High-resolution Synchrotron Powder Research Articles

Al-Si Order and Chemical Composition Model across Scapolite Solid Solutions with Evidence from Rietveld Structure Refinements

Scapolite forms solid solutions between the end members marialite, Na4[Al3Si9O24]Cl = Me0, and meionite, Ca4[Al6Si6O24]CO3 = Me100. Al-Si order and chemical composition models are proposed for the scapolite solid solutions. These models predict the chemical composition, Al-Si order, and average <T–O> distances between Me0–Me100. These models are based on the observed order of clusters and on two solid solutions that meet at Me75 coupled with predicted chemical compositions and <T–O> distances. The [Na4·Cl]3+ and [NaCa3·CO3]5+ clusters are ordered between Me0–Me75, whereas the clusters [NaCa3·CO3]5+ and [Ca4·CO3]6+ are disordered from Me75–Me100. To confirm the structural model, the crystal structure of 27 scapolite samples between Me6–Me93 has been obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements. The structure was refined in space group P42/n for all the samples. The <T–O> distances indicate that the T1 (=Si), T2 (=Al), and T3 (=Si) sites are completely ordered at Me37.5, where the 1:1 ratio of [Na4·Cl]3+:[NaCa3·CO3]5+ clusters are ordered and gives rise to antiphase domain boundaries (APBs) based on Cl-CO3 order instead of Al-Si order. The presence of APBs based on Cl-CO3 order and cluster order indicate that neither space group P42/n nor I4/m are correct for the structure of scapolite, but the lower symmetry space group P42/n is a good approximation for modeling the average structure of scapolite. The complete Al-Si order at Me37.5 changes in a regular and predictable manner toward the end members: Me0, Me75, and Me100. The observed unit cell and several structural parameters show a discontinuity at Me75, where the series is divided into two. There is no structural evidence to support any phase transition in the scapolite series. The T1 site contains only Si from Me0–Me37.5; from Me37.5–Me100, Al atoms enter the T1 site and the <T1–O> distance increases linearly to Me100.

Using synchrotron high-resolution powder X-ray diffraction for the structure determination of a new cocrystal formed by two active principle ingredients.

The crystal structure of a new 1:1 cocrystal of carbamazepine and S-naproxen (C15H12N2O·C14H14O3) was solved from powder X-ray diffraction (PXRD). The PXRD pattern was measured at the high-resolution beamline CRISTAL at synchrotron SOLEIL (France). The structure was solved using Monte Carlo simulated annealing, then refined with Rietveld refinement. The positions of the H atoms were obtained from density functional theory (DFT) ground-state calculations. The symmetry is orthorhombic with the space group P212121 (No. 19) and the following lattice parameters: a = 33.5486 (9), b = 26.4223 (6), c = 5.3651 (10) Å and V = 4755.83 (19) Å3.

Exploring the tuning patterns of cyclopentyl amine hydrate for potential application to CH4 storage

With the increasing demand on world energy consumption, natural gas has received attention as the near-term primary energy source that produces fewer CO2 emissions than other fossil fuels. Accordingly, developing eco-friendly gas storage and transport technology has remained a significant challenge. Gas hydrate is a crystalline inclusion compound formed by water molecules and has emerged as alternative option for natural gas storage. Despite the benefits of hydrates, relatively high pressure and low temperature formation conditions need to be mitigated for practical application. Here, we introduce cyclopentyl amine (CPA) as a new thermodynamic promoter for potential application for CH4 storage. We measured the synchrotron high-resolution powder diffraction and 13C NMR spectra, and the results revealed that the CPA could form sII hydrate without help gas. We also measured the pressure – temperature phase equilibria of CPA + CH4 hydrate, and the results revealed that CPA works as a thermodynamic promoter for CH4. Further, the tuning phenomena were observed below the stoichiometric concentration of the CPA. Specifically, the tuning factor was obtained as ~ 0.34, which exhibited a critical guest concentration was 0.9 mol%. We believe that these findings provide better understanding of the tuning phenomena for potential application to CH4 storage media.

Synthesis and Characterization of Magnetoelectric Ba7Mn4O15

We present the synthesis of a novel binary metal oxide material: Ba7Mn4O15. The crystal structure has been investigated by high-resolution powder synchrotron X-ray diffraction in the temperature range of 100–300 K as well as by powder neutron diffraction at 10 and 80 K. This material represents an isostructural barium-substituted analogue of the layered material Sr7Mn4O15 that forms its own structural class. However, we find that Ba7Mn4O15 adopts a distinct magnetic ordering, resulting in a magnetoelectric ground state below 50 K. The likely magnetoelectric coupling mechanisms have been inferred from performing a careful symmetry-adapted refinement against the powder neutron diffraction experiments, as well as by making a comparison with the nonmagnetoelectric ground state of Sr7Mn4O15.

Apatite, Ca10(PO4)6(OH,F,Cl)2: Structural Variations, Natural Solid Solutions, Intergrowths, and Zoning

Thirty-three samples from natural apatite (Ap) solid solutions, ideal structural formula [9]Ca12[7]Ca23([4]PO4)3[3](F,OH,Cl), (Z = 2) were examined with electron-probe microanalysis, synchrotron high-resolution powder X-ray diffraction (HRPXRD), and Rietveld refinements. Apatite has space group P63/m for the general chemical formula above. In Ap, the two different Ca sites are generally occupied by Ca, Mn, Sr, Na, or REE3+ cations; the P site is occupied by P, Si, or S, and the X is occupied by F, OH, Cl, O2−, or (CO3)2− anions. However, it may be possible for CO32− + F− anions to partially replace PO43− groups. In this study, the unit-cell parameters a, c, and c/a ratio, vary smoothly and non-linearly with the unit-cell volume, V. The data falls on two distinct trend lines. The average <P-O>[4] distance is nearly constant across the Ap series, whereas the average <O-P-O>[6] angle decreases linearly. The coordination numbers for the atoms are given in square brackets in the general chemical formula above. The average <Ca1-O>[9], <Ca2-O>[6], <Ca2-O,X>[7], and Ca2-X distances change non-linearly with increasing V. Although Cl− anion is larger than OH− and F− anions, the c unit-cell parameter in F-Ap is larger than that in Cl-Ap. In Cl-Ap, the Ca2 polyhedra are larger than in F-Ap, so the O and Cl anions are under-bonded, which cause the Ca1 polyhedra to contract and charge balance the anions. Alternatively, the Ca1 polyhedra are smaller in Cl-Ap than in F-Ap, so the Ca1 polyhedra in Cl-Ap cause the c axis to contract compared to that in F-Ap.

Structural Variations across Wolframite Solid Solutions, (Fe,Mn)WO4

The crystal structure of four samples from natural wolframite solid solutions, (Fe,Mn)WO4, was obtained with synchrotron high-resolution powder X-ray diffraction (HRPXRD) data, Rietveld refinements, space group P2/c, and Z = 2. Wolframite solid solutions extend from ferberite (FeWO4) to hübnerite (MnWO4). The W and (Mn,Fe) cations are in six-fold coordination. This study shows that the unit-cell parameters, a, b, c, and β angle, vary linearly with the unit-cell volume, V, across the wolframite series. The average <Mn,Fe–O> distance increases linearly because of larger Mn2+ (0.83 Å) replacing smaller Fe2+ (0.78 Å) cations, whereas the average <W–O> distance increases slightly because of the higher effective charge of the smaller Fe2+ cation. The distortions of the two types of polyhedra across the series are discussed.

Crystal Structure of an Anisotropic Pyrope Garnet That Contains Two Cubic Phases

The crystal structure of two different samples of pyrope garnet, ideally Mg3Al2Si3O12, from South Africa was refined using the Rietveld method, space group Ia3¯d, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Sample 1 from Wesselton Mine is a single cubic phase and is optically isotropic. Electron-probe microanalysis (EPMA) provided an average composition {Mg2.30Fe2+0.26Ca0.42Mn2+0.02}∑3[Al1.53Fe3+0.06Cr3+0.40Ti4+0.01Fe2+0.01]∑2Si3O12, which contains a significant amount of Cr cations. The unit-cell parameter (Å) and bond distances (Å) are a = 11.56197(1) Å, average <Mg-O> = 2.2985, Al-O = 1.9101(4), and Si-O = 1.6343(3) Å. Sample 2 from De Beers Diamond Mine has an average composition {Mg2.33Fe2+0.33Ca0.33Mn2+0.01}∑3[Al1.73Fe3+0.12Cr3+0.06Ti4+0.05Fe2+0.05]∑2Si3O12 and is a fine-scale intergrowth of two cubic phases. The weight percentage, unit-cell parameter (Å), and bond distances (Å) for phase 2a are 62.2(1)%, a = 11.56185(1) Å, average <Mg-O> = 2.3006, Al-O = 1.9080(4), Si-O = 1.6334(4) Å. The corresponding values for phase 2b are 37.8(1)%, a = 11.53896(1) Å, average <Mg-O> = 2.2954, Al-O = 1.9020(6), Si-O = 1.6334(6) Å. The two cubic phases in sample 2 cause the crystal to be optically anisotropic because of strain induce birefringence. The unit-cell parameter and bond distances for sample 1 are similar to those in phase 2a.

Structure determination of riboflavin by synchrotron high-resolution powder X-ray diffraction

The crystal structure of the stable form of vitamin B2 or riboflavin (C17H20N4O6) was solved using high-resolution powder X-ray diffraction (PXRD). The high-resolution PXRD pattern of riboflavin was recorded at room temperature at the European Synchrotron Radiation Facility (Grenoble, France). The starting structural model was generated using a Monte Carlo simulated annealing method. The final structure was obtained through Rietveld refinement. The positions of the H atoms belonging to hydroxy groups were estimated from computational energy minimizations. The symmetry is orthorhombic with the space group P212121 and the following lattice parameters: a= 20.01308, b= 15.07337 and c= 5.31565 Å.

Crystal Chemistry of Six Grossular Garnet Samples from Different Well-Known Localities

Two isotropic grossular (ideally Ca3Al2Si3O12) samples from (1) Canada and (2) Tanzania, three optically anisotropic grossular samples (3, 4, 5) from Mexico, and one (6) anisotropic sample from Italy were studied. The crystal structure of the six samples was refined in the cubic space group Ia3¯d, using monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and the Rietveld method. The compositions of the samples were obtained from electron microprobe analyses (EPMA). The HRPXRD traces show a single cubic phase for two isotropic samples, whereas the four anisotropic samples contain two different cubic phases that were also resolved using X-ray elemental line scans, backscattered electron (BSE) images, and elemental maps. Structural mismatch from two cubic phases intergrown in the birefringent samples gives rise to strain-induced optical anisotropy. Considering the garnet general formula, [8]X3[6]Y2[4]Z3[4]O12, the results of this study show that with increasing unit-cell parameter, the Y-O distance increases linearly and rather steeply, the average <X-O> distance increases just slightly in response to substitution mainly on the Y site, while the Z-O distance remains nearly constant. The X and Z sites in grossular contain Ca and Si atoms, respectively; both sites show insignificant substitutions by other atoms, which is supported by a constant Z-O distance and only a slight increase in the average <X-O> distance. The main cation exchange is realized in the Y site, where Fe3+ (ionic radius = 0.645 Å) replaces Al3+ (ionic radius = 0.545 Å), so the Y-O distance increases the most.

Linear Structural Trends and Multi-Phase Intergrowths in Helvine-Group Minerals, (Zn,Fe,Mn)8[Be6Si6O24]S2

Synchrotron high-resolution powder X-ray diffraction (HRPXRD) and Rietveld structure refinements were used to examine the crystal structure of single phases and intergrowths (either two or three phases) in 13 samples of the helvine-group minerals, (Zn,Fe,Mn)8[Be6Si6O24]S2. The helvine structure was refined in the cubic space group P4¯3n. For the intergrowths, simultaneous refinements were carried out for each phase. The structural parameters for each phase in an intergrowth are only slightly different from each other. Each phase in an intergrowth has well-defined unit-cell and structural parameters that are significantly different from the three endmembers and these do not represent exsolution or immiscibility gaps in the ternary solid-solution series. The reason for the intergrowths in the helvine-group minerals is not clear considering the similar radii, identical charge, and diffusion among the interstitial M cations (Zn2+, Fe2+, and Mn2+) that are characteristic of elongated tetrahedral coordination. The difference between the radii of Zn2+ and Mn2+ cations is 10%. Depending on the availability of the M cations, intergrowths may occur as the temperature, pressure, fugacity fS2, and fluid composition change on crystallization. The Be–Si atoms are fully ordered. The Be–O and Si–O distances are nearly constant. Several structural parameters (Be–O–Si bridging angle, M–O, M–S, average <M–O/S>[4] distances, and TO4 rotational angles) vary linearly with the a unit-cell parameter across the series because of the size of the M cation.

A Possible Radiation-Induced Transition from Monazite-(Ce) to Xenotime-(Y)

This study examines two pegmatitic monazite samples (2a and 4b, these numbers are related to a previous study) to determine their crystal chemistry and effects of internal radiation damage using synchrotron high-resolution powder X-ray diffraction and electron-probe micro-analysis. Both the huttonite and cheralite substitutions are discussed. Rietveld structure refinement of sample 2a shows three different phases [2a = monazite-(Ce), 2b = monazite-(Ce), and 2c = xenotime-(Y)] with distinct structural parameters. The changes among the unit-cell parameters between the two monazite-(Ce) phases is more pronounced in the a followed by the b and c unit-cell parameters. Sample 4a is a single-phase monazite-(Sm) that contains 0.164 apfu Th. Phase 2c with space group I41/amd arises from redistribution of La, Ce, Pr, Nd, Sm, Gd, Dy, Si, and Y atoms from those in monazite (space group P21/n). A possible cause for the phase transition from monazite-(Ce) to xenotime-(Y) is α-radiation events over a long geological time. However, other chemical processes cannot be ruled out as a cause for the transition.

Hard to Soft: Biogenic Absorbent Sponge-like Material from Waste Mussel Shells

Summary Mussel shells are a source of renewable CaCO3 and can be obtained from aquaculture waste streams. Valorization of waste shells relieves strain on landfills and aligns with the goals of a circular economy. Here, we present the preparation of an absorbent CaCO3 material, soft calcite (SC), prepared from waste blue mussel shells under mild conditions. Calcite is the only polymorph present in SC, despite the presence of aragonite in the shell. SC has a nest-like morphology, unlike the highly ordered calcite in the shell. SC is formed by the reassembly of calcite prisms and may be held together by the organic matrix “glue” from the original shell as evidenced by NMR spectroscopy. This inorganic sponge adsorbs dyes from aqueous solution and absorbs crude oil from seawater with good recyclability. Our results demonstrate how a natural structure can be modified in a sustainable fashion to yield a higher-value material.

Crystal Chemistry and Structural Variations for Zircon Samples from Various Localities

This study investigates the variations of structural parameters and chemistry of a partially metamict and seven detrital zircon samples from different localities using single-crystal X-ray diffraction, synchrotron high-resolution powder X-ray diffraction, and electron-probe micro-analysis techniques. The unit-cell parameters for the eight zircon samples vary linearly with increasing unit-cell volume, V. A zircon sample from the Canadian Arctic Islands has the smallest unit-cell parameters, bond distances, ideal stoichiometric composition, unaffected by α-radiation damage, and is chemically pure. A zircon sample from Jemaa, Nigeria has the largest unit-cell parameters because of the effect of α-radiation doses received over a long time (2384 Ma). All the samples show good correlations between Zr and Si apfu (atom per formula unit) versus unit-cell volume, V. The α-radiation doses in the samples are lower than ~3.5 × 1015 α-decay events/mg. Substitutions of other cations at the Zr and Si sites control the variations of the structural parameters. Relatively large unit-cell parameters and bond distances occur because the Zr site accommodates other cations that have larger ionic radii than the Zr atom. Geological age increases the radiation doses in zircon and it is related to V.

Structural variations across the nepheline (NaAlSiO4)–kalsilite (KAlSiO4) series

Abstract The crystal structures of 19 samples from the nepheline (NaAlSiO4; Ne)–kalsilite (KAlSiO4; Ks) series, previously prepared via ion-exchange, were examined using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements. Parent materials for the three series include a natural Monte Somma nepheline (series-1), synthetic Na nepheline (series-2), and high-Si synthetic nepheline (series-3), having excess Si mole percentages of 5.2, 1.7, and 12.5%, respectively. Three different structure-types were found to occur among the samples examined: nepheline (P63), tetrakalsilite (P63), and kalsilite (both P63 and P31c intergrowth). Trikalsilite was not observed in this study. Vacancies (☐) at the K site as well as Ca and K atoms at the Na1 site play an important role in the crystal-chemical behavior of nepheline solid solutions. Vacancies cause an elongation in the average <K-O>[9] distance in nepheline. When K atoms enter the Na1 site in nepheline, the average <(Na,K)-O>[7] distance increases linearly and is parallel to the average <(Na,K)-O>[9] distance in kalsilite and the grand mean of such distances in trikalsilite and tetrakalsilite. Before K atoms enter the Na1 site, the average <(Na,K)-O>[7] distance is constant because of the full occupancy of the Na1 site with Na atoms. Ca atoms at the Na1 site in the Monte Somma sample-1 cause a contraction in the <(Na,K)-O>[7] distance. In Na-rich nepheline samples, Na atoms in the large channels occupy a Na(K) site that is off the 63 axis and close to the usual K site. In natural nepheline samples, the K site in most cases contains K atoms and ☐, and the Na1 site is filled mainly with Na, minor Ca, and K atoms in K-rich samples. Nepheline from Monte Somma (sample-1) contains weak satellite reflections that are also present in some other kalsilite samples. Average <T-O> distances indicate a high degree of Al-Si disorder in nepheline but increasing Al-Si order in tetrakalsilite and kalsilite. Increasing the amount of K atoms beyond the ideal composition of K0.25Na0.75[AlSiO4] causes expansion in multiple structural parameters because of the larger size of K+ relative to Na+.

Oxabicyclic Guest Compounds as sII Promoters: Spectroscopic Investigation and Equilibrium Measurements.

In this study, we investigate three oxabicyclic compounds, 3,6-dioxabicyclo[3. 1.0]hexane (C4H6O2, ETHF), 7-oxabicyclo[2.2.1]heptane (C6H10O, 14ECH), and 7-oxabicyclo[4.1.0]heptane (C6H10O, 12ECH) as novel promoters for gas hydrates. According to the outcomes of powder X-ray diffraction (PXRD) and synchrotron high-resolution powder diffraction (HRPD), all CH4 hydrates formed with ETHF, 14ECH, and 12ECH were identified to be sII (Fd-3m) hydrates with corresponding lattice parameters of 17.195, 17.330, and 17.382 Å, respectively. It was also clearly demonstrated that CH4 molecules are accommodated in the sII-S cages through solid-state 13C NMR and Raman spectra. Consequently, we clarified that the three compounds observed are large guest molecules (LGMs) that occupy the sII-L cages. Moreover, the thermodynamic stability of each LGM + CH4 (and N2) hydrate system was remarkably improved compared to that of the simple CH4 (and N2) hydrate. In particular, 14ECH manifested several unique features compared to the other two promoters. First, the 14ECH + CH4 hydrate did not dissociate up to room temperature (298 K), even at a moderate pressure of approximately 60 bar. At a given pressure, 14ECH increased the dissociation temperature of the CH4 hydrate by ~18 K, indicating that its promotion capability is as strong as that of tetrahydrofuran (THF), currently considered to be the most powerful promoter. Second, more interestingly, it was revealed by further PXRD, NMR, and Raman analyses that 14ECH forms a simple sII hydrate in the absence of help gases. According to differential scanning calorimetry (DSC) outcomes, we revealed that the simple 14ECH hydrate dissociates at 270~278 K under ambient pressure. In addition to the thermodynamic stability, we also note that the 14ECH + CH4 hydrate presented a sufficiently high temperature of formation, requiring little additional cooling. Given these promising features, we expect that the 14ECH hydrate system can be adopted to realize hydrate-based technologies. We also believe that the LGMs introduced here have considerable potential to serve as alternates to conventional promoters and that they can be widely utilized in both engineering and scientific research fields.

Observation of distinct structural transformation between sI and sH gas hydrates and their kinetic properties during CO2 capture from N2 + CO2

sH hydrate has a deal of attention as an attractive medium of gas storage and separation due to its relatively milder formation condition and higher storage capacity, and thus sH hydrate has focused on the potential application in the post-combustion CO2 capture. Nonetheless, in-depth investigations on the thermodynamic stability of sH hydrate of N2 + CO2 and the possible occurrence of structural transformation during the process have not yet been addressed. Since the gas hydrate-based CO2 capture technology involves stepwise enrichments of CO2 through iterative formation-dissociation processes of CO2 hydrate, knowledge about the thermodynamic stability between sI and sH hydrates in the presence of various N2:CO2 ratios and their kinetic properties is important for designing the gas hydrate-based CO2 separation process. Here, we explored sI and sH hydrates of 3,3-dimethyl-1-butanol + N2 + CO2 focusing on their structural stability and structure-dependent kinetic properties. The experimental data including P–T phase equilibria, induction time, gas uptake, and CO2 separation factor are provided with a structural analysis via synchrotron high-resolution powder diffraction.

Crystal Chemistry of Birefringent Uvarovite Solid Solutions

The crystal chemistry of five optically anisotropic uvarovite samples from different localities (California, Finland, Russia, and Switzerland) were studied with electron-probe microanalysis (EPMA) and the Rietveld method. Monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data were used, and Rietveld refinement was carried out with the cubic space group, I a 3 ¯ d . The general formula for garnet is [8]X3[6]Y2[4]Z3[4]O12. Uvarovite has the ideal formula, Ca3Cr2Si3O12, which may be written as Ca3{Cr,Al,Fe}Σ2[Si3O12] because of solid solutions. HRPXRD traces show multiple cubic garnet phases in each sample that has a heterogeneous chemical composition. The optical and back-scattered electron (BSE) images and elemental maps contain lamellar and concentric zoning as well as patchy intergrowths. With increasing a unit-cell parameter for uvarovite solid solutions, the Z–O distance remains constant, and the average <X–O> distance increases slightly in response to the Cr3+ ⇔ Al3+ cation substitution in the Y site. The Y–O distance increases most because Cr3+ (radius = 0.615 Å) is larger than Al3+ (radius = 0.545 Å) cations. The Fe3+ (radius = 0.645 Å) cation is also involved in this substitution. Structural mismatch between the cubic garnet phases in the samples gives rise to strain-induced optical anisotropy.

Crystal structure of LaSr3Fe3O9 and its phase relation with LaSr3Fe3O10

The Ruddlesden-Popper phase LaSr3Fe3O10 and its reduced derivatives with nominal composition LaSr3Fe3O10–x, with 0 < x < 1.21, are described in detail with respect to crystal structure, magnetic order, thermal stability and likely vacancy mechanisms. The crystal structure of the LaSr3Fe3O9 (x = 1) phase is described. The LaSr3Fe3O10-δ phase prevails for x < 0.51, whereas LaSr3Fe3O9±ε prevails for x > 0.85. The phases are separated by a two-phase region 0.51 < x < 0.85. By combined Rietveld refinements of high-resolution powder synchrotron and neutron diffraction, the crystallographic and magnetic structure of LaSr3Fe3O9 is described. LaSr3Fe3O9 takes a layer like RP3 type structure with features of the brownmillerite structure in the triple perovskite type slabs; unit cell dimensions a = 28.7562(13) Å, b = 5.5280(2) Å, c = 5.4583(2) Å, space group Cmcm. LaSr3Fe3O9 is an antiferromagnet with TN > 350 K and with a G-type magnetic structure, magnetic space group PCbcm. Based on unit cell volume considerations, local oxygen vacancy ordering schemes are suggested.

Crystal Chemistry of Three Volcanic K-rich Nepheline Samples From Oldoinyo Lengai, Tanzania and Mount Nyiragongo, Eastern Congo, Africa

Natural nepheline (Nph) samples are intermediate members of solid solutions that extend from the pure Na end-member Na[AlSiO4] to the pure K end-member K[AlSiO4] (= kalsilite, Kls). We examined two volcanic nepheline (ideally, K2Na6[Al8Si8O32] or Nph0.75Kls0.25) samples from Oldoinyo Lengai, Tanzania (samples 1 and 2) and a third sample 3 from Mount Nyiragongo, Eastern Congo. The crystal structure of these three samples was obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements in space group P63. Satellite reflections were not observed in these samples. The EPMA composition of sample 1 is {K1.50Na0.19Ca0.070.23}Σ2(Na)6[Si8.35Al7.48Fe0.18]Σ16.00O32. The A site contains 0.23 vacancies () with some Na and Ca atoms, whereas the B site is completely filled with only Na atoms. From the EPMA formula, K(sof) = 0.844 and Na(sof) = 1.0. The refinement unit-cell parameters are a = 10.00247(1), c = 8.38396(1) A, V = 726.431(1) A3; K(sof) = 0.779(2) and Na(sof) = 0.988(2). The composition of sample 2 is {K1.670.33}Σ2(Na5.76K0.174Ca0.06)Σ6[Si8.44Al7.37Fe0.20]Σ16.01O32, so K(sof) = 0.835 and Na(sof) = 1.030. Sample 2 has a = 10.01769(2), c = 8.39586(2) A, V = 729.678(1) A3, K(sof) = 0.812(2), and Na(sof) = 0.995(2). Sample 3 is a three-phase (3a, 3b, 3c) intergrowth with an average formula of {K1.770.23}2(Na4.30K1.51Ca0.19)Σ6[Si8.13Al7.76Fe0.11]Σ16.00O32, K(sof) = 0.887, and Na(sof) = 1.208. A large amount of K and Ca atoms occur in the B site. Based on the weight percentages and sofs of the three phases, the Rietveld refinement gives an average K(sof) = 0.859 and Na(sof) = 1.145. The average distances indicate a high degree of Al-Si disorder in samples 1, 2, and phase 3b. These African samples extend natural nepheline solid solutions to about Nph0.55Kls0.45. The average distance increases linearly with Na(sof) and V across nepheline solid solutions because K atoms enter the Na site. K-rich nepheline seems to always contain vacancies in the A site.

High-Temperature Structural Evolution in the Ba3Mo(1- x)W xNbO8.5 System and Correlation with Ionic Transport Properties.

The evolution of the hybrid structure between 9R hexagonal perovskite and palmierite in the entire Ba3Mo(1- x)W xNbO8.5 solid solution (where x = 0, 0.25, 0.5, 0.75, and 1) was probed in the 100-900 K range by synchrotron high-resolution powder diffraction. Each sample exhibits a chemical-dependent structural model in the low-temperature regime (from 100 to 500 K) in which 9R and palmierite structures compete each other, the former being progressively favored as tungsten replaces molybdenum. Above 500 K, unit cell parameters and metal site occupancies start to converge toward a similar structural arrangement that is completely reached at 900 K. In fact, at this temperature, the entire solid solution discloses comparable unit cell and an almost enterely occupied M1 site, with a structure that is much closer to palmierite rather than 9R polytype. The present crystallographic results well explain the behavior of the material's bulk ionic conductivity, whose temperature evolution for different compositions depends from the contribution of tetrahedral units proper of the palmierite structure.