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Boehmite (γ-AlOOH) and gibbsite (α-Al-(OH)3) are important archetype (oxy)hydroxides of aluminum in nature that also play diverse roles across a plethora of industrial applications. Developing the ability to understand and predict the properties and characteristics of these materials, on the basis of their natural growth or synthesis pathways, is an important fundamental science enterprise with wide-ranging impacts. The present study describes bulk and surface characteristics of these novel materials in comprehensive detail, using a collectively sophisticated set of experimental capabilities, including a range of conventional laboratory solids analyses and national user facility analyses such as synchrotron X-ray absorption and scattering spectroscopies as well as small-angle neutron scattering. Their thermal stability is investigated using in situ temperature-dependent Raman spectroscopy. These pure and effectively defect-free materials are ideal for synthesis of advanced alumina products.

The magnetic structure of two natural samples of goethite (α-FeOOH) with varying crystallinity was analyzed at 15 and 300 K by neutron diffraction. The well crystallized sample has the Pb′nm color space group and remained antiferromagnetic up to 300 K, with spins aligned parallel to the c axis. The purely magnetic 100 peak, identifying this color space group, was clearly resolved. The nanocrystalline sample shows a phase transition to the paramagnetic state at a temperature below 300 K. This lowering of the Néel temperature may be explained by the interaction of magnetic clusters within particles. The nuclear structure, refined with the Rietveld and pair distribution function methods, is consistent with reports in the literature.

We characterize a concentrated 7.3 m CaCl2 solution, combining neutron diffraction with chloride isotopic substitution (Cl-NDIS) in null water and molecular dynamics (MD) simulations. We elucidate the solution structure, thermodynamic properties, and extent of ion pairing previously suggested as concentration-dependent and often not observed at lower concentrations. Our Cl-NDIS measurements designate the solvent-shared ion pairing as dominant and the contact ion pairing (CIP) as insignificant even under conditions close to the solubility limit. The MD models parameterized against neutron diffraction with calcium isotopic substitution (Ca-NDIS) overestimate CIP despite successfully reproducing most of the Cl-NDIS signal. This drawback originates from the fact that Ca2+–Cl− interactions were primarily "hidden" in the Ca-NDIS signal due to overlapping with Ca2+–Ow and Ca2+–Hw contributions to the total scattering. Contrary, MD models with moderate CIP and possessing generally good performance at high concentrations fail to reproduce the NDIS measurements accurately. Therefore, the electronic polarization, introduced in most of the recent MD models via scaling ionic charges, resolves some but not all parameterization drawbacks. We conclude that despite improving the quality of MD models "on average," the question "which model is the best" has not been answered but replaced by the question "which model is better for a given research." An overall "good" model can still be inappropriate or, in some instances, "bad" and, unfortunately, produce erroneous results. The accurate interpretation of several NDIS datasets, complemented by MD simulations, can prevent such mistakes and help identify the strengths, weaknesses, and convenient applications for corresponding computational models. [ABSTRACT FROM AUTHOR]

The structure of opal has long fascinated scientists. It occurs in a number of structural states, ranging from amorphous to exhibiting features of stacking disorder. Opal‐CT, where C and T signify cristobalite‐ and tridymite‐like interstratification, represents an important link in the length scales between amorphous and crystalline states. However, details about local atomic (dis)order and arrangements extending to long‐range stacking faults in opal polymorphs remain incompletely understood. Here, a multilevel modeling approach is reported that considers stacking states in correlation with the abundance of C and T segments as a high‐level structural parameter (i.e. not each atom). Optimization accounting for inter‐tetrahedral bond lengths and angles and the regularity of the silicate tetrahedra is included as lower levels of structural parameters. Together, a set of parameters with both coarse‐grained and atomistic features for different levels of structural details is refined. Structural disorder at the ∼10–100 Å distance scale is evaluated using experimental pair distribution function and diffraction datasets, comparing peak intensities, widths and asymmetry. This work presents a complete multilevel structural description of natural opal‐CT and explains many of the unusual features observed in X‐ray powder diffraction patterns. This modeling approach can be adopted generally for analyzing layered materials and their assembly into 3D structures. [ABSTRACT FROM AUTHOR]

The response of the mesolite crystal structure ([Na.sub.16][Ca.sub.16][Al.sub.48][Si.sub.72][O.sub.240] x 64 [H.sub.2]O) to dehydration was evaluated as a function of temperature and partial pressure of water (i.e., [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] or relative humidity, RH) using laboratory X-ray powder diffraction (XRD; CuK[alpha] radiation) and synchrotron X-ray pair distribution function (PDF) methods. At 425[degrees]C under low-[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] conditions ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] [less than or equal to] ~1.3 mbar), dehydrated mesolite preserved the long-range ordered aluminosilicate framework structure, which has not been previously observed. This new dehydrated phase, x-metamesolite, has unit-cell parameters [a 16.731(3) [Angstrom], b = 17.822(2) [Angstrom], c = 6.312(1) [Angstrom], V 1882.5(5) [[Angstrom].sup.3], and possible space group Fdd2] similar to those of other dehydrated natrolite phases (either [alpha]1-or [alpha]2-metanatrolite). Conversely, under high-[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] conditions ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] > ~1.3 mbar), dehydrated mesolite became amorphous (amorphous [T.sub.5][O.sub.10]) at 425[degrees]C. The local structure of amorphous [T.sub.5][O.sub.10] was characterized by PDF analyses, which showed the formation of twisted [T.sub.5][O.sub.10] nano-fibers [with dimensions (LWH) of ~6.9 x 6.9 x 6.3 [Angstrom]] resulting from breakage of the mesolite aluminosilicate framework. The two distinct high-temperature [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-dependent phase transition paths illustrate the importance of considering the combined effects of temperature and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] in mesolite. In addition, the low-temperature phase transition in mesolite, involving order-disorder of the extraframework cations, also showed a [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-dependent transition temperature. Although, there is no path dependence on [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] for this transition, the local arrangement of Na, Ca, and vacancies in disordered metamesolite (formed through the extraframework cation order-disorder phase transition) likely influences the thermal stability of the aluminosilicate framework during further heating. Keywords: Phase transition, mesolite, XRD data, crystal structure DOI: 10.2138/am.2010.3443

The crystal structure of the 2.5 hydrate MgSO4 phase was determined by simulated annealing from laboratory X-ray powder diffraction data measured from 2-140[degrees]2[theta] using CuK[alpha] radiation. The 2.5 hydrate is monoclinic, space group C2/c, with unit-cell parameters a - 18.8636(4) [Angstrom], b 12.3391(2) [Angstrom], c= 8.9957(2) [Angstrom], [beta] = 94.568(2)[degrees], V 2087.1(6) [[Angstrom].sup.3], and Z= 16. The model was refined using fundamental-parameters Rietveld refinement, converging to [R.sub.wp] = 8.89%, [R.sub.p] = 6.61%, [R.sub.exp] = 3.33%, [R.sub.Bragg] = 3.95%, and [chi square] = 2.67. The refined structure is consistent with a formula of 2.5 [H.sub.2]O. Bond-valence calculations for the refined model show that the structure is chemically sensible. In the refined structure, [[Mg(O,[H.sub.2]O).sub.6]] octahedra and [[SO.sub.4]] tetrahedra build up 2-D double-sheet slabs by sharing vertex O atoms, which are held together by inter-slab H-bonds involving [([SO.sub.4]).sup.2-] groups and [H.sub.2]O molecules coordinated with [Mg.sup.2+] cations to form the layer structure of the 2.5 hydrate phase. Keywords: Mg[SO.sub.4].2.5[H.sub.2]O, Mg[SO.sub.4].2.4[H.sub.2]O, crystal structure, simulated annealing, structure determination, powder diffraction, Rietveld refinement

The crystal structure of sanderite, MgS[O.sub.4] x 2[H.sub.2]O, was determined from laboratory X-ray powder diffraction data measured from 2-140 [degrees]2[theta] using CuK[alpha] radiation. Sanderite is orthorhombic, space group P[2.sub.1][2.sub.1][2.sub.1], with unit-cell parameters a = 8.8932(1) [Angstrom], b = 8.4881(1) [Angstrom], c = 12.4401(2) [Angstrom], V = 939.16(3) [Angstrom], Z = 8. The crystal structure model was determined using the charge flipping method and was refined using fundamental parameters Rietveld refinement method to [R.sub.wp] = 6.52%, [R.sub.exp]: 1.89%, and [chi square] = 3.43. Bond-valence calculations for the refined model show that the structure is chemically reasonable. In the refined structure, [Mg.sup.2+] cations are coordinated by four O atoms from [[S[O.sub.4]].sup.2-] groups and by two [H.sub.2]O molecules, forming distorted octahedra. By sharing vertex O atoms, [S[O.sub.4]] tetrahedral and [Mg[O.sub.4]([[H.sub.2]O).sub.2]] octahedra build up a 3-D framework. Keywords: Sanderite, MgS[O.sub.4] x 2[H.sub.2]O, crystal structure, charge flipping, structure determination, powder diffraction

The crystal structures of natrolite and its dehydrated high-temperature phases ([alpha]1- and ([alpha]2-metanatrolite) have been determined from powder X-ray diffraction measurements as a function of temperature and partial pressure of water ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) to characterize the phase transition behavior. The evolution of crystal structure as a function of temperature shows two different phase transitions, depending on the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], with [alpha]1-metanatrolite occurring at elevated [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [alpha]2-metanatrolite occurring at low [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. Our discovery of [alpha]2-metanatrolite implies the existence of more than one transition mechanism, which we correlate with the migration of [Na.sup.+] ions and the rate of evolution of [H.sub.2]O molecules. The transition behavior is rationalized in terms of two cooperating mechanisms: (1) dehydration-induced processes, which determine the phase transition temperature; and (2) thermally induced processes, which determine how the framework and its extraframework cations are modified. Keywords: Phase transition, natrolite, crystal structure, Rietveld refinement, TGA

H2W2O7, a metastable material synthesized via selective etching of the Aurivillius‐related Bi2W2O9, is demonstrated as an electrode for high power proton‐based energy storage. Comprehensive structural characterization is performed to obtain a high‐fidelity crystal structure of H2W2O7 using an iterative approach that combines X‐ray diffraction, neutron pair distribution function, scanning transmission electron microscopy, Raman spectroscopy, and density functional theory modeling. Electrochemical characterization shows a capacity retention of ≈80% at 1000 mV s–1 (1.5‐s charge/discharge time) as compared to 1 mV s–1 (≈16‐min charge/discharge time) with cyclability for over 100 000 cycles. Energetics from density functional theory calculations indicate that proton storage occurs at the terminal oxygen sites within the hydrated interlayer. Last, optical micrographs collected during in situ Raman spectroscopy show reversible, multicolor electrochromism, with color changes from pale yellow to blue, purple, and last, orange as a function of proton content. These results highlight the use of selective etching of layered perovskites for the synthesis of metastable transition metal oxide materials and the use of H2W2O7 as an anode material for proton‐based energy storage or electrochromic applications. [ABSTRACT FROM AUTHOR]

Nitrite (NO2−) is a prevalent nitrogen oxyanion in environmental and industrial processes, but its behavior in solution, including ion pair formation, is complex. This solution phase complexity impacts industries such as nuclear waste treatment, where NO2− significantly affects the solubility of other constituents present in sodium hydroxide (NaOH)-rich nuclear waste. This work provides molecular scale information into sodium nitrite (NaNO2) and NaOH ion-pairing processes to provide a physical basis for later development of thermodynamic models. Solubility isotherms of NaNO2 in aqueous mixtures with NaOH and total alkalinity were also measured. Spectroscopic characterization of these solutions utilized high-field nuclear magnetic resonance spectroscopy (NMR) and Raman spectroscopy, with additional solution structure detailed by X-ray total scattering pairwise distribution function analysis (X-ray PDF). Despite the NO2− deformation Raman band's insensitivity to added NaOH in saturated NaNO2 solutions, 23Na and 15N NMR studies indicated the Na+ and NO2− chemical environments change likely due to ion pairing. The ion pairing correlates with a decrease in diffusion coefficient of solution species as measured by pulsed field gradient 23Na and 1H NMR. Two-dimensional correlation analyses of the 2800–4000 cm−1 Raman region and X-ray PDF indicated that saturated NaNO2 and NaOH mixtures disrupt the hydrogen network of water into a new structure where the length of the O⋯O correlations is contracted relative to the typical H2O structure. Beyond describing the solubility of NaNO2 in a multicomponent electrolyte mixture, these results also indicate that nitrite exhibits greater ion pairing in mixtures of concentrated NaNO2 and NaOH than in comparable solutions with only NaNO2. [ABSTRACT FROM AUTHOR]

Despite widespread industrial importance, predicting metal solubilities in highly concentrated, multicomponent aqueous solutions is difficult due to poorly understood ion–ion and ion–solvent interactions. Aluminum hydroxide solid phase solubility in concentrated sodium hydroxide (NaOH) solutions is one such case, with major implications for ore refining, as well as processing of radioactive waste stored at U.S. Department of Energy legacy sites, such as the Hanford Site, Washington State. The solubility of gibbsite (α-Al(OH)3) is often not well predicted because other ions affect the activity of hydroxide (OH−) and aluminate (Al(OH)4−) anions. In the present study, we systematically examined the influence of key anions, nitrite (NO2−) and nitrate (NO3−), as sodium salts on the solubility of α-Al(OH)3 in NaOH solutions taking care to establish equilibrium from both under- and oversaturation. Rapid equilibration was enabled by use of a highly pure and crystalline synthetic nano-gibbsite of well-defined particle size and shape. Measured dissolved aluminum concentrations were compared with those predicted by an α-Al(OH)3 solubility model derived for simple Al(OH)4−/OH− systems. Specific anion effects were expressed as an enhancement factor (Alenhc) conveying the excess of dissolved aluminum. At 45 °C, NaNO2 and NaNO3-containing systems exhibited Alenhc values of 2.70 and 1.88, respectively, indicating significant enhancement. The solutions were examined by Raman and high-field 27Al NMR spectroscopy, indicating specific interactions including Al(OH)4−–Na+ contact ion pairing and Al(OH)4−–NO2−/NO3− ion–ion interactions. Dynamic evolution of the α-Al(OH)3 particles including growth and agglomeration was observed revealing the importance of dissolution/reprecipitation in establishing equilibrium. These studies indicate that incomplete ion hydration, as a result of the low water activity in these concentrated electrolytes, results in: (i) enhanced reactivity of the hydroxide ion with respect to α-Al(OH)3; (ii) increased concentrations of Al(OH)4− in solution; and (iii) stronger ion–ion interactions that act to stabilize the supersaturated solutions. This information on the mechanisms by which α-Al(OH)3 becomes supersaturated is essential for more energy-efficient aluminum processing technologies, including the treatment of millions of gallons of Al(OH)4−-rich high-level radioactive waste. [ABSTRACT FROM AUTHOR]