Altair Soria Pereira

Periodic Hartree-Fock linear combination of crystalline orbitals calculation of the structure, equation of state and elastic properties of titanium diboride

We have performed all-electron ab initio calculations for TiB2 in the athermal limit using the CRYSTAL95 code. The lattice parameters of the AlB2-type structure were optimized as a function of pressure. The fitting of a Murnaghan equation of state resulted in values of B0 = 292±1 GPa and B � 0 = 3.34±0.03 for the bulk modulus and its first derivative at zero pressure. The values for the linear bulk modulus along the a-axis and the c-axis are Ba0 = 1031 ± 3G Pa (B � a0 = 10.6 ± 0.2) and Bc0 = 675 ± 3G Pa(B � c0 = 8.8 ± 0.2), respectively. All five independent elastic constants were calculated, and the analysis of the elastic behaviour of titanium diboride indicates that this compound is more isotropic than one would suppose from its crystal structure. The discussion on the nature of the chemical bonds and the electronic charge transfer in titanium diboride gives some insight into its mechanical properties, such as its high hardness, despite an apparent layered structure. In this sense, the analysis of the charge-density distribution shows a non- negligible interaction between graphite-like boron planes along the c-axis, which increases with pressure, and suggests a three-dimensional picture for the TiB2 structure, instead of the traditional planar description.

Compressibility of AlB2-type transition metal diborides

The pressure behaviour of a series of transition metal borides has been studied both experimentally and by means of ab initio calculations. X-ray diffraction patterns measured up to ~50 GPa for VB2 and ZrB2 show no obvious phase transition. Bulk moduli of 322 and 317 GPa, respectively, were obtained using a Murnaghan equation of state. Hartree–Fock LCCO (linear combination of crystal orbitals) calculations performed for TiB2 have allowed its compression behaviour to be studied. The bulk modulus obtained (292 GPa) and the proposed important contribution of the interlayer interaction to the elastic behaviour under high pressure are consistent with the experimental results for the other borides.

Hardness and elasticity in cubic ruthenium dioxide

The Knoop hardness of the highly incompressible cubic phase of ruthenium dioxide was found to be 19–20 GPa from indentation tests. This value scales well with the shear modulus approximated by the elastic constant C44 of 144 GPa obtained from Brillouin scattering measurements. This work provides evidence that the shear modulus is a better indicator of hardness than the bulk modulus for ionic and covalent materials.

Structural characterisation of the pa3̄-type, high pressure phase of ruthenium dioxide

Cubic Pa3-type RuO2 was prepared from the ambient pressure, rutile-structured phase at 20 GPa and 1100 °C in a multianvil device. The structure of this Pa3 phase was refined by time-of-flight, neutron powder diffraction on the quenched sample yielding a cell constant a = 4.85892(3) A and an oxygen positional parameter u = 0.35115(8). The ruthenium cation is rhombohedrally coordinated with six anions at 1.9893(4) A and two more distant anions at 2.9552(4) A. The minimum interpolyhedral OO distance of 2.5045(5) A in this structure is the shortest known in any solid and is shorter than the intrapolyhedral OO distances, which are of 2.6208(5) A. These short distances are the origin of the very low compressibility of this oxide phase, which approaches that of diamond. The Raman spectrum of Pa3-type RuO2 is consistent with group-theoretical calculations.

Splitting of the resistive transition in MgB2

We report accurate measurements of the resistive transition in two different polycrystalline samples of superconducting MgB2. The results are described in terms of the temperature derivative of the resistivity, d ?/d T. A method is developed to analyse these data. A structure consisting of two closely separated maxima is observed in d ?/d T near the transition. This splitting is suppressed by applying magnetic fields above 10?kOe parallel to the current orientation. We discuss the possible mechanisms underlying the split resistive transition observed in our samples.

High pressure annealing of defects induced by ion implantation on graphite

Abstract The aim of this work was to investigate, by Raman spectroscopy, the effect of high pressure on the annealing of the surface disorder induced by hydrogen, carbon and nickel ion implantations on graphite samples. In the case of H implantation, the induced disorder decreased after annealing at 650°C for 7.7 GPa. For C implantation, the defect annealing started at 500°C for 7.7 GPa, while in the case of Ni implantation, it started at 850°C for the same pressure. For the three cases studied, there was practically no annealing effect for temperature treatments in vacuum up to 1100°C. The results indicated that high pressure plays an important role in the defect annealing, accelerating the graphitization process, probably due to the large volume of the defective ion implanted phase. Despite the annealing at high pressure being conducted deep inside the thermodynamic region of stability of diamond, the graphite phase was recovered, indicating that the nucleation barrier for diamond is indeed very high as compared to graphite.

An Innovative Series of Layered Nanostructured Aminoalkylsilica Hybrid Material

Uma nova serie de amostras de material hibrido a base de silica, contendo o grupo cationico amoniopropil ligado na forma pendente, foi obtida usando-se o metodo sol-gel de sintese, variandose a razao molar entre o precursor inorgânico e o precursor orgânico. A analise termogravimetrica mostrou que as amostras sao termicamente estaveis ate 260 °C. Os resultados obtidos por difratometria de raios X, microscopia eletronica de transmissao, ressonância magnetica nuclear de 29 Si e analise elementar sao compativeis com o modelo estrutural de camadas de silsesquioxano com distâncias basais de ate 5,4 nm, contendo silica amorfa no espaco entre as camadas. An innovative series of silica-based hybrid materials containing pendant cationic ammoniumpropyl groups was obtained using the sol-gel method, by varying the molar ratio of the inorganic and organic precursors. Thermogravimetric analysis showed that samples were thermally stable up to 260 °C. Results obtained by X-ray diffractometry, transmission electron microscopy, 29 Si nuclear magnetic resonance and elemental analysis were compatible with a silsesquioxane layered structural model showing basal distances up to 5.4 nm, containing amorphous silica in the interlayer space.

High-pressure study of Ti50Ni25Fe25 powder produced by mechanical alloying

A nanostructured Ti50Ni25Fe25 phase (B2) was formed by mechanical alloying and its structural stability was studied as a function of pressure. The changes were followed by X-ray diffraction. The B2 phase was observed up to 7 GPa; for larger pressures, the B2 phase transformed into a trigonal/hexagonal phase (B19) that was observed up to the highest pressure used (18 GPa). Besides B2 and B19, elemental Ni or a SS-(Fe,Ni) and FeNi3 were observed. With decompression, the B2 phase was recovered. Using in situ angle-dispersive X-ray diffraction patterns, the single line method was applied to obtain the apparent crystallite size and the microstrain for both the B2 and the B19 phases as a function of the applied pressure. Values of the bulk modulus for the B2, B19, elemental Ni or SS-(Fe,Ni) and FeNi3 phases were obtained by fitting the pressure dependence of the volume to a Birch–Murnaghan equation of state (BMEOS).

Effect of pressure on nanostructured Gd3Fe5O12

The effect of pressure on mechanically alloyed nanostructured Gd3Fe5O12 was investigated using in situ angle-dispersive X-ray diffraction measurements at room temperature. No structural phase transitions were observed, but an anomalous behavior in the volume vs. pressure curve, attributed to nonhydrostatic effects, was observed in the pressure range between 4.4 GPa and 9.1 GPa. The analysis of effect of pressure on the Wickoff 23c sites, occupied by Gd3+ ions, 16a and 24d sites, occupied by Fe3+ ions, and 96h sites, occupied by O2– ions, was performed. The value of the bulk modulus B0 for nanostructured Gd3Fe5O12 could not be determined due to nonhydrostatic effects associated with the silicone oil used as a pressure transmitting medium.

Environment and temperature effects on the phase stability of ZrO2 under high pressure

Abstract We investigated the effect of treatments at the temperatures of 120, 300 and 500°C during 24 h in water, on the phase stability of a ZrO 2 fine powder (particle size ∼ 30 nm) submitted to pressures up to 12 GPa. The transformed fraction from the monoclinic to the orthorhombic-I phase was monitored by in situ X-ray diffraction. A sample treated at 300°C/24 h in vacuum was also studied to investigate a possible environment influence. The experimental data are well described employing the statistical heterogeneous nucleation model proposed by Olson and Cohen, if the pressure dependence of the transition free-energy change is taken into account. The results indicate annealing of the surface nucleation defects, if the sample is treated in water. The behavior of the vacuum treated sample is almost the same as a non-treated sample.