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Nuclear energy is among the most viable alternatives to our current fossil fuel-based energy economy. The mass deployment of nuclear energy as a low-emissions source requires the reprocessing of used nuclear fuel to recover fissile materials and mitigate radioactive waste. A major concern with reprocessing used nuclear fuel is the release of volatile radionuclides such as xenon and krypton that evolve into reprocessing facility off-gas in parts per million concentrations. The existing technology to remove these radioactive noble gases is a costly cryogenic distillation; alternatively, porous materials such as metal-organic frameworks have demonstrated the ability to selectively adsorb xenon and krypton at ambient conditions. Here we carry out a high-throughput computational screening of large databases of metal-organic frameworks and identify SBMOF-1 as the most selective for xenon. We affirm this prediction and report that SBMOF-1 exhibits by far the highest reported xenon adsorption capacity and a remarkable Xe/Kr selectivity under conditions pertinent to nuclear fuel reprocessing.

Rhomboclase, ([H.sub.5][O.sub.2])Fe[(S[O.sub.4]).sub.2] x 2[H.sub.2]O, transforms to a solid crystalline phase, ([H.sub.3]O)Fe[(S[O.sub.4]).sub.2], upon dehydration. The structure of ([H.sub.3]O)Fe[(S[O.sub.4]).sub.2] is found to be the same as a recently reported structure determined from single-crystal diffraction by Peterson et al. (2009), who synthesized the same compound using a hydrothermal method. The phase boundary between rhomboclase and ([H.sub.3]O)Fe[(S[O.sub.4]).sub.2] as a function of temperature (7) and relative humidity (RH) was determined by environment-controlled in situ X-ray diffraction (XRD) method. The stability of ([H.sub.3]O)Fe[(S[O.sub.4]).sub.2] against rhomboclase was further evaluated under a simulated martian condition (constant 50% RH, -20[degrees]C, 6 mbar C[O.sub.2]). Both phases remained after 14 days with no observable transition. This result suggests that hydrate ferric sulfate minerals might not respond to diurnal RH fluctuation under the extremely slowed kinetics expected on the martian surface. Keywords: Rhomboclase, sulfate, hydrate, humidity, Mars, ferric sulfate DOI: 10.2138/am.2010.3470

Phases encountered in the hydration of monoclinic and trigonal anhydrous [Fe.sub.2](S[O.sub.4]).sub.3] and evaporation of [Fe.sub.2](S[O.sub.4]).sub.3] solutions at room temperature were determined using in situ and ex situ X-ray diffraction (XRD) under dynamic relative humidity (RH) control at room temperature (22-25[degrees]C). Both monoclinic and trigonal forms of [Fe.sub.2](S[O.sub.4]).sub.3remain anhydrous at 11% RH or below, and undergo the following phase evolution sequence: anhydrous [Fe.sub.2](S[O.sub.4]).sub.3] [arrow right] (ferricopiapite, rhomboclase) kornelite [right arrow] paracoquimbite at RH between 33 and 53% as a function of time. Evaporation of aqueous [Fe.sub.2](S[O.sub.4]).sub.3] solutions at 40% < RH < 60% results in precipitation of ferricopiapite and rhomboclase during evaporation, followed by a transition to kornelite and then paracoquimbite. Evaporation at RH < 33% produced an amorphous ferric-sulfate phase. The presence of some iron sulfate hydrates and their stability under varying RH are not only determined by the final humidity level, but also the intermediate stages and hydration history (i.e., either ferricopiapite or paracoquimbite can be a stable phase at 62% RH depending on the hydration history). The sensitivity to humidity change and pathdependent transitions of ferric sulfates make them potentially valuable indicators of paleo-environmental conditions and past water activity on Mars. The phase relationships reported herein can help in understanding the diagenesis of ferric sulfate minerals, and are applicable to geochemical modeling of mineral solubility in multi-component systems, an endeavor hindered by the need for fundamental laboratory studies of iron sulfate hydrates. Keywords: Ferric sulfate, humidity, ferricopiapite, rhomboclase, kornelite, paracoquimbite DOI: 10.2138/am.2009.3182

Arsenate ([As.sup.5+]) substitution in the hydroxylapatite structure was examined using a combination of crystallographic and spectroscopic techniques. Samples of hydroxylapatite, the [As.sup.5+]-substituted analog (synthetic johnbaumite), and five intermediate compositions were synthesized from solution. Synchrotron X-ray diffraction data show that all samples are single-phase, confirming complete substitution. No evidence is found for lowering of symmetry below [P6.sub.3]/m. Rietveld structure refinements show progressive expansion of the unit cell with increasing As substitution, which can be accounted for primarily by an average expansion of the tetrahedral site. Sizes of Ca polyhedra show little variation as a result of As substitution. NMR results show no evidence for local clustering of P[O.sub.4] tetrahedra. EXAFS confirms that the size of As-centered tetrahedra remains constant across the solid-solution series. Keywords: Apatite, hydroxylapatite, johnbaumite, arsenic, arsenate, NMR, EXAFS

Low-OH apatite of the compositional range [Ca.sub.4.99-5.06][(P[O.sub.4]).sub.2.98-3.00 [F.sub.0.51-0.48][Cl.sub.0.38-0.36]O[H.sub.0.14-0.12] was synthesized and characterized structurally by synchrotron-based single-crystal X-ray diffraction (XRD), and multiple nuclear magnetic resonance (NMR) spectroscopic techniques. The average structure is hexagonal with space group [P6.sub.3]/m. The presence of scattering in the single-crystal diffraction data set, which is incommensurate within the average hexagonal structure, suggests the presence of localized short-range monoclinic domains. Complex lineshapes in the [sup.31]P and [sup.19]F MAS NMR spectra are also consistent with the presence of an incommensurate phase. No evidence was detected for splitting of the Ca2 site into two distinct sites (as had been previously reported for hexagonal ternary apatites). Structure refinement and [sup.19]F {[sup.35]Cl} TRAPDOR NMR experiments verified intercolumnal neighboring of F and Cl atoms (inter-column distance of 2.62 [Angstrom]) within this low-OH- apatite suggesting that long-range neighboring ofF and Cl within the apatite anion channels is feasible. Keywords: Apatite, NMR spectroscopy, single-crystal XRD, Mars, Moon, PGE, synthesis

The structure of CaIr[O.sub.3] (Cmcm) has been refined at high pressure and at low temperature using time-of-flight neutron powder diffraction data. Evidence supporting deviation from space group Cmcm to Cmc[2.sub.1] is inconclusive. As CaIr[O.sub.3] (Cmcm) unit-cell volume changes, refinements indicate deformation of cation-centered coordination polyhedra, rather than tilting. Structure models demonstrate [Ca.sup.2+]-centered polyhedra are an order of magnitude more compressible than [Ir.sup.4+]-centered octahedra. Bond valence sums show significant chemical strain (over-bonding) of calcium and oxygen at ambient conditions. Implications for structure change in MgSi[O.sub.3] post-perovskite are discussed and a method for predicting the Clapeyron slope between perovskite and post-perovskite phases is proposed based on extrapolation of the volume-ratio between cation-centered polyhedra. Keywords: Post-perovskite, high pressure, structure, neutron diffraction, Rietveld refinement, bond valence, D' layer, CaIr[O.sub.3]

Analysis of pressure-temperature dependent monochromatic X-ray powder diffraction data yield the bulk modulus [[K.sub.T] = 180.2(28) GPa] and thermal expansion coefficients [[[alpha].sub.0] = 2.841(34) x [10.sup.-5] [K.sup.-1]; [[alpha].sub.1] = 3.37(48) x [10.sup.-9] [K.sup.-2] of CaIr[O.sub.3], the structure model for post-perovskite MgSi[O.sub.3]. CaIr[O.sub.3] is orthorhombic (Cmcm, space group 63, Z = 4) with best-fit unit-cell parameters, a = 3.14147(5) [Angstrom], b = 9.87515(19), c = 7.29711(11), and V= 226.3754(78) [[Angstrom],sup.3] at 1 bar and 300 K. The c-axis of Cair[O.sub.3] has a small compressibility and a large thermal expansion when compared to the other principal axes. Rietveld structure refinement reveals changes in CaIr[O.sub.3] as a function of temperature in terms of Ir[O.sub.6] octahedra distortion. Dissociation of Cair[O.sub.3] at high temperature has possible implications for the post-perovskite MgSi[O.sub.3] structure, Earth's lower mantle, and D' layer. Keywords: Post-perovskite, Cair[O.sub.3], pressure, temperature, structure, X-rays, Rietveld refinement, D' layer

Neighborite (NaMg[F.sub.3]) with the perovskite structure, transforms to a post-perovskite (ppv) phase between 27 and 30 GPa. The ppv phase is observed to the highest pressures achieved (56 GPa) at room temperature and transforms to an as yet unknown phase upon heating. Rietveld structure refinement using monochromatic synchrotron X-ray diffraction data provide models for the perovskite and postperovskite structures at high pressure. The refined models at 27(1) GPa indicate some inter-octahedral F-F distances rival the average intra-octahedral distance, which may cause instability in the perovskite structure and drive the transformation to the post-perovskite phase. The ratio of A-site to B-site volume ([V.sub.A]/[V.sub.B]) in perovskite structured NaMg[F.sub.3] ([ABX.sub.3]), spans from 5 in the zero-pressure high-temperature cubic perovskite phase to 4 in this high-pressure perovskite phase at 27(1) GPa, matching the [V.sub.A]/[V.sub.B] value in post-perovskite NaMg[F.sub.3]. Using Rietveld refinement on post-perovskite structure models, we observe discrepancies in pattern fitting, which may be described in terms of development of sample texture in the diamond-anvil cell, recrystallization, or a change of space group to [Cmc2.sub.1], a non-isomorphic subgroup of Cmcm--the space group describing the structure of CaIr[O.sub.3]. Keywords: Perovskite, post-perovskite, pressure, diamond anvil cell, X-ray diffraction, Rietveld modeling, structure

The structural behavior of a cancrinite, [Na.sub.5.96][Ca.sub.1.52] [[Al.sub.6][Si.sub.6][O.sub.24]][(C[O.sub.3]).sub.1.57] x 1.75[H.sub.2]O, was determined by using in situ synchrotron X-ray powder diffraction data [[lambda] = 0.91806(5) [Angstrom]] at room pressure and from 25 to 982 [degrees]C. The sample was heated at a rate of about 9.5 [degrees]C/min, and X-ray traces were collected at about 15 [degrees]C intervals. The satellite reflections in cancrinite were lost at about 504 [degrees]C, where a phase transition occurs. All the unit-cell parameters for cancrinite also show a discontinuity at 504 [degrees]C. Initially, the [Ca x C[O.sub.3]] clusters and their vacancies are ordered in the channels, and this ordering is destroyed on heating to give rise to the phase transition. Cancrinite loses water continuously until about 625 [degrees]C; thereafter an anhydrous cancrinite phase exists. From 25 to 952 [degrees]C, a minimal amount of C[O.sub.2] is lost from the structure. Over this temperature range, the average bridging angle, which is an indication of the degree of rotation of the tetrahedra, increases from 143.7(4) to 147.7(5)[degrees]. Rotations of the tetrahedra are caused by expansion of the Nal-O2 bond lengths. Keywords: Cancrinite, high-temperature structure, phase transitions, dehydration, Rietveld refinements, synchrotron radiation

The tetranatrolite-paranatrolite transformation has remained a key problem in understanding the paragenesis of zeolites in the natrolite family. It is accepted that when paranatrolite, approximate formula [Na.sub.16-x][Ca.sub.x][Al.sub.16+x][Si.sub.24-x][O.sub.80]* 24[H.sub.2]O, is removed from an aqueous environment and exposed to the atmosphere, it loses water and transforms to tetranatrolite, [Na.16-x][Ca.sub.x][Al.sub.16+x][Si.sub.24-x][O.sub.80]*n[H.sub.2]O (n [less than or equal to] 24). Here we show that this transformation is not only reversible, but that tetranatrolite exhibits two sequential pressure-induced hydrations leading first to paranatrolite and then to a superhydrated tetranatrolite above 0.2 and 3.0 GPa, respectively. We have previously reported similar behavior for the corresponding system with an ordered Si/Al distribution, i.e., natrolite itself, however the ordered version of paranatrolite exists over a much smaller pressure range. The pressure-induced transformations of natrolite and tetranatrolite thus further supports the supposition that paranatrolite is a distinct mineral species, with a pressure-stability field dependent upon composition. Keywords: Crystal structure, high pressure, phase transition, tetranatrolite, paranatrolite, synchrotron X-ray powder diffraction, pressure-induced hydration

The crystal structure of NaMg[F.sub.3] perovskite (neighborite) has been studied at 4 GPa and temperatures up to 1000 [degrees]C using the Rietveld structure-refinement method. In situ synchrotron X-ray powder diffraction data was collected using monochromatic radiation. The orthorhombic (Pbnm) to cubic (Pm3m) transition was observed when the temperature increased from 900 to 1000 [degrees]C. Structure refinements show that the ratio of polyhedral volumes of the A and B sites ([V.sub.A]/[V.sub.B]) of the orthorhombic phase increases with temperature, approaching the ideal value (5) for the cubic structure. However, this ratio becomes smaller at 4 GPa compared to the result from previous studies at the same temperature but ambient pressure, indicating that pressure makes it more difficult to transform from the orthorhombic phase to the cubic phase in this kind of perovskite.

We present a structural model for (K,Na)Mg[F.sub.3] perovskite using results from high-resolution synchrotron X-ray powder diffraction and nuclear magnetic resonance (NMR) spectroscopy. (K,Na)Mg[F.sub.3] perovskite is found to transition from orthorhombic (Pbnm) to tetragonal (P4/mbm) to cubic (Pm3m) as potassium concentration is increased. These phase transitions are not accompanied by a discontinuity in pseudo-cubic unit-cell volume and occur close to compositions ([K.sub.0.37]N[a.sub.0.63])Mg[F.sub.3] and ([K.sub.0.47]N[a.sub.0.53])Mg[F.sub.3], respectively. [sup.19F] NMR spectra indicate that the N[a.sup.+] and [K.sup.+] cations do not occupy the A cation site at random and end-member local environments are favored for all compositions. Based on results from both X-ray diffraction and NMR, we propose that diffuse diffraction is the result of strain between coexisting regions of different octahedra (Mg[F.sub.6]) tilts brought about by the ionic radius mismatch of N[a.sup.+] and [K.sup.+] cations. We suggest A-site cations group with like cations as neighbors to reduce excess volume and total strain.

Disorder in stoichiometric magnesioferrite, MgF[e.sub.2][O.sub.4], was determined from in situ synchrotron powder X-ray diffraction data [lambda] = 0.3738 [Angstrom] A] at 6, 5, and 3 GPa and temperatures up to 1430 K. The a unit-cell parameter increases linearly on heating at the three different pressures. Higher pressures cause a smaller cell volume, as expected. Cation order was analyzed in terms of the inversion parameter, x, {[sup.iv[[[Ms.sub.1-x][F.sub.x]].sup.vi] [[[Mg.sub.x2][F[e.sub.1-x/2].sub.2][O.sub.4]} and the order parameter Q = 1 - (3/2)x. As pressure increases, the inversion parameter increases in inverse MgF[e.sub.2][O.sub.4] spinel. O'Neill and Navrotsky (1983) and Landau models were used to describe the equilibrium non-convergent ordering process in MgF[e.sub.2][O.sub.4], and they both fit the data well.

NMR and Pair Distribution Function (PDF) analysis represent two complementary techniques, where the local environments and short-range ordering in positive electrode materials are investigated. The PDF method, like EXAFS, is used to determine bond distances, provide insights into the oxidation states of cations and local distortions that are not captured in the average models of structure obtained by diffraction.