First-principles calculations and Raman scattering evidence for local symmetry lowering in rhombohedral ilmenite: temperature- and pressure-dependent studies.

ATiO3-type materials may exist in two different crystalline forms: the perovskite and ilmenite. While many papers have devoted their attention to evaluating the structural properties of the perovskite phase, the structural stability of the ilmenite one still remains unsolved. Here, we present our results based on the lattice dynamics and first-principles calculations (density functional theory) of the CdTiO3 ilmenite phase, which are confronted with experimental data obtained through micro Raman spectroscopy that is a very good tool to probe the local crystal structure. Additional Raman bands, which are not foreseen from group-theory for the ilmenite rhombohedral structure, appeared in both low temperature (under vacuum condition) and high-pressure (at room temperature) spectra. The behavior can be explained by considering the local loss of inversion symmetry operation which reduces the overall space group from [Formula: see text] ([Formula: see text]) to [Formula: see text] ([Formula: see text]). Our results can also be extended to other ilmenite-type compositions.

Temperature dependence of the Raman spectrum of 1-(4-chlorophenyl)-3-(2-thienyl)prop-2-en-1-one.

The heterocyclic chalcone containing thiophene ring 1-(4-chlorophenyl)-3-(2-thienyl)prop-2-en-1-one, C13H9ClOS was synthesized and investigated using experimental techniques such as nuclear magnetic resonance (1H and 13C NMR), Fourier transform infrared spectroscopy (FTIR) at room temperature, differential scanning calorimeter (DSC) from room temperature to 500K and Raman scattering at the temperature range 10-413K in order to study its structure and vibrational properties as well as stability and possible phase transition. Density functional theory (DFT) calculations were performed to determine the vibrational spectrum viewing to improve the knowledge of the material properties. A reasonable agreement was observed between theoretical and experimental Raman spectrum taken at 10K since anharmonic effects of the molecular motion is reduced at low temperatures, leading to a more comprehensive assignment of the vibrational modes. Increasing the temperature up to 393K, was observed the typical phonon anharmonicity behavior associated to changes in the Raman line intensities, line-widths and red-shift, in special in the external mode region, whereas the internal modes region remains almost unchanged due its strong chemical bonds. Furthermore, C13H9ClOS goes to melting phase transition in the temperature range 393-403K and then sublimates in the temperature range 403-413K. This is denounced by the disappearance of the external modes and the absence of internal modes in the Raman spectra, in accordance with DSC curve. The enthalpy (ΔH) obtained from the integration of the endothermic peak in DSC curve centered at 397K is founded to be 121.5J/g.

Lattice dynamics and pressure-induced phase transitions in α-BaTeMo2O9.

Orthorhombic α-BaTeMo2O9 nonlinear optical single crystals were investigated at ambient pressure by micro-Raman and infrared spectroscopy with a focus on the polarization properties of the vibrational modes. These results were analyzed based on classical lattice dynamics calculations, allowing us to propose the normal-mode symmetries and assignments. In addition to the ambient-pressure studies, high-pressure Raman scattering studies were performed. These studies showed the onset of a reversible first-order phase transition near 3.5 GPa. The pressure dependence of Raman bands provides strong evidence that the phase transition involves significant distortion of the TeOx (x = 3,4) polyhedra, whereas the MoO6 octahedra are less affected. A large increase in the number of observed bands points to lower symmetry of the high-pressure phase.

Synthesis of (Ca,Nd)TiO3 powders by complex polymerization, Rietveld refinement and optical properties.

Neodymium calcium titanate, (Ca(0.99)Nd(0.01))TiO(3) powders were synthesized by the complex polymerization method and heat treated at different temperatures for 2 h under air atmosphere. The structural evolution of these powders as a function of heat treatment temperature was analyzed by X-ray diffraction (XRD) and micro-Raman (MR) spectroscopy. The optical properties were investigated by Ultraviolet-visible (UV-vis) absorption spectroscopy and Photoluminescence (PL) measurements. XRD patterns, Rietveld refinement and MR spectra indicated that the powders heated treated at 750 degrees C for 2 h present an orthorhombic structure without secondary phases. UV-vis measurements suggested the presence of intermediary energy in disordered (Ca(0.99)Nd(0.01))TiO(3) powders. Broad and narrow bands were observed in the PL spectra of these powders when excited with 350 nm wavelength. The broad bands were associated to the structural defects and/or p-d electronic transitions while, the narrow bands were ascribed to f-f transitions arising from Nd(3+) ions.

Synthesis, growth process and photoluminescence properties of SrWO4 powders.

SrWO(4) powders were synthesized by the co-precipitation method and processed in a microwave-hydrothermal (MH) at 140 degrees C for different times. The obtained powders were analyzed by X-ray diffraction (XRD), micro-Raman (MR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, field-emission gun scanning electron microscopy (FEG-SEM), ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectra showed that the SrWO(4) powders present a scheelite-type tetragonal structure without the presence of deleterious phases. FT-IR spectra exhibited a high absorption band situated at 831.57 cm(-1), which was ascribed to the WO antisymmetric stretching vibrations into the [WO(4)] tetrahedron groups. FEG-SEM micrographs suggested that the processing time is able to influence in the growth process and morphology of SrWO(4) powders. UV-vis absorption spectra revealed different optical band gap values for these powders. A green PL emission at room temperature was verified in SrWO(4) powders when excited with 488 nm wavelength.

Morphology and Photoluminescence of HfO(2) Obtained by Microwave-Hydrothermal.

In this letter, we report on the obtention of hafnium oxide (HfO(2)) nanostructures by the microwave-hydrothermal method. These nanostructures were analyzed by X-ray diffraction (XRD), field-emission gum scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDXS), ultraviolet-visible (UV-vis) spectroscopy, and photoluminescence (PL) measurements. XRD patterns confirmed that this material crystallizes in a monoclinic structure. FEG-SEM and TEM micrographs indicated that the rice-like morphologies were formed due to an increase in the effective collisions between the nanoparticles during the MH processing. The EDXS spectrum was used to verify the chemical compositional of this oxide. UV-vis spectrum revealed that this material have an indirect optical band gap. When excited with 488 nm wavelength at room temperature, the HfO(2) nanostructures exhibited only one broad PL band with a maximum at around 548 nm (green emission).

Photoluminescence of barium titanate and barium zirconate in multilayer disordered thin films at room temperature.

The emission of wide band photoluminescence showed a synergic effect on barium zirconate and barium titanate thin films in alternate multilayer system at room temperature by 488 nm exiting wavelength. The thin films obtained by spin-coating were annealed at 350, 450, and 550 degrees C for 2 h. The X-ray patterns revealed the complete separation among the BaTiO3 and BaZrO3 phases in the adjacent films. Visible and intense photoluminescence was governed by BaZrO3 thin films in the multilayer system. Quantum mechanics calculations were used in order to simulate ordered and disordered thin films structures. The disordered models, which were built by using the displacement of formers and modifier networks, showed a different symmetry in each system, which is in accordance with experimental photoluminescence emission, thus allowing to establish a correlation among the structural and optical properties of these multilayered systems.

Toward an understanding of intermediate- and short-range defects in ZnO single crystals. A combined experimental and theoretical study.

A joint use of experimental and theoretical techniques allows us to understand the key role of intermediate- and short-range defects in the structural and electronic properties of ZnO single crystals obtained by means of both conventional hydrothermal and microwave-hydrothermal synthesis methods. X-ray diffraction, Raman spectra, photoluminescence, scanning electronic and transmission electron microscopies were used to characterize the thermal properties, crystalline and optical features of the obtained nano and microwires ZnO structures. In addition, these properties were further investigated by means of two periodic models, crystalline and disordered ZnO wurtzite structure, and first principles calculations based on density functional theory at the B3LYP level. The theoretical results indicate that the key factor controlling the electronic behavior can be associated with a symmetry breaking process, creating localized electronic levels above the valence band.

Lead and aluminum bonding in Pb-AI metaphosphate glasses.

The bonding properties of cations in phosphate glasses determine many short- and medium-range structural features in the glass network, hence influencing bulk properties. In this work, Pb-Al-metaphosphate glasses (1 - x)Pb(PO(3))(2).xAI(PO(3))(3) with 0 < or = x < or = 1 were analyzed to determine the effect of the substitution of Pb by AI on the glass structure in the metaphosphate composition. The glass transition temperature and density were measured as a function of the Al concentration. The vibrational and structural properties were probed by Raman spectroscopy and nuclear magnetic resonance of (31)P, (27)AI, and (207)Pb. Aluminum incorporates homogeneously in the glass creating a stiffer and less packed network. The average coordination number for AI decreases from 5.9 to 5.0 as x increases from 0.1 to 1, indicating more covalent AI-O bonds. The coordination number of Pb in these glasses is greater than 8, showing an increasing ionic behavior for compositions richer in AI. A quantitative analysis of the phosphate speciation shows definite trends in the bonding of AIO(n) groups and phosphate tetrahedra. In glasses with x < 0.48, phosphate groups share preferentially only one nonbridging O corner with an AIO(n) coordination polyhedron. For x > 0.48 more than one nonbridging O can be linked to AIO(n) polyhedra. There is no corner sharing of O between AIO(n) and PbO(n) polyhedra nor between AIO(n) themselves throughout the compositional range. The PbO(n) coordination polyhedra show considerable nonbridging O sharing, with each O participating in the coordination sphere of at least two Pb. The bonding preferences determined for Al are consistent with the behavior observed in Na-AI and Ca-AI metaphosphates, indicating this may be a general behavior for ternary phosphate glasses.

The pressure-induced phase transition of mechanically alloyed nanocrystalline GaSb.

Ga K-edge energy dispersive x-ray absorption spectroscopy and Raman spectroscopy measurements were employed to follow the pressure-induced semiconductor-metal phase transition of nanocrystalline GaSb produced by mechanical alloying up to 26 GPa. The results showed a slight increase of the phase transition pressures for both as-milled (8 GPa) and annealed (10 GPa) GaSb samples, as compared to that for the bulk one. The extended x-ray absorption fine structure analysis of the zinc blende (ZB) pressure domain (<10 GPa) showed that the microscopic compressibility of the bonds in the as-milled/annealed samples is higher/lower than the crystalline bulk modulus (56 GPa). The comparison between x-ray absorption near edge structure regions of the spectra and multiple scattering calculations suggests that the ZB structure evolves to a short-range chemically ordered ß-Sn structure for pressures as high as 8 GPa. Raman measurements confirm the semiconductor-metal phase transitions of ZB-GaSb between 8 and 11 GPa for both as-milled and annealed samples, showing that the semiconductor character was not recovered on releasing the pressure down to 3.9 and 1.8 GPa, indicating a very strong hysteresis effect (or even irreversible transitions). The well-known transverse effective charge reduction with pressure was also observed. Furthermore, resonance behaviour is clearly seen for transverse optical phonons and the resonance maxima peak occurs at about 1.2 GPa, corresponding to 2.11 eV in the E(1) scale, smaller by 0.3 eV than the incident photon energy.

Mechanical alloying: a pressure induced reaction for obtaining zinc blende GaSb and multiphase states.

The cubic zinc blende GaSb phase was produced by a mechanical alloying technique, which is a solid state route based on the action of non-hydrostatic pressures. The thermal stability of this phase was tested using the differential scanning calorimetry technique and, in order to clarify the results, an annealing process was performed. Comparing x-ray diffraction patterns for as-prepared and annealed samples, the improvement in crystallinity of the cubic phase and Sb segregation and/or crystallization can be easily seen. Optical phonons frequencies were measured for both as-milled and annealed samples by means of the Raman spectroscopy technique. Raman profiles of as-milled samples showed typical zinc blende GaSb optical modes and revealed new features that can be associated with multiphase states.

Structural phase transition and dynamical properties of PbTiO3 simulated by molecular dynamics.

The temperature- and pressure-induced structural phase transition in PbTiO3 is studied with the isoenthalpic-isobaric molecular-dynamics method, using an effective two-body interaction potential. The tetragonal to cubic transformation is successfully reproduced with both temperature and pressure. The behaviour of lattice parameters, vibrational density of states, and phonon anharmonicity with temperature and pressure are in very good agreement with experimental data. Two- and three-body correlations were analysed through pair distribution functions, coordination numbers and bond-angle distributions.

The effect of the cation substitution on the structural and vibrational properties of Cs2NaGaxSc(1-x)F6 solid solution.

Raman scattering and x-ray diffration were used to characterize the structural and vibrational properties of the Cs2NaGaxSc(1-x)F6 solid solutions, for x ranging from 0.0 to 1.0. The Raman spectra, taken at room and low temperature, allow us to follow the phase evolution in detail and indicate the breaking of the local symmetry since low Ga concentration levels. Five compositions were studied by x-ray diffraction: x = 0.0, 0.2, 0.5, 0.8, and 1.0. A cubic space group, Fm3m, was found to x = 0.0 and x = 0.2 and a trigonal one was found to x = 0.5, 0.8, and 1.0. Details of both phases are presented and the correlation between x-ray diffraction and Raman scattering is discussed.

Reverse Monte Carlo simulations, Raman scattering, and thermal studies of an amorphous Ge30Se70 alloy produced by mechanical alloying.

The short- and intermediate-range orders of an amorphous Ge30Se70 alloy produced by mechanical alloying were studied by reverse Monte Carlo simulations of its x-ray total structure factor, Raman scattering, and differential scanning calorimetry. The simulations were used to compute the G(Ge-Ge) (RMC)(r), G(Ge-Se) (RMC)(r), and G(Se-Se) (RMC)(r) partial distribution functions and the S(Ge-Ge) (RMC)(K), S(Ge-Se) (RMC)(K), and S(Se-Se) (RMC)(K) partial structure factors. We calculated the coordination numbers and interatomic distances for the first and second neighbors and the bond-angle distribution functions Theta(ijl)(cos theta). The data obtained indicate that the structure of the alloy has important differences when compared to alloys prepared by other techniques. There are a high number of Se-Se pairs in the first shell, and some of the tetrahedral units formed seemed to be connected by Se-Se bridges.

Synthesis of SnO2 nanoribbons by a carbothermal reduction process.

This communication describes, for the first time, the growth of SnO2 nanoribbons by a controlled carbothermal reduction process. An analysis of the transmission electron microscopy image revealed that these nanoribbons have a well-defined shape, with a typical width in the range of 70-300 nm. In general, the nanostructured ribbons were more than 100 microns in length. The results reported here support the hypothesis that this ribbon-like nanostructured material grows by a vapor-solid process. This study introduces two hypotheses to explain the SnO2 nanoribbon growth process.