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Raman scattering data on ammonium halides in 1 and 2 disordered phases are reported. The effects of disorder and that of short range ordering is discussed in terms of group theoretical arguments.
The transformation of quartz to stishovite has been studied by X-ray and optical examination of a series of experimentally shock-loaded specimens of a quartz-copper mixture. Shock pressures of 68 to 260 kb and peak temperatures of 320 to 870 K were achieved. Stishovite was identified from quartz shock-loaded above 90 kb; the quantity increases with increasing pressure, but is not dependent on temperature. The formation of stishovite under shock conditions appears to be intimately related to a short-range order phase.
An investigation of the phase transformation and the austenite stabilization in a high strength austenite has been made. An Fe-29Ni-4.3Ti austenite age-hardened by gamma-prime (Ni3Ti) precipitates showed a further increase of strength after martensitic and reverse martensitic phase transformations. The stability of ausaged austenite as well as ausaged and transformation-strengthened austenite was improved significantly through an isothermal treatment at 500 C. The Ms temperature of the strengthened austenite was restored to nearly that of annealed austenite while the austenite was hardened to R(C) 41 through precipitation and phase transformations. The observed austenite stabilization is attributed to the formation of GP zones or short-range order of less than about 10A in size.
Integral analysis of the intensity of the electron density distribution curve in molten crystal hydrates provided by X-ray analysis, permits the following conclusions on the structure of the complex SO and NO ions, and the short-range order in the structure of the solution. The SO4 ion in the solution has a tetrahedral structure with an S to O distance equal to 1.5 A. For the NO3 in the solution, a planar triangular shape is probable, with an N to O distance equal to 1.2 A. Preferential distances between each of the oxygens of the SO ion and the nearest molecules of water proved near to the corresponding distances in solid crystal hydrates. For an (H2SO4)(H2O) solution, the average number of water molecules surrounding each oxygen atom of the SO4 (--) ion was on the order of 1.3 molecules. Hence the preferential distances between the water molecules and the oxygen atoms of the SO ion, and the preference of their mutual position, correspond to the fixed position of these same elements of the structure in the solid crystal hydrate.
The physical properties of quasi-one-dimensional conductors are profoundly influenced by slight changes in structure. The unusual structural features of this class of materials include only short range order of the anions and structure modulation. The nature of disorder and structure modulation can affect the character of the metal-to-insulator transition at low temperature, and may allow us to make some predictions of low temperature behavior based on room temperature structural properties.
Such complexities of the Wustite structure as nonstoichiometry, ferric iron variable site distribution, long and short range ordering, and exsolution, yield complex physical properties. Magnesiowustite, a phase which has been suggested to occur in the earth's lower mantle, is also expected to exhibit many of these complexities. Geophysical models including the properties of (Mg, Fe)O should accordingly take into account the uncertainties associated with the synthesis and measurement of iron-rich oxides. Given the variability of the Fe(1-x)O structure, it is important that future researchers define the structural state and extent of exsolution of their samples.
Several surface modificaation techniques were performed in situ in an ultrahigh vacuum as part of a program to develop electrical contacts on the (100) face of cubic SiC. The Auger electron spectroscopy line shapes and peak-to-peak heights of the Si LVV and CKLL transitions indicated changes in surface stoichiometry, bonding, and short range order. Changes in the low-energy electron diffraction pattern identified changes in the symmetry of long range ordering on the surface. Heating above 1050 C depleted the surface of Si with an activation energy of 120 kcal/mol, resulting eventually in a disordered graphitic layer which was several atomic layers thick. Bombardment by Ar ions of energies greater than 1000 eV enhanced the Si to C ratio on the surface and destroyed the LEED pattern. Long range order was recovered by simultaneous heating and ion bombardment. Finally, adsorption of Cr on the Ar ion damaged surface and subsequent desorption left a reconstructed, p(2 x 1)Si enriched surface. Similar treatments with Al did not produce a surface reconstruction.
Extended X ray absorption fine structure (EXAFS) is a powerful technique for probing the local atomic structure of battery and fuel cell materials. The major advantages of EXAFS are that both the probe and the signal are X rays and the technique is element selective and applicable to all states of matter. This permits in situ studies of electrodes and determination of the structure of single components in composite electrodes, or even complete cells. EXAFS specifically probes short range order and yields coordination numbers, bond distances, and chemical identity of nearest neighbors. Thus, it is ideal for structural studies of ions in solution and the poorly crystallized materials that are often the active materials or catalysts in batteries and fuel cells. Studies on typical battery and fuel cell components are used to describe the technique and the capability of EXAFS as a structural tool in these applications. Typical experimental and data analysis procedures are outlined. The advantages and limitations of the technique are also briefly discussed.
The mineralogical nature of Mars soil is far from being understood, nor are the formation time and weathering processes known. Quantitatively, the two major mineral-forming elements in Mars soil are silicon and iron, constituting 44 and 19 percent of the soils as SiO4 and Fe2O3, respectively. The silicate phases have been studied only briefly, mostly because of their limited spectral fingerprinting in the VIS and NIR. Much attention was given to the iron minerals in the soil, due to their pronounced absorption in the VIS and NIR, making them easily detectable by telescopic observations. The available information on Mars soil mineralogy, mostly obtained by remote sensing, is reviewed, and it is hypothesized that it leads to the suggestion that nanophase short-range-order (amorphous) phases of the silicates and iron oxides abound in the soil.
Recent experiments using scanning tunneling microscopy show evidence for the formation of surface alloys of otherwise immiscible metals. Such is the case for Au deposited in Ni(11), where experiments by Pleth Nielsen et al. indicate that at low Au coverage (less than 0.5 ML), Au atoms replace Ni atoms in the surface layer forming a surface alloy while the Ni atoms form islands on the surface. In this work, we present results of a theoretical modeling of this phenomenon using the recently developed BFS method for alloys. We provide results of an extensive analysis of the growth process which strongly support the conclusions drawn from the experiment; at very low coverages, there is tendency for dimer formation on the overlayer, which later exchange positions with Ni atoms in the surface layer, thus accounting for the large number of substituted dimers. Ni island formations as well as other alternative short range order patterns are discussed.
Recent experiments using scanning tunneling microscopy show evidence for the formation of surface alloys of otherwise immiscible metals. Such is the case for Au deposited in Ni(110), where experiments by Pleth Nielsen el al.indicate that at low Au coverage (less than 0. 5 ML), Au atoms replace Ni atoms in the surface layer forming a surface alloy while the Ni atoms form islands on the surface. In this paper, we present results of a theoretical modeling of this phenomenon using the recently developed Bozzolo-Ferrante-Smith method for alloys. We provide results of an extensive analysis of the growth process that strongly support the conclusions drawn from the experiment: at very low coverages, there is a tendency for dimer formation on the overlayer, which later exchange positions with Ni atoms in the surface layer, thus accounting for the large number of substituted dimers. Ni island formation as well as other alternative short-range-order patterns are discussed.
The temporal evolution of the nanostructure of a model Ni-base superalloy (Ni-5.2 at.% Al-14.2 at.% Cr) is studied experimentally employing three-dimensional atom-probe (3DAP) microscopy in conjunction with kinetic Monte Carlo (KMC) simulations at 600 C. It is demonstrated that not only can the mean compositions of individual gamma' (Ni3Al with the Li2 structure) precipitates be measured but the Ni, Al, and Cr concentration profiles within the precipitates can also be determined for precipitates with a mean radius ( ) as small as 0.85 nm. The three asymptotic tinie dependencies of the Lifshitz-Slyzov-Wagner (LSW) theory of coarsening (Ostwald ripening) are measured and found to deviate from its theoretical predictions: possible explanations for these discrepancies are discussed. At 0.25 hr. there is 3DAP microscope evidence for the presence of precipitates of another nickel-rich phase. ="Ni3Cr" (Ni3Cr(1-x)Al(x)), which exhibits short-range order (SRO) and that is metastable with respect to Ni3Al. This metastable phase is also found by KMC simulations and has the composition Ni3Cr(1-x)Al(x), which is Ni-2.91 at.% Al-21.98 at.% Cr at 16 hours. Our results demonstrate that the decomposition of gamma the primary gamma (FCC) phase results in the concurrent formation of an ordered phase and a disordered phase by 0.25 hours.
The chemical pathways leading to gamma'(L1(sub 2)) nucleation from nondilute Ni-5.2 Al-14.2 Cr at. %, gama(fcc), at 873 K are followed with radial distribution functions and isoconcentration surface analyses of direct-space atom-probe tomographic images. Although Cr atoms initially are randomly distributed, a distribution of congruent Ni3Al short-range-order domains (SRO), [R] approx. equals 0.6 nm, results from Al diffusion during quenching. Domain site occupancy develops as their number density increases leading to Al-rich phase separation by gamma'-nucleation, [R]=0.75 nm, after SRO occurs.
Both primary (volcanic/impact glasses) and secondary (opal/silica, allophane, hisingerite, npOx, S-bearing) amorphous phases appear to be major components of martian surface materials based on orbital and in-situ measurements. A key observation is that whereas regional/global scale amorphous components include altered glass and npOx, local scale amorphous phases include hydrated silica/opal. This suggests widespread alteration at low water-to-rock ratios, perhaps due to snow/ice melt with variable pH, and localized alteration at high water-to-rock ratios. Orbital and in-situ measurements of the regional/global amorphous component on Mars suggests that it is made up of at least three phases: npOx, amorphous silicate (likely altered glass), and an amorphous S-bearing phase. Fundamental questions regarding the composition and the formation of the regional/global amorphous component(s) still remain: Do the phases form locally or have they been homogenized through aeolian activity and derived from the global dust? Is the parent glass volcanic, impact, or both? Are the phases separate or intimately mixed (e.g., as in palagonite)? When did the amorphous phases form? To address the question of source (local and/or global), we need to look for variations in the different phases within the amorphous component through continued modeling of the chemical composition of the amorphous phases in samples from Gale using CheMin and APXS data. If we find variations (e.g., a lack of or enrichment in amorphous silicate in some samples), this may imply a local source for some phases. Furthermore, the chemical composition of the weathering products may give insight into the formation mechanisms of the parent glass (e.g., impact glasses contain higher Al and lower Si [30], so we might expect allophane as a weathering product of impact glass). To address the question of whether these phases are separate or intimately mixed, we need to do laboratory studies of naturally altered samples made up of mixed phases (e.g., palagonite) and synthetic single phases to determine their short-range order structures and calculate their XRD patterns to use in models of CheMin data. Finally, to address the timing of the alteration, we need to study rocks on the martian surface of different ages that may contain glass (volcanic or impact) with MSL and future rovers to better understand how glass alters on the martian surface, if that alteration mechanism is universal, and if alteration spans across long periods of time or if there is a time past which unaltered glass remains.
The Chemistry and Mineralogy (CheMin) instrument onboard the Mars Curiosity rover has detected abundant amounts (approx. 25-30 weight percentage) of X-ray amorphous materials in a windblown deposit (Rocknest) and in a sedimentary mudstone (Cumberland and John Klein) in Gale crater, Mars. On Earth, X-ray amorphous components are common in soils and sediments, but usually not as abundant as detected in Gale crater. One hypothesis for the abundant X-ray amorphous materials on Mars is limited interaction of liquid water with surface materials, kinetically inhibiting maturation to more crystalline phases. The objective of this study was to characterize the chemistry and mineralogy of soils formed in the Antarctica Dry Valleys, one of the driest locations on Earth. Two soils were characterized from different elevations, including a low elevation, coastal, subxerous soil in Taylor Valley and a high elevation, ultraxerous soil in University Valley. A variety of techniques were used to characterize materials from each soil horizon, including Rietveld analysis of X-ray diffraction data. For Taylor Valley soil, the X-ray amorphous component ranged from about 4 weight percentage in the upper horizon to as high as 15 weight percentage in the lowest horizon just above the permafrost layer. Transmission electron microscopy indicated that the presence of short-range ordered (SRO) smectite was the most likely candidate for the X-ray amorphous materials in the Taylor Valley soils. The SRO smectite is likely an aqueous alteration product of mica inherited from granitic materials during glaciation of Taylor Valley. The drier University Valley soils had lower X-ray amorphous contents of about 5 weight percentage in the lowest horizon. The X-ray amorphous materials in University Valley are attributed to nanoparticles of TiO2 and possibly amorphous SiO2. The high abundance of X-ray amorphous materials in Taylor Valley is surprising for one of the driest places on Earth. These materials may have been physically and chemical altered during soil formation, however, the limited interaction with water and low temperatures may result in the formation of "immature" X-ray amorphous or SRO materials. Perhaps, a similar process contributes to the formation of the high content of X-ray amorphous materials detected on Mars.
Mineralogical and geochemical data returned by orbiters and landers over the past 15 years have substantially enhanced our understanding of the history of aqueous alteration on Mars. Here, we summarize aqueous processes that have been implied from data collected by landed missions. Mars is a basaltic planet. The geochemistry of most materials has not been “extensively” altered by open-system aqueous processes and have average Mars crustal compositions. There are few examples of open-system alteration, such as Gale crater’s Pahrump Hills mudstone. Types of aqueous alteration include (1) acid-sulfate and (2) hydrolytic (circum-neutral/alkaline pH) with varying water-to-rock ratios. Several hypotheses have been suggested for acid-sulfate alteration including (1) oxidative weathering of ultramafic igneous rocks containing sulfides; (2) sulfuric acid weathering of basaltic materials; (3) acid fog weathering of basaltic materials; and (4) near-neutral pH subsurface solutions rich in Fe (sup 2 plus) that rapidly oxidized to Fe (sup 3 plus) producing excess acidity. Meridiani Planum’s sulfate-rich sedimentary deposit containing jarosite is the most “famous” acid-sulfate environment visited on Mars, although ferric sulfate-rich soils are common in Gusev crater’s Columbia Hills and jarosite was recently discovered in the Pahrump Hills. An example of aqueous alteration under circum-neutral pH conditions is the formation of Fe-saponite with magnetite in situ via aqueous alteration of olivine in Gale crater’s Sheepbed mudstone. Circum-neutral pH, hydrothermal conditions were likely required for the formation of Mg-Fe carbonate in the Columbia Hills. Diagenetic features (e.g., spherules, fracture filled veins) indicate multiple episodes of aqueous alteration/diagenesis in most sedimentary deposits. However, low water-to-rock ratios are prominent at most sites visited by landed missions (e.g., limited water for reaction to form crystalline phases possibly resulting in large amounts of short-range ordered materials and little physical separation of primary and secondary materials). Most of the aqueous alteration appears to have occurred early in the planet’s history; however, minor aqueous alteration may be occurring at the surface today (e.g., thin films of water forming carbonates akin to those discovered by Phoenix).
The Antarctic Dry Valleys (ADV) comprise the largest ice-free region of Antarctica. Precipitation usually occurs as snow, relative humidity is frequently low, and mean annual temperatures are about -20°C [1]. Substantial work has focused on soil formation in the ADVs [2], however, little work has focused on the mineralogy of secondary alteration phases. The dominant weathering process in the ADV region is physical weathering, however, chemical weathering has been well documented [3]. The occurrence of chemical weathering processes are suggested by the presence of clay minerals and iron and titanium oxides in soil. Previously we have investigated soils from two sites in the ADV’s and have shown evidence of chemical weathering by the presence of clay minerals (vermiculite, smectite), short-range ordered (SRO) and/or X-ray amorphous materials, and Fe- and Tioxides as well as the presence of discrete calcite crystals [4, 5]. The Chemistry and Mineralogy (CheMin) instrument onboard the Mars Curiosity rover has detected abundant amounts (approx. 25-30 wt. %) of X-ray amorphous materials in a windblown deposit or “soil” (Rocknest) and in a sedimentary rocks [6,7,8]. The occurrence of large amounts of X-ray amorphous materials in Mars sediments is surprising because these materials are usually present in small quantities in terrestrial environments. The objective of this study is to further characterize the chemistry and mineralogy, specifically the secondary alteration mineralogy and the presence of X-ray amorphous material, of soils from two sites we have previously studied, a subxerous soil in Taylor Valley, and an ultraxerous soil in University Valley. While the chemical alteration processes and mineralogy of the ADV has been documented previously, there has been limited discussion on the occurrence and formation of X-ray amorphous and SRO materials in Antarctica soils. The process of aqueous alteration in the ADVs may have implications for pedogenic processes on Mars, and may lead to a better understanding to the abundance of amorphous material found in sediments in Gale crater.
The CheMin XRD instrument onboard the Curiosity rover has discovered abundant quantities of X-ray amorphous materials (25-40 wt. %) in windblown deposits and sedimentary rocks on Mars. The large amounts of X-ray amorphous materials on Mars is surprising because these materials are usually not found in large abundances in terrestrial soils. We have previously identified short-range ordered or X-ray amorphous materials in Antarctica Dry Valleys (ADV) soils. The ADV are one of the driest and coldest environments on Earth and provide an ideal process analog environment for Mars. The objective of this study was to further characterize the chemistry and mineralogy of ADV soils, specifically to quantify and characterize the X-ray amorphous materials. Two soils were characterized from different elevations in the ADV - a low elevation, coastal, subxerous soil in Taylor Valley and a high elevation, ultraxerous soil in University Valley. Rietveld analysis of XRD patterns generated with a CheMin-like analog instrument indicated up to 40 and 15 wt. % X-ray amorphous materials in the Taylor and University Valley soils, respectively. The high abundance of X-ray amorphous materials in Taylor Valley is surprising for one of the driest places on Earth. These materials may have been physically and chemical altered during soil formation, however, the limited interaction with water and low temperatures may have resulted in the formation of “immature” X-ray amorphous or SRO materials. Perhaps, a similar process contributes to the formation of the high content of X-ray amorphous materials detected on Mars.