61 research outputs found
Transições de Fase em Compostos com Estrutura de Pirocloro de Defeito em Altas Pressões
In this paper the influence of pressure and temperature on four compounds with defect pyrochlore structure (NH4NbWO6, RbNbWO6, CsNbWO6, and p-WO3) is explored by means of X-ray diffraction, vibrational (Raman and infrared absorption) spectroscopy and computer simulations. Several structural transitions were observed, including an unusual insertion reaction with volume increase at high pressures. This latter transition is further explored to reveal the influence on the transition pressure of the nature and ionic radius of the cation residing inside the cages formed by the pyrochlore framework. http://dx.doi.org/10.18226/23185279.v1iss1p24Neste artigo, a influência da pressão e da temperatura sobre quatro compostos com estrutura de pirocloro de defeito (NH4NbWO6, RbNbWO6, CsNbWO6 e p-WO3) e explorada por meio de difração de raios X, espectroscopia vibracional (Raman e de absorção no infravermelho) e simulação computacional. Foram observadas varias transições estruturais e uma inusitada reação de inserção com aumento de volume em altas pressões. Este comportamento e racionalizado em termos da natureza e do raio iônico do cation presente no interior das cavidades da estrutura destes pirocloros. http://dx.doi.org/10.18226/23185279.v1iss1p2
Pressure-induced amorphization and polyamorphism in one-dimensional single crystal TiO2 nanomaterials
The structural phase transitions of single crystal TiO2-B nanoribbons were
investigated in-situ at high-pressure using the synchrotron X-ray diffraction
and the Raman scattering. Our results have shown a pressure-induced
amorphization (PIA) occurred in TiO2-B nanoribbons upon compression, resulting
in a high density amorphous (HDA) form related to the baddeleyite structure.
Upon decompression, the HDA form transforms to a low density amorphous (LDA)
form while the samples still maintain their pristine nanoribbon shape. HRTEM
imaging reveals that the LDA phase has an {\alpha}-PbO2 structure with short
range order. We propose a homogeneous nucleation mechanism to explain the
pressure-induced amorphous phase transitions in the TiO2-B nanoribbons. Our
study demonstrates for the first time that PIA and polyamorphism occurred in
the one-dimensional (1D) TiO2 nanomaterials and provides a new method for
preparing 1D amorphous nanomaterials from crystalline nanomaterials.Comment: 4 figure
Elastic properties of mono- and polycrystalline hexagonal AlB2-like diborides of s, p and d metals from first-principles calculations
We have performed accurate ab initio total energy calculations using the
full-potential linearized augmented plane wave (FP-LAPW) method with the
generalized gradient approximation (GGA) for the exchange-correlation potential
to systematically investigate elastic properties of 18 stable, meta-stable and
hypothetical hexagonal (AlB2-like) metal diborides MB2, where M = Na, Be, Mg,
Ca, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ag and Au. For monocrystalline
MB2 the optimized lattice parameters, independent elastic constants (Cij), bulk
modules (B), shear modules (G) are obtained and analyzed in comparison with the
available theoretical and experimental data. For the first time numerical
estimates of a set of elastic parameters of the polycrystalline MB2 ceramics
(in the framework of the Voigt-Reuss-Hill approximation), namely bulk and shear
modules, compressibility, Young's modules, Poisson's ratio, Lame's coefficients
are performed.Comment: 24 pages, 3 figure
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