A de Bernabé

Effect of stoichiometry on the dynamic mechanical properties of LiNbO3

Acoustic and optic phonons have been measured by Brillouin and Raman spectroscopies in LiNbO3 samples with different stoichiometries. Consequently, the elastic constants have been determined by Brillouin scattering as a function of the stoichiometry. The C44 elastic constant presents, as a function of the stoichiometry, a clear minimum for the [Li]/[Nb]=1 in the melt sample. This behavior must be related to the existence and competition of two kind of defects, i.e., extended defects induced when growing samples not at the congruent composition, and point defects present in nonstoichiometric samples. The optic phonons seem to be dominated by the point defects. The linear increase of the phonon lifetimes, with respect to an ideal stoichiometric sample, can be due to the scattering by the static imperfections and to an increase of the anharmonic scattering. Several Raman peaks, which appear when the [Li]/[Nb] ratio decreases, are related to the presence of Nb in antisites. The width of some particular phonon...

STUDY OF INTERFACES IN CO/CU MULTILAYERS BY LOW-ANGLE ANOMALOUS X-RAY DIFFRACTION

The innovative method of combining specular and off-specular low-angle x-ray diffraction, along with the anomalous scattering effect, has been used to characterize magnetron-sputtered Co/Cu multilayers. The anomalous dispersion of Co is employed to increase the electron density contrast between the cobalt and copper layer. The use of a simulation program has been proven to be a straightforward and reliable method to analyze x-ray low-angle diffraction patterns in such a nonperfectly ordered metallic multilayer system. This method has been successfully applied to data obtained from synchrotron experiments and the results compared with those performed using a standard laboratory diffractometer. The combination of both specular and off-specular scans has ensured the obtention of a single set of simulation parameters for the structure of the multilayer and its interfaces. In addition, the off-specular scans have permitted us to confirm, in a rather complex system, the validity of the distorted wave born appro...

Elastic constants of InxGa1-xAs and InxGa1-xP determined using surface acoustic waves

The elastic constant tensors of thin layers of InxGa1-xAs and InxGa1-xP with compositions around x = 0.5 have been determined using surface acoustic waves measured by Brillouin light scattering spectroscopy. Contrary to the case for the usual estimates, the experimentally obtained elastic constants are different from the ones calculated by simple linear combination of those of the corresponding constituent binary compounds. An averaging model combining a parallel-serial disposition of springs with the elastic constants corresponding to the binary compounds has been used to explain the elastic constants of the ternary alloys. This model explains perfectly the elastic properties of the InxGa1-xAs system, while those of the InxGa1-xP system are not reproduced.

Structural study of the mechanically alloyed Fe–Co–Cu nanocrystalline system

Abstract The local order around Fe, Co and Cu atoms has been investigated at room temperature by X-ray absorption spectroscopy (XAS) and the long range order has been studied by X-ray diffraction (XRD) in Fe–Co–Cu nanocrystalline alloys prepared by mechanical alloying. In order to determine the time evolution of the alloying process, samples have been studied after several processing times. The joint analysis of XAS and XRD data shows a picture for the alloy formation in which, Co alloys with Cu in the first stages of the process but the iron enters the alloy phase much later than the cobalt does. On the other hand, a refinement of the XRD data permits the identification of a residual iron phase previously detected by Mossbauer spectroscopy.

Anomalous Elastic Properties of Si/Ge Superlattices: The Role of Interfaces

The elastic constant tensors of Si m /Ge n superlattices with different modulation wavelengths have been determined by Brillouin light scattering spectroscopy, and their Youngs moduli measured by the nanoindentation technique. Two sets of samples have been studied: m = n and m = 4n. The C 33 elastic constant obtained for Si n /Ge n superlattices (where bilayers are built-up by an equal number of Si and Ge monolayers) are some 13% above the predictions of the continuum elasticity theory, using the C ij values of bulk Si and Ge. On the other hand, the elastic constants tensor of Si 4n /Ge n superlattices (where bilayers are built-up by four times more Si monolayers than Ge ones) matches perfectly the predictions. A thorough structural analysis by low-angle X-ray reflectivity, high-angle diffraction and X-ray absorption spectroscopy has been carried out and a deep characterization of the microstructure is presented here. The unexpected value of C 11 in Si n /Ge n superlattices can be attributed to the compressed Si-Ge alloy found at the wide interfaces present in that set of superlattices.

A Brillouin light-scattering investigation of the glass-transition in low molecular weight network glass-formers

The temperature dependence of the phase velocity and attenuation of hypersonic longitudinal waves across the glass transition is monitored in two samples of monohydric alcohols by means of Brillouin (inelastic) light scattering. The behaviour with temperature of parameters characterizing the elastic and relaxational responses of the samples is discussed in some detail.

EXAFS determination of the Nd3+ lattice position in Nd:LiNbO3: influence of lithium niobate stoichiometry and Mg2+ and Zn2+ co-doping

Abstract A set of Nd–Mg and Nd–Zn co-doped LiNbO 3 samples has been investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy. We investigated the lattice position of the Nd 3+ ion site to understand the effect of this location on the optical properties of co-doped LiNbO 3 . We determined its lattice position by analyzing the local order around neodymium ions. For Mg 2+ and Zn 2+ concentrations higher than the characteristic Li deficiency in congruent LiNbO 3 crystals, the Nd 3+ host position is near the Li-octahedron. Surprisingly, for Mg 2+ concentrations smaller than the congruent Li deficiency, Nd 3+ occupies a vacancy position, associated with an empty structural octahedron between the Li- and Nb- lattice sites.

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.

Gd thin films : relationship between elastic properties and microstructure

Abstract A set of gadolinium thin films was prepared over Si(100) substrates by DC-operated planar magnetron sputtering deposition. This paper reports a change in the film texture by changing the sputtering working Ar gas pressure. An X-ray study of their structural properties shows that films have (0002) and (1010) orientations. On the other hand, their elastic properties have been studied by Surface Brillouin Light Scattering spectroscopy that provides the velocity of the surface acoustic waves. A simulation that takes into account values of thin film density and elastic constants tensor identical to the corresponding for bulk gadolinium shows good agreement between theory and experiment for (0002)-textured samples. Nevertheless, (1010)-textured films seem to have slightly different values of the elastic constant.

Anomalous temperature dependence of the low-frequency vibrational density of states in vitreous B2O3☆

Abstract Inelastic neutron-scattering measurements of v-B 2 O 3 reveal an upshift in energy of the first moment of the vibrational density of states for frequencies below ≈ 15 meV as temperature increases from 20 to 300 K. Such a stiffening is at odds with predictions based on some current approaches. Instead, the present result and related anomalies in specific heat and other microscopic and transport properties can be understood in terms of anharmonic motions at low frequencies. The atomic origin of the anharmonicity, and its consequence for the thermodynamical behavior are examined by molecular-dynamics simulations.