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Amorphous calcium carbonate (ACC) occurs as a precursor to geological and biogenic calcium carbonate (CaCO3), yet its transformation pathways and reaction mechanisms remain inconsistent and controversial. In this study, we investigated the transformation of ACC to calcite under both solution and dry conditions, in the presence and absence of impurity ions, utilizing operando time-resolved synchrotron X-ray diffraction (TRXRD) and reactive transport modeling. Results demonstrate that TRXRD techniques allow us to differentiate dissolution-reprecipitation versus solid-state transformation mechanisms for amorphous to crystalline phase transitions. Specifically, we observe that in environments with abundant water, ACC transforms to calcite through a dissolution-reprecipitation mechanism. This features an activation energy of 63 ± 2 kJ/mol and unit cell volume contraction during calcite crystal growth. Conversely, under water-limited conditions, ACC to calcite transformation proceeds via a solid-state transformation mechanism, with an activation energy of 210 ± 2 kJ/mol, three times greater than the dissolution-reprecipitation route, and a unit cell expansion during crystalline calcite growth. To illustrate the magnitude of these effects, the rates of calcite growth were similar during dissolution-reprecipitation at 3 °C [0.00207(35) s–1] and solid-state transformation at 280 °C [0.00134(11) s–1]. Moreover, the incorporation of an impurity, strontium, significantly retards the rate of calcite growth while expanding its unit cell but whose incorporation is history dependent. Reactive transport modeling of the dissolution–precipitation kinetics suggests that ACC must be dissolving as compact aggregates. These various transformation mechanisms drive diverse geological and biological carbonate formations, impacting their use as paleoenvironmental markers and functional materials synthesis.

Accurate structural models are of paramount importance for elucidating structure–property relationships in functional materials. Spinels (AB2O4) form a highly important family of materials with complex crystal structures, and subtle structural details have a critical bearing on understanding their physical properties. In some spinels, the space group symmetry is debated, and in general, point defects such as cation inversion and interstitials add complexity. Most studies of spinels concern powder materials, and this challenges deep structural characterization. In fact, most published spinel structures have dubious atomic displacement parameters (ADPs), which is a typical sign of problematic structural description in the refinement of diffraction data. Here, we use various X-ray and neutron diffraction techniques to establish a benchmark crystal structure for the essentially defect-free spinel ferrite ZnFe2O4, which is a widely studied frustrated magnet. It is shown that the appearance of Fd3̅mforbidden reflections in the ZnFe2O4single-crystal neutron diffraction data is an artifact of multiple scattering rather than the loss of inversion symmetry. We then provide benchmark ADPs and demonstrate how strongly these parameters affect the refined cation inversion. The ADPs reported here may be used as reference data to test the soundness of refined structural models, possibly to constrain those based on suboptimal data quality, and thereby provide a more accurate fundamental understanding of the structure–property relationship in spinel-type materials.

The challenge of growing rare-earth (RE) sesquioxide crystals can be overcome by tailoring their structural stability and melting point via composition engineering. This work contributes to the advancement of the field of crystal growth of high-entropy oxides. A compound with only small REs (Lu,Y,Ho,Yb,Er)2O3maintains a cubic C-type structure upon cooling from the melt, as observed via in-situhigh-temperature neutron diffraction on aerodynamically levitated samples. On the other hand, a compound with a mixture of small and large REs (Lu,Y,Ho,Nd,La)2O3crystallizes as a mixture of a primary C-type phase with an unstable secondary phase. Crystals of compositions (Lu,Y,Ho,Nd,La)2O3and (Lu,Y,Gd,Nd,La)2O3were grown by the micro-pulling-down (mPD) method with a single monoclinic B-type phase, while a powder of (Lu,Y,Ho,Yb,Er)2O3did not melt at the maximum operating temperature of an iridium-rhenium crucible. The minimization of the melting point of the two grown crystals is attributed to the mismatch in cation sizes. The electron probe microanalysis reveals that the general element segregation behavior in the crystals depends on the composition.

High-entropy aluminum garnets were grown as bulk single crystals using the micro-pulling-down method, taking the synthesis of complex ceramics a step further from the conventional preparation of polycrystalline materials. We studied the effects of growth parameters on the elemental distribution in high optical quality crystals of (Lu1/6Y1/6Ho1/6Dy1/6Tb1/6Gd1/6)3Al5O12containing six cations (yttrium and rare-earths) taken in equimolar amounts. A single garnet structure was confirmed by powder X-ray diffraction. Electron microprobe measurements were obtained to correlate the radial distribution of rare-earth elements with pulling rates and molten zone height. The nature of the elemental distribution in the radial direction was associated with ionic radius: smaller rare-earths concentrated in the center of the crystal, while larger rare-earths segregated toward the outer edge of the cylindrical crystal. Faster pulling rates led to a flattening of the concentration profiles toward the nominal concentration, promoting a more homogeneous radial elemental distribution, while varying the molten zone height did not have a significant effect. The demonstrated success with crystal growth enables the practical availability of single crystals of multicomponent aluminum garnets for further discovery of new phenomena and applications.

We report the discovery of four new compounds of Eu2+-doped scintillators [Cs3Rb]SrI6, [Cs3K]SrI6, [Cs3Rb]CaI6, and [Cs3K]CaI6. A compositional engineering strategy dictated by the structural systematics of the base compounds Cs4CaI6and Cs4SrI6(space group R3̅c-hexagonal basis) was conducted to confirm an ideal solid solution for this system. Substitution of smaller monovalent cations Rb1+and K1+for Cs1+, maintained the crystal structure, producing an ordered substitution. Crystals were initially grown doped with 0.5% Eu to compare scintillation properties, and a Eu2+optimization study determined that 7% Eu was the ideal concentration. A 56 mm3crystal of composition [Cs3.5Rb0.5]SrI6:Eu 7% gave the best measured scintillation performance with an absolute light yield of 75 000 ph/MeV and energy resolution of 3.3% at 662 keV.

The exchange of carbon dioxide for methane in natural gas hydrates is an attractive approach to harvesting CH4for energy production while simultaneously sequestering CO2. In addition to the energy and environmental implications, the solid solution of clathrate hydrate (CH4)1-x(CO2)x·5.75H2O provides a model system to study how the distinct bonding and shapes of CH4and CO2influence the structure and properties of the compound. High-resolution neutron diffraction was used to examine mixed CO2/CH4gas hydrates. CO2-rich hydrates had smaller lattice parameters, which were attributed to the higher affinity of the CO2molecule interacting with H2O molecules that form the surrounding cages, and resulted in a reduction in the unit-cell volume. Experimental nuclear scattering densities illustrate how the cage occupants and energy landscape change with composition. These results provide important insights on the impact and mechanisms for the structure of mixed CH4/CO2gas hydrate.

The lithium aluminosilicate mineral petalite (LiAlSi4O10) has been studied with high-pressure single-crystal X-ray diffraction (HP-XRD) up to 5 GPa. Petalite undergoes two fully reversible pressure-induced first-order phase transitions, not previously reported in the literature, at ca. 1.5 and 2.5 GPa. The first of these transforms the low-pressure α-phase of petalite (P2/c) to an intermediate β′-phase that then fully converts to the high-pressure β-phase at ca. 2.5 GPa. The α → β transition is isomorphic and is associated with tripling of the unit-cell volume. Analysis of the HP-XRD data show that although the fundamental features of the petalite structure are retained through this transition, there are subtle alterations in the internal structure of the silicate double-layers in the β-phase relative to the α-phase. Measurement of the unit-cell parameters of petalite as a function of pressure, and fitting of the data with third-order Birch-Murnaghan equation of state, has provided revised elastic constants for petalite. The bulk moduli of the α- and β-phases are 49(1) and 35(3) GPa, respectively. These values indicate that the compressibility of the α-phase of petalite lies between those of the alkali feldpsars and alkali feldspathoids, whereas the β-phase has a compressibility more comparable with layered silicates. Structure analysis has shown that the compression of the α-phase is facilitated by the rigid body movement of the Si2O7units from which the silicate double-layers are constructed.

Joint refinement of powder neutron diffraction data of hydrogenated and deuterated brucite over the temperature range of 10-295 K clearly shows significant isotope effects in the structural parameters. Mg(OH)2has a 0.31% larger volume than that of Mg(OD)2at room temperature, which is mostly due to the c-axis expansion of Mg(OH)2as compared to Mg(OD)2. The isotope effect in the a-axis has the opposite, but smaller, behavior as compared to the c-axis. These differences are slightly enhanced with reduction of the cell volume upon cooling. The temperature dependence of the isotropic atomic displacement parameters (ADPs) with the single site model show that the ADP of the H atom is approximately larger than that of the D atom by the amount expected from the reduced-mass difference, but this difference is not evident with the split site model. Despite the shorter c-axis of the deuterated form, nearest-neighbor D···D distances are longer than the H···H distances, because the O-H distances are longer than the O-D distances, which necessarily places the H atoms closer together by 0.03 Å or more within the interlayer space. This study showcases an example of when a joint Rietveld refinement is ideally suited, by combining data for the deuterated and hydrogenated forms of brucite. The approach reduces the number of least-squares variables, and reduces the systematic errors. It can be a general method to analyze isotope effects in materials studied by neutron diffraction.

We demonstrate for the first time the crystal growth of high-entropy rare-earth (RE) aluminum perovskites (REAlO3) using the micro-pulling-down method to inform future exploration of functional crystals. To determine how composition affects phase formation, we formulate equiatomic compositions containing five REs from the following list: Lu, Yb, Tm, Er, Y, Ho, Dy, Tb, Gd, Eu, Sm, Nd, Pr, Ce, La. To test whether combinations of REs with similar ionic radii may favor a single phase, compositions containing REs with consecutive or nonconsecutive ionic radius values were formulated. Powder and single-crystal X-ray diffraction indicate that crystals containing only REs with similar ionic radii that form orthorhombic single-RE REAlO3are a single phase. Crystals containing REs with dissimilar ionic radii or mixtures of REs that form orthorhombic, rhombohedral, and tetragonal single-RE REAlO3are a mixture of phases. The elemental distribution in single-phase crystals analyzed via electron probe microanalysis confirms no evidence of preferential incorporation of any of the constituent REs. The distribution and composition of secondary phases were analyzed via scanning electron microscopy and energy dispersive spectroscopy; secondary phases were seen as a small region in the center of the crystals with branching features closer to the outer surface.

A distinct crossover between the high temperature and low temperature normal state dc transport properties of epitaxial (Ca, Sr)CuO2 single crystal thin films has been observed. Defect-doped (Ca, 4;Sr)CuO2 films with dc conductivity behavior ranging from insulating to metallic-like were obtained by growing stoichiometric and cation-deficient material under various film growth conditions. The results indicate that 2D variable-range hopping is the dominant transport mechanism at low temperatures. The high temperature behavior depends on the magnitude of the conductivity, resulting in either activated conduction or a ln ρ ∼ 1/T1/2 dependence that is reminiscent of thermally-activated tunneling in a granular metal/insulator matrix. The distinct crossover in behavior suggests the possibility of gap formation in the temperature range 150–200 K.

A distinct crossover between the high temperature and low temperature normal state dc transport properties of epitaxial (Ca, Sr)CuO2 single crystal thin films has been observed. Defect-doped (Ca, Sr)CuO2 films with dc conductivity behavior ranging from insulating to metallic-like were obtained by growing stoichiometric and cation-deficient material under various film growth conditions. The results indicate that 2D variable-range hopping is the dominant transport mechanism at low temperatures. The high temperature behavior depends on the magnitude of the conductivity, resulting in either activated conduction or a ln ρ ~ 1/T^1/2 dependence that is reminiscent of thermally-activated tunneling in a granular metal/insulator matrix. The distinct crossover in behavior suggests the possibility of gap formation in the temperature range 150–200 K.

The amblygonite-montebrasite series of minerals, common constituents of granitic pegmatites and topaz-bearing granites, show complete solid solution with ideal composition LiAlPO4(F, OH). These compounds are ideal for studying F ↔ OH solid solution in minerals because natural members of the series generally show little deviation from the ideal composition. In this study, we used powder and single-crystal neutron diffraction and solid-state 6Li MAS, CP MAS, and REDOR NMR techniques to study the effect of F ↔ OH substitution on the series. Lattice parameters refined from single-crystal neutron diffraction data show increasing b and decreasing a, c, and V with increasing F/(F + OH). The volume is highest for the OH end-member because of the presence of an additional atom (H). The a and c parameters decrease with increasing F/(F + OH) because the O-H vector is close to the a-c plane and the Al-OH/F vectors are approximately parallel to c.

The amblygonite-montebrasite series of minerals, common constituents of granitic pegmatites and topaz-bearing granites, show complete solid solution with ideal composition LiAlPO4(F, OH). These compounds are ideal for studying F ↔ OH solid solution in minerals because natural members of the series generally show little deviation from the ideal composition. In this study, we used powder and single-crystal neutron diffraction and solid-state 6Li MAS, CP MAS, and REDOR NMR techniques to study the effect of F ↔ OH substitution on the series. Lattice parameters refined from single-crystal neutron diffraction data show increasing b and decreasing a, c, and V with increasing F/(F + OH). The volume is highest for the OH end-member because of the presence of an additional atom (H). The a and c parameters decrease with increasing F/(F + OH) because the O-H vector is close to the a-c plane and the Al-OH/F vectors are approximately parallel to c.

The crystal structures of natural F-bearing chondrodite [Mg4.64Fe0.28Mn0.014Ti0.023(Si1.01O4)2F1.02OH0.97] from the Tilley Foster mine (Brewster, New York), F-bearing titanian clinohumite [Mg8.805Fe0.006Ti0.214(Si0.993O4)4F0.484OH0.516] from Kukh-i-Lal (Tadjikistan) and F-free titanian hydroxylclinohumite [Mg7.378Fe1.12Mn0.052Ni0.014Ti0.453(Si0.996O4)4OH1.0] from Val Malenco (Italy) were refined in space group P21/b (unique axis a) from single-crystal neutron diffraction data, collected on a four-circle diffractometer at the High Flux Isotope Reactor at Oak Ridge National Laboratory. Accurate H atom positions were determined at 295 K, 100 K, and 20 (10) K. Only one H position of approximately 50% occupancy was observed for each structure, which confirms a disordered H model. Time-of-flight single-crystal neutron data were also collected at 295 K and 20 K for the Val Malenco clinohumite as an additional check on space group symmetry. The crystal structure of the Kukh-i-Lal clinohumite was further investigated by X-ray single-crystal refinement at 295 K and by piezoelectric measurements. A few, very weak, symmetry-forbidden reflections were observed for each crystal at both ambient and lower temperatures. The same reflections were observed by all methods used. No temperature dependence is indicated, as no additional peaks appear at low temperature, and the intensity of the reflections are sample dependent. It appears that the real structure is made up of P21and Pb domains so that violations are due to ordering of both H and Ti. No distinct piezoelectric effect was observed that would indicate the absence of a center of symmetry. This points to the simultaneous presence of various enantiomorphic domains, which cancels the piezoelectric effect of individual domains. The decrease in unit-cell volume with F substitution in clinohumites can be explained by the higher concentration of H-site vacancies and the coupled cationic and anionic substitution on the M3 and O/F site.

Abstract: High-energy ball milling with subsequent annealing is used to synthesize the intermetallic compound ζ-FeZn13. The mechanically alloyed phase in the as-milled state is determined to be nonequilibrium, or metastable. Transmission electron microscopy (TEM) studies show a highly defective microstructure with undefined grain areas, and the alloy can be described as a mechanical mixture of elemental Fe and Zn, based on neutron diffraction measurements. Characteristic stages associated with its transformation to the equilibrium state are identified based on differential scanning calorimetry (DSC) measurements. The activation energies corresponding to these stages are 128, 202, and 737 kJ/mole, respectively, with increasing transformation temperatures. The first stage is related to limited atomic diffusion or rearrangements, such as recovery, during thermal treatment, while the second stage depicts continued recrystallization and long-range atomic diffusion leading to a stable phase formation. The third and final stage marks structural decomposition of the equilibrium structure due to phase transition. Neutron diffraction of the equilibrium alloy confirmed that the structure is C2/m, with lattice parameters of a=13.40995 Å, b=7.60586 Å, c=5.07629 Å, and β=127 deg 18 minutes. The atomic positions of Fe and Zn compared well to reported values.

Improved structural parameters for the lanthanide orthovanadates (including also YVO₄ and ScVO₄ and new results for PrVO₄, TmVO₄, and the LuVO₄ end members) have been determined by employing a full-profile Rietveld structure analysis of neutron powder-diffraction data. High-quality powders of the zircon-type MVO₄ vanadates were prepared by a homogeneous coprecipitation in molten urea. The zircon structure has space group symmetry I4₁/amd, Z = 4, with 3 atoms occupying the asymmetric unit. In all, 22 parameters were refined with final agreement values of R_p = 0.0313-0.0421, R_wp = 0.0361-0.0511, and G-of-F = 1.210-1.962. The average V-O bond length for all of the MVO₄ samples is 1.709(2) Å, and the V-O distance exhibits a small systematic shortening with decreasing M atom size. The oxygen positional parameters, the cell dimensions, and the M-O distances vary systematically with the metal-ion atomic number. The anisotropic thermal motion of the oxygen atom has the largest amplitude normal to the shared polyhedral edge between the VO₄ tetrahedron and MO₈ bisdisphenoid. Empirical relations are given for the cell dimensions and oxygen atom position as a function of the size of the lanthanide ion which can be used to estimate structural parameters for other zircon-type lanthanide vanadate end-members and vanadate solid solutions. Copyright 1994, 1999 Academic Press

The MCuSi₄O₁₀ (M = Ca, Sr, Ba) compounds possess the gillespite (BaFeSi₄O₁₀) structure, which is tetragonal, P4/ncc, Z = 4, Data sets of 2365 step intensities were recorded for the title compounds with λ = 1.400 Å at T 295 K. For M = Ca, a = 7.3017(3) Å, c = 15.1303(6) Å, V_cell = 806.66(9) ų, R_exp = 0.0453, R_wp = 0.0515, for 487 reflections. For M = Sr, a = 7.3707(2) Å, c = 15.5904(6) Å, V_cell = 846.9(1) ų, R_exp = 0.0433, R_wp = 0.0489, for 521 reflections. For M = Ba, a = 7.4409(3) Å, c = 16.1367(8) Å, V_cell = 893.4(1) ų, R_exp = 0.0466, R_wp = 0.0522, for 546 reflections. The gillespite structure contains an unbranched single silicate layer, which can be viewed as a tesselation of corner-linked Si₄O₁₀ rings. The Cu in MCuSi₄O₁₀ is in square-planar coordination with the nonbridging oxygens of the corrugations of the silicate layer. The alkaline-earth atom, in distorted cubic coordination, joins together adjacent [CuSi₄O₁₀]^2- slabs. Increasing the size of the alkaline-earth atom increases the cell dimensions anisotropically (i.e., Δc > Δa) due to the inflexibility of the silicate anion. Room-pressure experiments show no evidence for oxygen intercalation or polytypism. The lattice mismatch between the [CuSi₄O₁₀]^2- slabs of the gillespite structure type and the square-planar CuO₂ layers of the superconducting cuprates is small, which suggests possible applications of these materials as exotic thin-film substrates for the high T_c cuprates or other novel structures having cuprate and silicate layers. Copyright 1993, 1999 Academic Press

Well-crystallized Li4Mn5O12powder was prepared by heating a eutectic mixture of lithium acetate LiOAc and manganese nitrate Mn(NO3)2in an O2atmosphere. The structure of Li4Mn5O12crystallites was found to be cubic spinel using Rietveld refinement of both neutron and X-ray powder diffraction profiles. We confirmed that lithium ions occupy both the tetrahedral sites 8aand part of the octahedral sites 16d, but not the 16csites in the space groupFd3m, while all the manganese ions occupy the 16dsites. The lattice parameter was found to be sensitive to synthesis temperature as a result of the variation in manganese valence. The presence of Mn3+leads to the formation of a stoichiometric spinel Li[LixMn2−x]O4in whichxdecreases from 1/3 (Li4Mn5O12) to 0 (LiMn2O4), with concomitant formation of Li2MnO3depending on the synthesis conditions.

Well-crystallized Li4Mn5O12powder with grain size of 0.1–0.4 μm was prepared by heating a eutectic mixture of lithium acetate (LiOAc), and manganese nitrate (Mn(NO3)2), in an oxygen atmosphere. The structure of Li4Mn5O12crystallites was found to be a cubic spinel using Rietveld refinement of both neutron and X-ray powder diffraction data. We confirmed that lithium ions occupy both the tetrahedral 8asites, and part of the octahedral 16dsites, but not the 16csites in the space group Fd3m, while all the manganese ions occupy the 16dsites. The lattice parameter was found to be sensitive to the synthesis temperature as a result of the variation in manganese valence. Samples prepared at 500 °C showed better electrode performance. A rechargeable capacity of about 135 mAh/g for the cell Li/Li4Mn5O12was obtained in the 2.5–3.6 V range of cell voltages.

Charge density waves (CDWs) are modulations of the electron density and the atomic lattice that develop in some crystalline materials at low temperatures. We report an unusual example of a CDW in BaFe2Al9below 100 K. In contrast to the canonical CDW phase transition, temperature-dependent physical properties of single crystals reveal a first-order phase transition. This is accompanied by a discontinuous change in the size of the crystal lattice. In fact, this large strain has catastrophic consequences for the crystals causing them to physically shatter. Single-crystal X-ray diffraction reveals superlattice peaks in the low-temperature phase signaling the development of a CDW lattice modulation. No similar low-temperature transitions are observed in BaCo2Al9. Electronic structure calculations provide one hint to the different behavior of these two compounds; the d-orbital states in the Fe compound are not completely filled. Iron compounds are renowned for their magnetism, and partly filled d-states play a key role. It is therefore surprising that BaFe2Al9develops a structural modulation at low temperature instead of magnetic order.

AbstractAn important step towards the construction of a new neutron source for the Unitedstates tookplaceonMarch3 and 4,1998. About 250 attendees representing all of the major US neutron scattering facilities joined forces to present a convincing case for the importance of neutron scattering research for science, education and industry. The $1.3 billion Spallation Neutron Source (SNS) project will be built by a consortium of Department of Energy (DOE) laboratories, Oak Ridge National Laboratory (ORNL), Brookhaven National Laboratory (BNL), Argonne National Laboratory (ANL). Los Alamos National Laboratory (LANL) and Lawrence Berkeley National Laboratory (LBNL). LBNL will be responsible for designing and building the SNS's front-end system, which includes an ion source, beam formation and control hardware, and low-energy beam transport and acceleration systems. LANL will be responsible for the linac, which accelerates the H-beam from 2.5 to 1000 million electron volts. ANL and ORNL will be responsible for developing the neutron scattering instrumentation and moderator designs. BNL will be responsible for the accumulator ring structure, which bunches and intensities the ion beam for delivery onto the mercury target to produce the pulsed neutron beams. ORNL will be responsible for the coordination, installation and operation of the new source, project management, conventional construction, and design and construction of the liquid mercury target.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

AbstractA local meeting entitled the SNSl HFIR Workshop was held from February 26-27, 1998, and was jointly sponsored by Oak Ridge Associated Universities, Oak Ridge National Laboratory (ORNL), Southeastern Universities Research Association and the University of Tennessee. The purpose was primanly to promote opportunities for neutron science at the existing HFIR and the proposed SNS.