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

All-sky Kinematics and Chemistry of Monoceros Stellar Overdensity

We explore the kinematic and chemical properties of Monoceros stellar overdensity by combining data from 2MASS, WISE, APOGEE, and $\text{Gaia}$. Monoceros is a structure located towards the Galactic anticenter and close to the disk. We identified that its stars have azimuthal velocity in the range of $200 < v_{\phi}\,{\rm(km\,s^{-1})}< 250$. Combining their kinematics and spatial distribution, we designed a new method to select stars from this overdensity. This method allows us to easily identify the structure in both hemispheres and estimate their distances. Our analysis was supported by comparison with simulated data from the entire sky generated by $\texttt{Galaxia}$ code. Furthermore, we characterized, for the first time, the Monoceros overdensity in several chemical-abundance spaces. Our results confirm its similarity to stars found in the thin disk of the Galaxy and suggest an $\textit{in situ}$ formation. Furthermore, we demonstrate that the southern (Mon-S) and northern (Mon-N) regions of Monoceros exhibit indistinguishable chemical compositions.Comment: Paper accepted for publication in Ap

The Chemodynamical Nature of the Triangulum-Andromeda Overdensity

We present a chemodynamical study of the Triangulum-Andromeda overdensity (TriAnd) employing a sample of 31 candidate stars observed with the GRACES high-resolution ($R$=40,000) spectrograph at the Gemini North (8.1 m) telescope. TriAnd is a stellar substructure found toward the outer disk of the Milky Way, located at $R_{\rm GC}\sim 18$ kpc from the Sun, toward Galactic latitude $b \sim 25${\deg}. Most stars in our sample have dynamical properties compatible with a disk stellar population. In addition, by applying an eccentricity cut, we are able to detect a stellar contamination that seems to be consistent with an accreted population. In chemical abundance space, the majority of our TriAnd candidates are similar to the outer thin-disk population, suggesting that the overdensity has an \textit{in situ} origin. Finally, the found accreted halo interlopers spatially overlapping with TriAnd should explain the historical discussion of the overdensity's nature due to its complex chemical patterns.Comment: Published in The Astrophysical Journal (ApJ