Rafael J. Jiménez-Riobóo

Two-level systems and boson peak remain stable in 110-million-year-old amber glass.

The two most prominent and ubiquitous features of glasses at low temperatures, namely the presence of tunneling two-level systems and the so-called boson peak in the reduced vibrational density of states, are shown to persist essentially unchanged in highly stabilized glasses, contrary to what was usually envisaged. Specifically, we have measured the specific heat of 110 million-year-old amber samples from El Soplao (Spain), both at very low temperatures and around the glass transition Tg. In particular, the amount of two-level systems, assessed at the lowest temperatures, was surprisingly found to be exactly the same for the pristine hyperaged amber as for the, subsequently, partially and fully rejuvenated samples.

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.

On the existence of a second phase transition in ferroelectrics with Aurivillius-type structure through the study of the Young's modulus

The possible existence of a second phase transition in Aurivillius-structure-type ferroelectric compounds is discussed. For this purpose, measurements of the complex elastic (Youngs) modulus, dielectric constant, DC conductivity and Raman spectra as a function of temperature have been performed. The results suggest that the anomalies that appear in macroscopic complex elastic modulus, DC conductivity and, probably, those of the 25 cm-1 vibration mode of Raman spectra near the temperature of 300 °C do not correspond to a ferro-paraelectric or ferro-ferroelectric phase transition. Anomalies in the dielectric constant have only been found at the phase transition temperatures. Brillouin measurements show that the 300 °C anomaly is of extrinsic character, which could be connected to shallow point defects caused by the Bi3+ instability in the Bi2 O2 layer. With increasing temperature, the defects are ionized increasing the DC conductivity and changing the elastic properties of the material by pinning domain walls or dislocations. Electron spin resonance (ESR) spectra, obtained after heating the samples at T = 500 °C, show the line of the almost free electron from the ionized defects.

Photochemical solution processing of films of metastable phases for flexible devices: the β-Bi 2 O 3 polymorph

The potential of UV-light for the photochemical synthesis and stabilization of non-equilibrium crystalline phases in thin films is demonstrated for the β-Bi2O3 polymorph. The pure β-Bi2O3 phase is thermodynamically stable at high temperature (450–667 °C), which limits its applications in devices. Here, a tailored UV-absorbing bismuth(III)-N-methyldiethanolamine complex is selected as an ideal precursor for this phase, in order to induce under UV-light the formation of a –Bi–O–Bi– continuous network in the deposited layers and the further conversion into the β-Bi2O3 polymorph at a temperature as low as 250 °C. The stabilization of the β-Bi2O3 films is confirmed by their conductivity behavior and a thorough characterization of their crystal structure. This is also supported by their remarkable photocatalytic activity. Besides, this processing method has allowed us for the first time the preparation of β-Bi2O3 films on flexible plastic substrates, which opens new opportunities for using these materials in potential applications not available until now (e.g., flexible photocatalytic reactors, self-cleaning surfaces or wearable antimicrobial fabrics). Therefore, photochemical solution deposition (PCSD) demonstrates to be not only an efficient approach for the low temperature processing of oxide films, but also an excellent alternative for the stabilization of metastable phases.

Low-temperature properties of monoalcohol glasses and crystals

We discuss our work on simple aliphatic glass-forming monoalcohols at low temperatures, including experiments on specific heat, thermal conductivity, Brillouin scattering and x-ray diffraction. The family of simple monoalcohols is an interesting model system for exploring molecular glass-forming liquids, the low-temperature universal properties of glasses, and even the glass transition phenomenon itself. More specifically, we examine the role of the molecular aspect ratio in the kinetics of vitrification/crystallization, the reported appearance of particular cases of polymorphism (in ethanol) and polyamorphism (in butanol), and, especially, the influence of positional isomerism and the location of the hydrogen bond on the lattice dynamics and, therefore, on the universal low-temperature properties of glasses.

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.

Influence of the short-range structural properties on the elastic constants of Si/Ge superlattices

The elastic constant tensor of Sim / Gen superlattices with different modulation wavelengths has been determined by Brillouin light scattering spectroscopy. The C 11 elastic constant obtained for Sin / Gen (where bilayers are built up by an equal number of Si and Ge monolayers) and Sin / Ge4n superlattices are 13% above the predictions of the continuum elasticity theory, using the Cij values of bulk Si and Ge. On the other hand, the elastic constant tensor of Si4n / Gen and Si3n / Gen superlattices matches perfectly the predictions. A structural analysis by extended x-ray absorption fine structure spectroscopy of the interfaces has been carried out and the unexpected different elastic behaviour can be explained by the observed strains at the superlattice interfaces.

Do tunneling states and boson peak persist or disappear in extremely stabilized glasses

We review and concurrently discuss two recent works conducted by us, which apparently give opposite results. Specifically, we have investigated how extreme thermal histories in glasses can affect their universal properties at low temperatures, by studying: (i) amber, the fossilized natural resin, which is a glass which has experienced a hyperaging process for about one hundred million years; and (ii) ultrastable thin-film glasses of indomethacin. Specific heat Cp measurements in the temperature range 0.07 K < T < 30 K showed that the amount of two-level systems, assessed from the linear term at the lowest temperatures, was exactly the same for the pristine hyperaged amber glass as for the subsequently rejuvenated samples, whereas just a modest increase of the boson-peak height (in Cp/T3) with increasing rejuvenation was observed, related to a corresponding increase of the Debye coefficient. On the other hand, we have observed an unexpected suppression of the two-level systems in the ultrastable glass of i...

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.

In- and out-of-plane longitudinal acoustic-wave velocities and elastic moduli in h-BN from Brillouin scattering measurements

The elastic constants of high-quality, single-crystal hexagonal boron nitride (h-BN) have been measured by means of high resolution Brillouin spectroscopy using a modified reflected light microscope. The sound propagation velocity in the c-axis direction and perpendicular to the c-axis have been obtained from the Brillouin frequency shift, with a proper account taken of the vast difference between the ordinary and extraordinary refractive indices recently reported in the highly anisotropic layered h-BN crystal. The elastic constants c11 and c33 obtained from the Brillouin experiments are somewhat lower than previous determinations based on inelastic x-ray measurements and confirm the overestimation of the h-BN elastic constants by ab initio calculations based on density functional perturbation theory in the local density approximation.