As-rich Phases Research Articles

Milling-driven nanonization of AsxS100-x alloys from second glass-forming region: The case of higher-crystalline arsenicals (51

Nanonization accompanied by amorphization processes driven by high-energy mechanical milling are studied in AsxS100-x alloys from higher-crystallinity domain (51<x<56) employing multiexperimental approach.Principal disproportionality in these arsenicals under milling comprises molecular-to-network transformations resulting in β-As4S4 polymorph supplemented by isocompositional amorphous phase. Nanosized state is signalized in broadened Raman-active bands of β-As4S4 crystallites (~40–50 nm), the milling-driven nanonization being accompanied by disappearing of other crystalline phases (pararealgar, χ-phase As4S4, low-temperature dimorphite α-As4S3). At high As content in studie alloys, milling facilitates disappearing of low-temperature α-As4S3 at cost of nanosized high-temperature β-As4S4 and α′-As4S3 phases. These transformations are clarified with heat capacity measurement showing depressing and splitting trend in melting and interphase-transition temperatures from normal-crystalline α′-As4S3 to plastic-crystalline β-As4S3. Intermediate-range ordering of the amorphized AsS phase is enhanced under milling, this phase dominating in network derivatives from As4S4 cage-like molecules, while generation of competitive As-rich phase being expected at higher As content.

Combining Ultrafast Calorimetry and Electron Microscopy: Reversible Phase Transformations in SeTeAs Alloys.

Reversible amorphous–crystalline phase transitions are studied using complementary ultrafast differential scanning calorimetry and transmission electron microscopy techniques, which together allow a wealth of thermal and structural properties to be determined. The SeTe(As) system is investigated because these chalcogenide based materials have favorable properties as a phase-change memory material and in optical systems. Using calorimetry, we find that the addition of 10 at. % As to SeTe alloys strongly increases their glass forming ability, increasing both glass transition and crystallization temperatures while reducing critical quench rate. Ex situ investigation of SexTe90–xAs10 using electron microscopy and elemental mapping reveals a two-phase lamellar segregation mechanism, where a trigonal SeTe-phase and an amorphous As-rich phase are formed. These findings demonstrate the power of combining thermal and structural analysis techniques.

Morphologies of mixed organic/inorganic/aqueous aerosol droplets

Despite major progress in the understanding of properties of tropospheric aerosol particles, it remains challenging to understand their physical state and morphology. To obtain more detailed knowledge of the phases, phase transitions and morphologies of internally mixed organic/inorganic aerosol particles, we evaluated liquid-liquid phase separation (LLPS), deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of 33 organic/ammonium sulfate (AS)/H2O systems from our own and literature data. The organic fraction consists of single compounds or mixtures with up to ten aliphatic and/or aromatic components with carboxylic acid, hydroxyl, carbonyl, ether, and ester functionalities, covering O : C ratios between 0.29 and 1.33. Thirteen out of these 33 systems did not show LLPS for any of the studied organic-to-inorganic mixing ratios, sixteen underwent LLPS showing core-shell morphology, and four showed both core-shell and partially engulfed configurations depending on the organic-to-inorganic ratio and RH. In all cases the organic fractions of the systems with partially engulfed configurations consisted of dicarboxylic acids. AS in mixed organic/AS/H2O particles deliquesced between 70 and 84% RH. AS effloresced below 58% RH or remained in a one-liquid-phase state. AS in droplets with LLPS always showed efflorescence with ERH between 30 and 50% RH, providing clear evidence that the presence of LLPS facilitates AS efflorescence. Spreading coefficients of the organic-rich phase on the AS-rich phase for systems containing polyethylene glycol 400 (PEG-400) and a mixture of dicarboxylic acids are in agreement with the optically observed morphologies of droplets deposited on the hydrophobic substrate. Analysis of high resolution elastic Mie resonance spectra allowed the detection of LLPS for single levitated droplets consisting of PEG-400/AS/ H2O, whereas LLPS was difficult to detect in (2-methylglutaric acid + 3-methylglutaric acid + 2,2-dimethylsuccinic acid)/AS/H2O. Measured Mie spectra of PEG-400/AS/H2O at 93.5% and at 80.9% RH agreed with computed Mie spectra for a homogeneous and a core-shell configuration, respectively, confirming the results obtained from droplets deposited on a hydrophobic substrate. Based on the presented evidence, we therefore consider the core-shell morphology to be the prevalent configuration of liquid-liquid-phase-separated tropospheric organic/AS/H2O particles.

Sulfides and Sulfarsenides from the Rosie Nickel Prospect, Duketon Greenstone Belt, Western Australia

Recent exploration in the Duketon greenstone belt, Yilgarn craton, Western Australia, led to the discovery of a new occurrence of high-grade Ni-PGE (platinum group element) sulfide mineralization associated with komatiite; this is referred to as the Rosie Ni Prospect. The mineralization consists predominantly of disseminated and brecciated semimassive to massive base metal sulfide with 0.5 to 5 cm thick sulfarsenide-bearing lenses. This pilot study focuses on the petrology, mineralogy, and trace element mineral chemistry of sulfides and sulfarsenides, and the mineralogy of minor PGE-rich minerals (sperrylite, melonite, and bismuthotel-lurides) in selected samples representing different parts of the orebody, with a particular emphasis on the sulfarsenide-rich lenses. Our mineral chemistry and mineralogical studies indicate that As-rich phases (either as a melt or as primary minerals) played a critical role in collecting and concentrating PGEs from the komatiitic magma. The concentrations of trace elements within the sulfarsenides and sulfides from the different mineralization types reflect the interaction between the silicate and sulfide liquids. The concentration of PGEs in the As-rich minerals is a function of the volume of sulfide melt with which they have interacted. The smaller the proportion of the sulfarsenide relative to sulfide in the rock is, the higher the PGE concentration in the sulfarsenide will be. In situ Se analysis of the base metal sulfides from the different ore types indicates that Se concentrations in pentlandite and pyrrhotite from sulfarsenide-rich lenses are an order of magnitude higher than those of sulfides found in As-poor samples. This correlation between the Se concentrations in the sulfide minerals and the As concentration in the whole rock indicates that the processes which led to As enrichment at Rosie also contributed to Se enrichment. The particular As-Se enrichment is inferred to have been triggered by the erosion and assimilation of sulfidic sediments enriched in organic matter (now observed as shales and/or black shales) by the komatiitic magma flows, leading to the formation of immiscible S-As-rich melt, where PGEs partition preferentially into the As-rich phases.

Alteration of As-bearing phases in a small watershed located on a high grade arsenic-geochemical anomaly (French Massif Central)

At a watershed scale, sediments and soil weathering exerts a control on solid and dissolved transport of trace elements in surface waters and it can be considered as a source of pollution. The studied subwatershed (1.5 km2) was located on an As-geochemical anomaly. The studied soil profile showed a significant decrease of As content from 1500 mg kg−1 in the 135–165 cm deepest soil layer to 385 mg kg−1 in the upper 0–5 cm soil layer. Directly in the stream, suspended matter and the <63 μm fraction of bed sediments had As concentrations greater than 400 mg kg−1. In all these solid fractions, the main representative As-bearing phases were determined at two different observation scales: bulk analyses using X-ray absorption structure spectroscopy (XAS) and microanalyses using scanning electron microscope (SEM) and associated electron probe microanalyses (EPMA), as well as micro-Raman spectroscopy and synchrotron-based micro-scanning X-ray diffraction (μSXRD) characterization. Three main As-bearing phases were identified: (i) arsenates (mostly pharmacosiderite), the most concentrated phases As in both the coherent weathered bedrock and the 135–165 cm soil layer but not observed in the river solid fraction, (ii) Fe-oxyhydroxides with in situ As content up to 15.4 wt.% in the deepest soil layer, and (iii) aluminosilicates, the least concentrated As carriers. The mineralogical evolution of As-bearing phases in the soil profile, coupled with the decrease of bulk As content, may be related to pedogenesis processes, suggesting an evolution of arsenates into As-rich Fe-oxyhydroxides. Therefore, weathering and mineralogical evolution of these As-rich phases may release As to surface waters.

Fabrication of cobalt-doped SrFe2As2 superconducting thin films

We have fabricated Co-doped bilayered SrFe2As2 thin films on Al2O3 (0001) and LaAlO3 (001) substrates by using a pulsed laser deposition (PLD) technique with a 248-nm-wavelength KrF excimer laser. In order to fabricate Co-doped SrFe2As2 thin films, we used homemade target with an As-rich phase. We tried to find optimum growth condition by changing various experimental conditions, such as substrate temperature and laser energy density. The superconducting transition temperatures were influenced by growth temperature, but hardly affected by laser energy density. The best sample showed the Tc of ∼20K with a narrow superconducting transition width of 2.2K.

Microextraction in a tetrabutylammonium bromide/ammonium sulfate aqueous two-phase system and electrohydrodynamic generation of a micro-droplet

Microextraction of methyl orange in the aqueous two-phase system (ATPS) formed by dissolving tetrabutylammonium bromide (TBAB) and ammonium sulfate (AS) is reported. Methyl orange was transported from the AS-rich phase to TBAB-rich phase across the interface of the two immiscible phases. The electrohydrodynamic effect on the shape of the interface of two immiscible flows was also observed by applying dc voltage at the T-junction of the microchannel and the generation of a droplet of AS-rich phase was observed when the potential difference between positive and negative electrodes exceeds a threshold voltage. The minimum voltage necessary for the droplet generation depends on pH due to the degree of dissociation and charge accumulation.

Thermo-chemical properties and electrical resistivity of Zr-based arsenide chalcogenides

abstractTernary phases in the systems Zr–As–Se and Zr–As–Te were studied using single crystals of ZrAs1.40(1)Se0.50(1) and ZrAs1.60(2)Te0.40(1) (PbFCl-type of structure, space group P4/nmm) as well as ZrAs0.70(1)Se1.30(1) and ZrAs0.75(1)Te1.25(1) (NbPS-type of structure, space group Immm). The characterization covers chemical compositions, crystal structures, homogeneity ranges and electrical resistivities. At 1223 K, the Te-containing phases can be described with the general formula ZrAsxTe2–x, with 1.53(1)≤x≤1.65(1) (As-rich) and 0.58(1)≤x≤0.75(1) (Te-rich). Both phases are located directly on the tie-line between ZrAs2 and ZrTe2, with no indication for any deviation. Similar is true for the Se-rich phase ZrAsx–ySe2–x with 0.70(1)≤x≤0.75(1). However, the compositional range of the respective As-rich phase ZrAsx–ySe2–x (0.03(1)≤y≤0.10(1); 1.42(1)≤x≤1.70(1)) is not located on the tie-line ZrAs2–ZrSe2, and exhibits a triangular region of existence with intrinsic deviation of the composition towards lower non-metal contents. Except for ZrAs0.75Se1.25, from the homogeneity range of the Se-rich phase, all compounds under investigation show metallic characteristics of electrical resistivity at temperatures >20 K. Related uranium and thorium arsenide selenides display a typical magnetic field-independent rise of the resistivity towards lower temperatures, which has been explained by a non-magnetic Kondo effect. However, a similar observation has been made for ZrAs1.40Se0.50, which, among the Zr-based arsenide chalcogenides, is the only system with a large concentration of intrinsic defects in the anionic substructure.

Time-resolved reflectance difference spectroscopy of InAs growth under alternating flow conditions

We report a time-resolved reflectance difference spectroscopy (RDS) study of the growth of InAs during alternating flow conditions typical of atomic layer epitaxy (ALE). The precursors used were trimethylindium (TMIn) and tertiarybutylarsine (TBAs). For ALE growth we observe that the InAs surface remains As-rich for an appreciable fraction of the 1 monolayer (ML) TMIn pulse. This is similar to results obtained for the growth of GaAs by ALE using trimethylgallium. Three distinct RDS spectra are observed, two corresponding to As-rich phases, and one which is In-rich. In both InAs and GaAs the extra As layer appears to play a key role in maintaining self-limiting behaviour.

Determination of the surface structures of the GaAs(001)-(2×4) As-rich phase

Field-ion scanning tunneling microscopy (STM) and in situ reflection high-energy electron diffraction (RHEED) have been used to study the atomic structures of the As-rich GaAs(001) surfaces grown by molecular-beam epitaxy and migration-enhanced epitaxy. The (2\ifmmode\times\else\texttimes\fi{}4) \ensuremath{\alpha}, \ensuremath{\beta}, and \ensuremath{\gamma} phases and the c(4\ifmmode\times\else\texttimes\fi{}4) phase have been investigated in a systematic manner, controlling the As surface coverage. The high-resolution STM images show that the (2\ifmmode\times\else\texttimes\fi{}4) \ensuremath{\alpha}, \ensuremath{\beta}, and \ensuremath{\gamma} phases all have the same unit structure in the outermost surface layer, which consists of two As dimers and two As dimer vacancies. Various structure models proposed for the (2\ifmmode\times\else\texttimes\fi{}4) phases are examined based on the STM observations and dynamical calculation of the RHEED spot intensities. We now propose the following model: The \ensuremath{\alpha} phase is the two-As-dimer model proposed by Farrell and Palmstrom with relaxation incorporated by Northrup and Froyen. The surface coverage of the \ensuremath{\alpha} phase is 0.5 ML of As. The \ensuremath{\beta} phase is the two-As-dimer model proposed by Chadi. The surface coverage of the \ensuremath{\beta} phase is 0.75 ML of As. The so-called \ensuremath{\gamma} phase is characterized by its small domains separated by open areas. The domains consist of the same local structure as the \ensuremath{\beta} phase and the open areas between them have a disordered As double-layer structure similar to that of the c(4\ifmmode\times\else\texttimes\fi{}4) phase. The \ensuremath{\gamma} phase is, thus, no more than the mixture of the \ensuremath{\beta} phase and the c(4\ifmmode\times\else\texttimes\fi{}4) phase with the surface As coverage varying between 1.75 and 0.75 ML depending on the growth conditions. Our structure model for the As-rich GaAs(001) surface is consistent with most of the observations reported.

Platinum-group minerals in the middle group of chromitite layers at Marikana, western Bushveld Complex: indications for collection mechanisms and postmagmatic modification

The platinum-group minerals in a drill core taken through the middle group of chromitite layers in the Critical Zone at Marikana in the western Bushveld Complex were found to consist mainly of laurite as inclusions in chromite grains. The platinum-group minerals containing Pt, Pd, and Rh are concentrated in the intercumulus silicates and frequently associated with base-metal sulphides. Up to about 20% of all platinum-group minerals in the investigated chromitite layers contain sub stantial amounts of As. The base-metal sulphides are strongly modified in the postmagmatic stage, which led to a significant loss of Fe and S, in this way concentrating Cu, Ni, and the platinum-group elements by factors of up to 10. Interaction between chromite and base-metal sulphides cannot account for all the Fe lost in chromite-poor samples, and the importance of additional processes is indicated. Inclusions in chromite and orthopyroxene indicate the formation of discrete platinum-group minerals and As-rich phases before the formation of an immiscible sulphide melt. Resorption of earlier formed platinum-group minerals into the immiscible sulphide melt and postmagmatic sulphidation destroyed most of the evidence of the early formed platinum-group minerals.

Reactions in Pd/GaAs(001) contacts at 7×108 Pa pressure

In this letter, we present the transmission electron microscopy and x-ray diffraction studies of Pd/GaAs (001) contacts that were annealed in an argon ambient with a pressure of 7×108 Pa. Our observations show that this ambient inhibits not only the decomposition of As-rich phases but the reactions accompanying the As sublimation for the Pd/GaAs (001) contacts and that the reactions are described as 4Pd+GaAs→Pd4GaAs (between 25 and 200 °C) and 3Pd+2GaAs→2PdGa+PdAs2 (between 350 and 800 °C).