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Ionic crystals consist of two or more different kinds of ions that usually have different sizes. The packing of these ions into a crystal structure is more complex than the packing of metal atoms that are the same size.
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Metallic solids such as crystals of copper, aluminum, and iron are formed by metal atoms. The structure of metallic crystals is often described as a uniform distribution of atomic nuclei within a “sea” of delocalized electrons. The atoms within such a metallic solid are held together by a unique force known as metallic bonding that gives rise to many useful and varied bulk properties.
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Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
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This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
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For any given polymer, the weight average molecular weight (Mw) is higher than, if not equal to, the number average molecular weight (Mn). The only situation in which the weight average molecular weight and the number average molecular weight are equal is when a polymer consists only of chains with equal molecular weight. However, this never happens in a synthetic polymer, since it is difficult to control the polymerization process up to a molecular level with accuracy to a hundred percent.
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Intrinsic semiconductors are highly pure materials with no impurities. At absolute zero, these semiconductors behave as perfect insulators because all the valence electrons are bound, and the conduction band is empty, disallowing electrical conduction. The Fermi level is a concept used to describe the probability of occupancy of energy levels by electrons at thermal equilibrium. In intrinsic semiconductors, the Fermi level is positioned at the midpoint of the energy gap at absolute zero. When...
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Distribución de politipos en el carburo de silicio circunstelar.

T L Daulton1, T J Bernatowicz, R S Lewis

  • 1Materials Science Division, Argonne National Laboratory, Argonne IL, 60439-4838, USA. tdaulton@nrlssc.navy.mil

Science (New York, N.Y.)
|June 8, 2002
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Los espectros infrarrojos astronómicos del carburo de silicio (SiC) son controvertidos. El análisis directo del SiC presolar de los meteoritos revela sólo dos politipos comunes, simplificando los modelos anteriores.

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Área de la Ciencia:

  • * Astroquímica y ciencias de los materiales.
  • * Caracterización de los granos presolares.
  • * Evolución estelar y nucleosíntesis.

Sus antecedentes:

  • * Se debate la estructura cristalográfica del carburo de silicio circumestelar (SiC), con espectros infrarrojos astronómicos que sugieren una mezcla compleja de politipos.
  • * Estudios anteriores se basaron en el análisis espectral indirecto, lo que condujo a la incertidumbre sobre las estructuras reales de SiC formadas en las salidas estelares.

Objetivo del estudio:

  • * Para determinar directamente la distribución politipo del carburo de silicio circunstelar (SiC).
  • * Para resolver la controversia en torno a las clases cristalográficas del SiC inferidas de datos astronómicos.
  • * Comprender las condiciones de formación del SiC en entornos estelares.

Principales métodos:

  • * Análisis estructural directo mediante microscopía electrónica de transmisión (TEM).
  • * Examen de granos de carburo de silicio (SiC) presolar extraídos del meteorito carbonáceo Murchison.
  • * Identificación y cuantificación de los politipos de SiC presentes en las muestras.

Principales resultados:

  • * Solo se identificaron dos politipos de SiC: el 3C cúbico y el 2H hexagonal.
  • * También se observaron crecimientos entre estos dos politipos.
  • * La simplicidad estructural observada contrasta con la amplia variedad de politipos sugeridos por los análisis espectrales.

Conclusiones:

  • * La limitada distribución de politipos se atribuye a las bajas presiones en las salidas circumestelares.
  • * Las bajas temperaturas de condensación del carburo de silicio en estos entornos favorecen la formación de sólo 3C y 2H politipos.
  • * Este hallazgo simplifica nuestra comprensión de la formación de SiC en estrellas evolucionadas.