in the rotator phase of C17H36. are discussed in terms of the anomalous temperature dependence of certain elastic tensor coefficients and of related n-telomers.

Existence of a second glass transition in the mixed alkali cyanides

Abstract The temperature dependence of the elastic constants of KxNa1-x(CN) mixed crystals is investigated by means of high resolution Brillouin spectroscopy. Clear hints are found for the existence of an additional classical glass transition. X-Ray measurements support the observations of elastic properties.

Ferroelastic phase transition in (NH4)LiH3(SO4)4: Brillouin scattering studies and Landau theory modeling

Abstract High-resolution Brillouin spectroscopy was used to study the elastic properties of (NH4)4LiH(SO4)4. The temperature dependence of eleven Brillouin modes from acowtic phonons, propagating in the [100], [001], [110] and [101] directions were measured in the temperature range from 140 to 300 K. The greatest elastic anomaly was observed in the plane perperdicular to the [001] crystallographical direction. The soft elastic constant c2S was found to vanish at TC ≃ 232 K. In order to explain the observed elastic anomalies a Landau-type free energy expansion is postulated and discussed.

Elastic properties of breakaway glass and liquid by Brillouin scattering up to 170 °C

The compressional VP wave velocity, refractive index n, and acoustic attenuation coefficient Γ of breakaway glass and liquid have been measured by high-resolution Brillouin spectroscopy up to 170 °C in platelet and backscattering geometries. A marked break in the compressional wave velocity-temperature relationship is observed at the glass transition temperature of 52 °C consistent with the glass transition temperature observed in ultrasonic experiments on the same sample. The temperature dependence of the compressional wave velocity determined by Brillouin scattering and ultrasonic interferometry are compared.

Elastic constants of the orientationally disordered phase of C2F6 by Brillouin spectroscopy

The elastic constants and elasto-optic coefficient ratios of the orientationally disordered β phase of C2F6 have been determined using the technique of high resolution Brillouin spectroscopy. Just below the triple point, at 169 K, the values of the elastic constants (in units of kbar) are C11=26.99±0.10, C12=19.32±0.10, and C44=4.44±0.04. The temperature dependence of all three elastic constants was also determined and appears to be linear for the temperature range investigated, with ΔC11/ΔT=−0.335±0.010, ΔC12/ΔT=−0.236±0.010, and ΔC44/ΔT=0.005±0.011 kbar/K. Values of the ratios of acoustic velocities in high symmetry directions in the crystal indicate near perfect elastic isotropy and that moderately strong rotation–translation coupling is present in β-C2F6.

The ferroelastic phase transition in (NH4)4LiH3(SO4)4: Brillouin scattering studies and Landau theory modeling

High-resolution Brillouin spectroscopy was used to study the elastic properties of (NH4)4LiH3(SO4)4 (ALHS). The temperature dependences of eleven Brillouin modes from acoustic phonons, propagating in the (100), (001), (110) and (101) directions, were measured in the temperature range from 140 to 300 K. The greatest elastic anomaly was observed in the plane perpendicular to the (001) crystallographic axes. The soft elastic constant c2S was found to vanish at T approximately=232 K. A Landau-type free-energy expansion is postulated and discussed, in order to explain the observed elastic anomalies.

Elasticity of single-crystal MgAl2O4 spinel up to 1273 K by Brillouin spectroscopy

The adiabatic single-crystal elastic constants, C ij , of stoichiometric magnesium aluminate spinel (MgAl2O4) have been measured up to 1273 K by highresolution Brillouin spectroscopy, using a 6-pass tandem Fabry-Perot interferometer and an argon ion laser (514.5 nm). Two platelet samples were employed for probing the acoustic phonons along [100] and [110] directions by platelet and backscattering geometries. The measured temperature dependences of the elastic moduli show a distinct anomaly at 923 K in the shear modulus C s = (C11-C12)/2 (along [110] direction) and the longitudinal modulus C11 (along [100] direction). This anomaly is consistent with the order-disorder phase transition, resulting from the atomic exchange between Mg at the tetrahedral site and Al at the octahedral site, which has been well documented recently (Peterson et al. 1991; Millard et al. 1992) by neutron powder diffraction and 27Al magic-angle spinning NMR. The values of the temperature derivatives of v p , v s , and K s , in the temperature range 300–923 K, calculated by the Voigt-Reuss-Hill approximation are -0.40ms−1 K−1, -0.26ms−1 K−1, and -1.89 x 10−2GPaK−1.

Ferroelastic phase transition in K3Na(SeO4)2: Brillouin-scattering studies and theoretical modeling.

High-resolution Brillouin spectroscopy was used to study the elastic properties of ferroelastic ${\mathrm{K}}_{3}$Na(${\mathrm{SeO}}_{4}$${)}_{2}$. The temperature dependence of nine Brillouin modes of the acoustic phonons propagating in the [100], [010], [001], and [011] directions were measured in the temperature range 290--380 K. These results indicate a 3\ifmmode\bar\else\textasciimacron\fi{}m\ensuremath{\rightarrow}2/m transition at ${\mathit{T}}_{\mathit{c}}$=334 K followed, by approximately 12 K, by a transition involving an unknown mode Q. Mode softening in the ${\mathit{c}}_{66}$ and ${\mathit{c}}_{44}$ elastic constants results from the main first-order transition while an additional softening in ${\mathit{c}}_{33}$ and ${\mathit{c}}_{44}$ is due to coupling with Q. A Landau-type free-energy expansion has been postulated and discussed in order to explain the qualitative features of the system.

Cw stimulated Brillouin gain spectroscopy of liquids

We demonstrate the use of cw stimulated Brillouin gain (SBG) spectroscopy to measure high resolution Brillouin spectra of liquids. Results are reported for thirteen liquids at room temperature, all obtained in a polarized (VV) configuration with a scattering angle of 178°. The observed Brillouin frequencies compare favorably with those predicted on the basis of previous spontaneous Brillouin light scattering measurements. The narrow Brillouin bands of carbon disulfide (70 MHz half-width at half-maximum) are easily resolved in the SBG experiment, and the sensitivity is sufficient to detect the Mountain mode in the SBG spectrum of carbon tetrachloride.

Elastic properties of Rb4LiH3(SO4)4 and K4LiH3(SO4)4

High-resolution Brillouin spectroscopy was used to study the elastic properties of Rb4LiH3(SO4)4 and K4LiH3(SO4)4 crystals. In contrast to earlier studies of the former compound, the present results indicate that the ferroelastic phase transition, which occurs at approximately 115 K, is of the 4 to 2 type rather than 4 mm to mm2 as previously believed. An appropriate theoretical model using a two-component order parameter free energy expansion has been presented based on the Landau theory. K4LiH3(SO4)4 does not appear to be ferroelastic in the temperature range examined and only exhibits a slight change in slope of the temperature plots for elastic constants associated with longitudinal phonons.

Acoustic, opto-acoustic, and thermal properties investigated around the phase transitions of NaCN and Na(CN)xCl1-x mixed crystals.

The elastic and opto-acoustic properties of sodium cyanide crystals NaCN and sodium cyanide--sodium chloride mixed crystals Na(CN${)}_{\mathit{x}}$${\mathrm{Cl}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ (1\ensuremath{\ge}x\ensuremath{\ge}0.63) were investigated below room temperature using high-resolution Brillouin spectroscopy. Employing special scattering techniques, we were able to measure the elastic behavior in both the high- and low-temperature phases. Changing from systems with high ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ concentrations, which undergo structural phase transitions of first order, to systems with low ${\mathrm{CN}}^{\mathrm{\ensuremath{-}}}$ concentrations, which show glass transitions, the discontinuities of the extremely soft shear stiffness of these cubic crystals gradually vanish. Using a space- and angular-resolving Brillouin technique, we report data on the inhomogeneity of the Czochralski-grown mixed crystals. The consequences of these inhomogeneities for the phase-transition behavior have been studied by thermal analyses. We try to describe our shear elastic stiffness data in the framework of current theoretical approaches.