Metalloid | EPFL Graph Search

A metalloid is a type of chemical element which has a preponderance of properties in between, or that are a mixture of, those of metals and nonmetals. There is no standard definition of a metalloid and no complete agreement on which elements are metalloids. Despite the lack of specificity, the term remains in use in the literature of chemistry. The six commonly recognised metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. Five elements are less frequently so classified: carbon, aluminium, selenium, polonium and astatine. On a standard periodic table, all eleven elements are in a diagonal region of the p-block extending from boron at the upper left to astatine at lower right. Some periodic tables include a dividing line between metals and nonmetals, and the metalloids may be found close to this line. Typical metalloids have a metallic appearance, but they are brittle and only fair conductors of electricity. Chemically, they behave mostly as nonmetals. They can

Speciation of aqueous tellurium(IV) in hydrothermal solutions and vapors, and the role of oxidized tellurium species in Te transport and gold deposition

Pascal Grundler, Lothar Helm, Yuan Tian

Despite the close association between tellurium (Te) and gold (Au) in many epithermal, orogenic, and intrusion-related hydrothermal ore deposits, the hydrothermal chemistry of Te remains poorly understood. We studied the protonation/ deprotonation and structure of tellurous acid ((H2TeO3)-O-IV) species in aqueous solutions and in water vapor as a function of pH from room-temperature to 505 degrees C using a number of methods: TeO2(s) solubility in solution and steam, potentiometry, NMR spectroscopy, and in situ XAS spectroscopy. The solubility of TeO2(s) increases by up to a factor 80 between ambient temperature and 200 degrees C. As the temperature increases, the pH range over which the neutral species H2TeO3(aq) dominates to the detriment of the ionic species H3TeO3+, HTeO3-and TeO32-expands. The structure of these species is a trigonal pyramid with the Te atom at its apex, indicating a stereochemically active electron pair. The Te-O bond lengths increase with increasing protonation (i.e., decreasing pH). Although hydrated tellurite species such as TeO2(H2O)(g) and TeO2(H2O)(2)(g) exist in significant concentrations in vapors equilibrated with TeO2(s), these species are unlikely to play a significant role in natural systems, because of the high solubility of Te(IV) in the liquid phase under these conditions. Solubility calculations conducted with the new and existing properties confirm the importance of reduced species for the vapor transport of Te, with a partitioning coefficient (Kd = Te concentration in vapor/Te concentration in liquid) up to >10(5) in favor of the vapor, and ppm concentrations of Te in reduced vapors at 300 degrees C. Thermodynamic calculations show that slightly basic, mildly reduced fluids that can transport Au efficiently as Au(HS)(2)(-) can also carry significant Te (e.g., similar to 100 ppb at 300 degrees C). The calculations also suggest that under magmatic hydrothermal conditions, large amounts of Te can be transported as Te(IV) complexes in oxidized fluids (coexisting with SO2(g)). Reduction of Te(IV) caused by processes such as fluid-rock interaction or fluid mixing will lead to a dramatic decrease in the solubilities of both Te and Au, and to the precipitation of telluride minerals. (C) 2013 Elsevier Ltd. All rights reserved.

Elsevier2013

Electronic structural details of donor-vacancy complexes in Si-doped Ge and Ge-doped Si

Alexandra Sarabando de Carvalho

We present a detailed picture of the electronic structure of donor-vacancy complexes in Ge-doped silicon and Si-doped germanium Clusters to mimic Si-rich and Ge-rich SiGe alloys, respectively Jahn-Teller effects and electrical levels were investigated in both Si-rich and Ge-rich end compositions It is shown that while Ge atoms act as efficient traps for mobile E-centers in Si-rich alloys, Si atoms in Ge-rich material do not have this ability A detailed linear combination of atomic orbitals model is proposed for VP-Ge and VP-Si complexes in Si1-xGex and Ge1-xSix, where the orbital filling order is driven by the larger electron affinity of Si atoms when compared to Ge. (C) 2009 Elsevier B.V. All rights reserved

2010

Direct-write laser-induced self-organization and metallization beyond the focal volume in tellurite glass

Yves Bellouard, Gözden Torun

We report on a self-organized process initiated on a potassium-tungsten-tellurite glass surface under repetitive femtosecond laser irradiation in a regime where cumulative effects lead to a localized melting. Specifically, we show that self-organized periodic patterns consisting of parallel nanoplanes perpendicular to the laser polarization are forming and extending beyond the zone under direct laser exposure. Examination of the modified regions revealed a phase change from a glassy tellurium-oxide to a crystalline tellurium. In addition, we observe that this self-organization process, associated with elemental redistribution and deoxygenation, is triggered by the optical-field strength. We suggest that early self-organized nanostructures formed by a local-field enhancement is subsequently reinforced by a metallization event in an open-air atmosphere.

2021

Speciation of aqueous tellurium(IV) in hydrothermal solutions and vapors, and the role of oxidized tellurium species in Te transport and gold deposition

Pascal Grundler, Lothar Helm, Yuan Tian

Elsevier2013