Rb Sites Research Articles

Electric field gradients at rubidium sites in rubidium uranyl nitrate and quadrupole moments of 85Rb and 87Rb

The electronic structure along the axial direction of the uranyl ion in rubidium uranyl nitrate (RbUO2(NO3)3) has been probed by nuclear quadrupole resonance and nuclear magnetic resonance measurements of 85Rb and 87Rb quadrupole couplings. The parameter of interest in these experiments is the electric field gradient at the Rb sites, which may be compared with values computed by various theoretical approaches. From this analysis a refinement of the ratio of the quadrupole moments of 85Rb and 87Rb has also been obtained.

Uranyl silicate nanotubules in Rb2[(UO2)2O(Si3O8)]: synthesis and crystal structure

Abstract A new rubidium uranyl silicate, Rb2(UO2)2O(Si3O8) (1), was obtained using high-temperature approach from the melt in silica tubes. Its crystal structure was solved by direct methods: hexagonal, P6/m, a = 27.7992(7), c = 7.2346(2) Å, V = 4841.8(3) Å3, R 1 = 0.033. The structure of 1 represents a new structure type with unprecedented topology not observed before among U(VI) oxides and oxysalts. It is comprised of layers with large voids derived from the U3O8 structure formed exclusively by pentagonal UrO5 bipyramids. The low-occupied Rb sites are located in the interlayer space. The SiO4 silicate tetrahedra in the structure of 1 share vertices to form rolled [Si6O16]8− chains. The nanotubules [(UO2)(Si6O16)]6− penetrate through both U3O8-derived layers and Rb interlayer. These tubules are attached to the U3O8 derived sheets via uranyl-uranyl interactions and edge-sharing between silicate tetrahedra and UrO5 bipyramids.

Data-Driven Investigation of Tellurium-Containing Semiconductors for CO2 Reduction: Trends in Adsorption and Scaling Relations.

Light-assisted conversion of CO2 into liquid fuels is one of several possible approaches to combating the rise of carbon dioxide emissions. Unfortunately, there are currently no known materials that are efficient, selective, or active enough to facilitate the photocatalytic CO2 reduction reaction (CO2RR) at an industrial scale. In this work, we employ density functional theory to explore potential tellurium-containing photocathodes for the CO2RR by observing trends in adsorption properties arising from over 350 *H, 200 *CO, and 110 *CHO surface–adsorbate structures spanning 39 surfaces of 11 materials. Our results reveal a scaling relationship between *CHO and *H chemisorption energies and charge transfer values, while the scaling relation (typically found in transition metals) between *CO and *CHO adsorption energies is absent. We hypothesize the scaling relation between *H and *CHO to be related to the lone electron located on the bonding carbon atom, while the lack of scaling relation in *CO we attribute to the ability of the lone pair on the C atom to form multiple bonding modes. We compute two predominant orbital-level interactions in the *CO-surface bonds (either using s or p orbitals) in addition to bonding modes involving both σ and π interactions using the Crystal Orbital Hamiltonian Population analysis. We demonstrate that bonds involving the C s orbital are more chemisorptive than the C p orbitals of CO. In general, chemisorption trends demonstrate decreased *H competition with respect to *CO adsorption and enhanced *CHO stability. Finally, we uncover simple element-specific design rules with Te, Se, and Ga sites showing increased competition and Zn, Yb, Rb, Br, and Cl sites showing decreased competition for hydrogen adsorption. We anticipate that these trends will help further screen these materials for potential CO2RR performance.

High-performance narrow-band green-emitting phosphor enabled by substituting Na with Rb

Exploring green phosphors with satisfactory peak position (525–535 nm) and full width at half-maximum (FWHM) to replace the commercial narrow-band green β-SiAlON:Eu2+ phosphor (λem = 540 nm, FWHM = 54 nm) is crucial for realizing wide-gamut full-color liquid crystal display (LCD) devices. Here, taking the UCr4C4-type tetragonal phase RbNa3(Li3SiO4)4:3%Eu2+ (RN3(LSO)4:Eu2+) blue phosphor as starting model, we substitute Na+ with Rb+ to synthesize a series of Rb1+xNa2.97-x(Li3SiO4)4:3%Eu2+ (0.1 ≤ x ≤ 2.5) (R1+xN2.97-x(LSO)4:Eu2+) phosphors that exhibit tunable emission from blue to green. We demonstrate that Rb+-substitution triggers tetragonal-to-monoclinic phase transition, making Eu2+ ions enter two new, cuboid-like Rb and Na sites and resultant green emission. With optimized content of Rb+, Rb3Na(Li3SiO4)4:3%Eu2+ (R3N(LSO)4:Eu2+) green phosphor not only shows appropriate peak position (∼526 nm) and FWHM (∼43 nm) compared to β-SiAlON:Eu2+ phosphor, but also high internal quantum efficiency (IQE) of 90.2% and superior thermal stability (integrated emission intensity at 150 °C is 105.7% of room-temperature performance). Using the green R3N(LSO)4:Eu2+ phosphor, white light-emitting diode prototype shows a wide-color-gamut (111.2% National Television System Committee, NTSC). This work demonstrates that as-prepared green R3N(LSO)4:Eu2+ phosphor has a potential application in LCD backlight display, and may stimulate more researches on exploring high-performance narrow-band UCr4C4-type phosphors.

On the origin of the highest ozone episodes in Spain

The 2000–2015 occurrences of the highest ozone (O3) pollution episodes in Spain were evaluated to investigate their origin. To this end, data series available for urban and regional background (UB and RB), traffic (TR) and industrial (IN) sites were analysed separately and intercompared. Results evidenced that during these 16years mean O3 levels in the RB sites did not change significantly, and remained constantly high. However, there is a clear increase at the TR and UB sites. Although sensitivity analysis is needed to interpret the cause of this increasing trend, this might be caused probably by the lower O3 titration intensity due to the preferential abatement of NO vs NO2, as supported from the neutral trend of OX (NO2+O3) at these sites. We found that the exceedances of the hourly information threshold for O3 (>180μg/m3) are recorded mostly at UB and IN sites located in seven areas of Spain (specific hotspots or at the tail end of large urban plumes), and that these increased during summer heatwaves (i.e. 2003 and 2015).Although the external contribution of regional-to-subcontinental transported O3 might be relevant during the highest O3 episodes in the Western Mediterranean, our results evidenced that in the above specific areas, regional-local O3 production decisively contributes to the exceedances of the information threshold. Also that the human protection threshold and the AOT40 are more frequently exceeded in the Central, Southern and Mediterranean sides of the Iberian Peninsula.The design of effective episode abatement measures is quite complex in those conditions, due to both the nonlinearity of the chemical processes of O3 formation and destruction, and to the interplay with the complex meteorological setting, causing frequent recirculation and in situ aging of air masses. However, the combination of meteorological forecasting of the main recirculation processes and sensitivity analysis of NOX/VOC emission abatement measures might be powerful tools to evaluate the effectiveness of potential O3 mitigation strategies.Finally we would like to highlight that the current UB, RB, IN and TR classification (somewhat subjective) is not adequate to interpret the origin of O3 exceedances in complex areas of Southern Europe. Thus, a UB station recording exceedances, and located in a small city in the tail end of an urban plume of a large city, receives not only the contribution from its own UB, but mainly from the specific high O3 RB caused by the urban plume transport.

High-resolution air quality simulation over Europe with the chemistry transport model CHIMERE

Abstract. A modified version of CHIMERE 2009, including new methodologies in emissions modelling and an urban correction, is used to perform a simulation at high resolution (0.125° × 0.0625°) over Europe for the year 2009. The model reproduces the temporal variability of NO2, O3, PM10, PM2.5 better at rural (RB) than urban (UB) background stations, with yearly correlation values for the different pollutants ranging between 0.62 and 0.77 at RB sites and between 0.52 and 0.73 at UB sites. Also, the fractional biases (FBs) show that the model performs slightly better at RB sites than at UB sites for NO2 (RB = −33.9%, UB = −53.6%), O3 (RB = 20.1%, UB = 25.2%) and PM10 (RB = −5.50%, UB = −20.1%). The difficulties for the model in reproducing NO2 concentrations can be attributed to the general underestimation of NOx emissions as well as to the adopted horizontal resolution, which represents only partially the spatial gradient of the emissions over medium-size and small cities. The overestimation of O3 by the model is related to the NO2 underestimation and the overestimated O3 concentrations of the lateral boundary conditions. At UB sites, CHIMERE reproduces PM2.5 better than PM10. This is primarily the result of an underestimation of coarse particulate matter (PM) associated with uncertainties in secondary organic aerosol (SOA) chemistry and its precursor emissions (Po valley and Mediterranean basin), dust (south of Spain) and sea salt (western Europe). The results suggest that future work should focus on the development of national bottom-up emission inventories including a better account for semi-volatile organic compounds and their conversion to SOA, the improvement of the CHIMERE urban parameterization, the introduction into CHIMERE of the coarse nitrate chemistry and an advanced parameterization accounting for windblown dust emissions.

A 3D experiment that provides isotropic homonuclear correlations of half-integer quadrupolar nuclei

Two 3D experiments, capable of producing enhanced resolution two-spin double-quantum (DQ) homonuclear correlations for half-integer quadrupolar nuclei, are described. The first uses a split-t1 MQMAS sequence followed by a sandwiched oR3 symmetry-based dipolar recoupling sequence to directly excite DQ coherences. In this case an isotropic single-quantum (SQ) coherence starts the homonuclear DQ excitation. In the second experiment a single strong pulse is used to create triple quantum (TQ) coherence followed by a further single pulse conversion to zero-order before a non-sandwiched oR3 DQ sequence. The first experiment is demonstrated using 87Rb in RbNO3, with three Rb sites in a ∼5ppm range, and the second to 11B in caesium triborate, CsB3O5, with two three-coordinated sites separated by ∼2ppm and one four-coordinated boron site. In both cases, all sites are clearly resolved and their connections observed. The second experiment has higher sensitivity and a good signal to noise is obtained in a reasonable time despite the long T1 relaxation time of 11B in this material.

A Structural Study of Ion Permeation in OmpF Porin from Anomalous X-ray Diffraction and Molecular Dynamics Simulations

OmpF, a multiionic porin from Escherichia coli, is a useful protypical model system for addressing general questions about electrostatic interactions in the confinement of an aqueous molecular pore. Here, favorable anion locations in the OmpF pore were mapped by anomalous X-ray scattering of Br(–) ions from four different crystal structures and compared with Mg(2+) sites and Rb(+) sites from a previous anomalous diffraction study to provide a complete picture of cation and anion transfer paths along the OmpF channel. By comparing structures with various crystallization conditions, we find that anions bind in discrete clusters along the entire length of the OmpF pore, whereas cations find conserved binding sites with the extracellular, surface-exposed loops. Results from molecular dynamics simulations are consistent with the experimental data and help highlight the critical residues that preferentially contact either cations or anions during permeation. Analysis of these results provides new insights into the molecular mechanisms that determine ion selectivity in OmpF porin.

Structural properties of mixed (NH4)2−xRbxZnCl4 (x=0, 1, and 2) crystals studied by 1H and 87Rb nuclear magnetic resonance

The structures of mixed (NH4)2−xRbxZnCl4 (x=0, 1, and 2) single crystals were determined by x-ray diffraction. In addition, the nuclear magnetic resonance (NMR) spectra and spin–lattice relaxation times of the 1H and 87Rb nuclei in the three crystals were determined using NMR spectroscopy. We analyzed the crystallographic structures of these crystals by considering the occupation probabilities (NH4 or Rb) of the two cationic sites. Our NMR results confirm that there are two crystallographically inequivalent NH4 sites, NH4(1) and NH4(2), in (NH4)2ZnCl4, and two crystallographically inequivalent Rb sites, Rb(1) and Rb(2), in Rb2ZnCl4. However, only one NH4 site and two Rb sites were observed in NH4RbZnCl4. The NH4 and Rb occupation rates of each of the two available sites, A1 and A2, were determined from the NMR signals.

CsHgInS3: a New Quaternary Semiconductor for γ-ray Detection

The new layered compound CsHgInS3 was synthesized using solid state and flux synthesis techniques. The compound is a semiconductor and shows promising properties for X-ray and γ-ray detection. It features a layered structure that crystallizes in the monoclinic space group C2/c with cell parameters: a = 11.2499(7) Ǻ, b = 11.2565(6) Ǻ, c = 22.146(1) Ǻ, β = 97.30(5)°, V = 2781.8(4) Ǻ3, and Z = 8. CsHgInS3 is isostructural to Rb2Cu2Sn2S6, where the Hg, In, and Cs atoms occupy the Cu, Sn, and Rb sites, respectively. Large single crystals with dimension up to 5 mm were grown with a vertical Bridgman method as well as a horizontal traveling heater method. CsHgInS3 has a γ-ray attenuation length comparable to commercial Cd1–xZnxTe and a band gap value of 2.30 eV. The electrical resistivity of CsHgInS3 is anisotropic with values of 98 GΩ cm and 0.33 GΩ cm perpendicular and parallel to the (001) plane, respectively. The mobility-lifetime product (μτ) of electrons and holes estimated from photoconductivity measureme...

ChemInform Abstract: Variable-Temperature 87Rb Magic-Angle Spinning NMR Spectroscopy of Inorganic Rubidium Salts

Temperature dependencies of the 87Rb quadrupole coupling constants (CQ), asymmetry parameters (ηQ), and isotropic chemical shifts (δiso) have been observed and determined for RbCl, RbClO4, Rb2SO4, and RbNO3 from variable-temperature (VT) 87Rb magic-angle spinning (MAS) NMR spectra for the temperature range from about −100 to +165 °C. VT 87Rb multiple-quantum MAS NMR spectra of the trigonal form for RbNO3 have assisted the retrival of CQ, ηQ, and δiso for the three Rb sites of this phase. The results demonstrate that CQ(87Rb) and δiso(87Rb) are highly temperature-dependent for all samples over the studied temperature range. Linear correlations of δiso with temperature, corresponding to temperature coefficients in the range −0.014 to −0.036 ppm/°C (i.e., increasing shielding with increasing temperature), are observed for all samples. An exception is the Rb(2) site in Rb2SO4 for which a small linear decrease in shielding is observed with increasing temperature. For RbClO4 and Rb2SO4 the three CQ(87Rb) values...

R 87 b and C133s nuclear magnetic resonance study of the structural properties of mixed RbxCs2−xZnCl4 (x=, 1, and 2) crystals

The structures of mixed RbxCs2−xZnCl4 (x=0, 1, and 2) single crystals grown by using the slow evaporation method were determined with x-ray diffraction. In addition, the NMR spectra and the spin-lattice relaxation times, T1, of the R87b and C133s nuclei of the three crystals were determined by using nuclear magnetic resonance (NMR) spectroscopy. The spin-lattice relaxation times, T1, of the Cs-based compounds are very much longer than those of the Rb-based compounds. The differences between the T1 values of the Rb and Cs nuclei are due to the difference between their electric quadrupole moments. Further, we analyzed the crystallographic structures of these crystals by considering the occupation probabilities (Rb or Cs) of the two cationic sites. Our NMR experimental results confirm that there are two crystallographically inequivalent Rb sites, Rb(1) and Rb(2), in Rb2ZnCl4 and two crystallographically inequivalent Cs sites, Cs(1) and Cs(2), in Cs2ZnCl4. And, only one Rb site and only one Cs site in RbCsZnCl4 was obtained. The Cs and Rb occupation rates of each of the two available sites, A1 and A2, were determined from the NMR signals.

Study on the paramagnetic relaxation mechanisms by R87b nuclear magnetic resonance in Rb2MnCl4 crystals of A2BX4 type

The spin-lattice relaxation time, T1, for R87b nuclei in Rb2MnCl4 crystals was investigated, and a phase transition was found near 383 K (=TC). The R87b spectrum changes near TC from two central resonance lines due to Rb(1) and Rb(2) to one Rb central resonance line, and this change is due to changes in the local symmetry of the Rb sites near TC. The central resonance lines for Rb nuclei in Rb2MnCl4 crystals are shifted to higher frequencies by the paramagnetic ions. Further, the relaxation times for the Rb(1) and Rb(2) nuclei in Rb2MnCl4 crystals are more or less continuous near TC. Below TC, the spin-lattice relaxation rates, T1−1, for Rb(1) and Rb(2) are proportional to T−2, which implies that these relaxations occur mainly via Raman processes. We also compare our results for Rb2MnCl4 with those reported previously for Rb2BCl4 (B=Co and Zn) and CsBCl3 (B=Mn, Cu, Zn, and Cd) crystals.

The diphyllosilicate Rb2(VO)2[Si8O19

Dirubidium divanadyl phyllooctasilicate, Rb(2)(VO)(2)[Si(8)O(19)], is the first known anhydrous diphyllosilicate containing V(IV). The structure consists of silicate double layers which are separated by [V(2)O(8)](8-) dimers and is related to that of the compounds A(2)Cu(2)[Si(8)O(19)] (A = Rb or Cs), although the title compound crystallizes in a noncentrosymmetric orthorhombic space group. The silicate double layers contain four tetrahedrally coordinated Si sites in general positions and 12 O sites, nine in general positions and the other three on mirror planes. The vanadyl dimers have two square-pyramidally coordinated V sites (site symmetry m). There are two different 10- and 12-fold coordinated Rb sites with site symmetry m, one of which is a split position located between the dimers in the interlayer space, while the other is in a channel within the silicate layer.

Syntheses and structural properties of four Rb-aluminosilicate zeolites

Four types of Rb-aluminosilicate zeolites were hydrothermally synthesized in pure phase for the first time from Rb-aluminosilicate gels without using any organic structure-directing agent (SDA) under stirring conditions. The crystal structure of each zeolite was refined by Rietveld analysis of the X-ray diffraction (XRD) data. These crystal structures were confirmed to be Rb-mordenite, Rb-merlinoite, a new aluminosilicate zeolite with an ATT framework topology, and Rb-offretite denoted by RMA-1, RMA-2, RMA-3, and RMA-4, respectively. The Si/Al ratio of RMA-1 with an MOR topology varied from 5.3 to 8.0; however, the variation of the Si/Al ratios of the other zeolites was rather small. The crystal system of RMA-2 was tetragonal with space group I4/mmm, where two Rb sites were distributed at the center of an 8-membered ring (MR). On the other hand, two Rb sites in RMA-3 were located at the center of the 8-MR in small two cages. The structure of RMA-4 was identified as the OFF type with a local disorder or defect, which included a small amount of an intergrown ERI phase.

Solid-State 87Rb NMR Study in Powdered RbMnCl3

Structural phase transition at 290 K and the implication on the intermediate phase above 290 K in powdered RbMnCl3 are observed by using a solid-state 87Rb NMR spectroscopy. Quadrupole coupling constants (e2qQ/h), the asymmetry parameters (η), and the relative peak intensities for two physically nonequivalent Rb sites, Rb(I) and Rb(II), are determined from nonlinear least-squares fits to the 87Rb NMR powder patterns in the temperature range from 260 to 330 K. Quadrupole coupling constants and the asymmetry parameters are examined for the detection of the phase transition resulting in a significant structural change in the Rb(II) site. In addition, changes in the relative peak intensity between the Rb(I) and Rb(II) sites seem to suggest the existence of an anomalous intermediate phase, which is complemented by the differential scanning calorimetry and X-ray diffraction studies.

Electronic structure ofK3Ba3C60andRb3Ba3C60superconductors

We have studied the electronic structure of the superconducting fullerides ${\mathrm{K}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}$ using the local-density approximation in the density functional theory. Their conduction-band profiles are found to be quantitatively very similar to each other although the lattice constant of ${\mathrm{K}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}$ is definitely smaller than that of ${\mathrm{Rb}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}.$ The density of states at Fermi level of ${\mathrm{K}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}$ is slightly larger than that of ${\mathrm{Rb}}_{3}{\mathrm{Ba}}_{3}{\mathrm{C}}_{60}.$ In addition, ${\mathrm{C}}_{60}$ states are found to be hybridized not only with Ba states but also with K (Rb) states. These results are in sharp contrast to ${A}_{3}{\mathrm{C}}_{60}$ superconductors $(A=\mathrm{K}$ and Rb). The hybridization is expected to play an important role in their superconducting properties since carriers are found to be not only on ${\mathrm{C}}_{60}$ but also around K (Rb) sites as well as Ba sites.

Variable-Temperature 87Rb Magic-Angle Spinning NMR Spectroscopy of Inorganic Rubidium Salts

Temperature dependencies of the 87Rb quadrupole coupling constants (CQ), asymmetry parameters (ηQ), and isotropic chemical shifts (δiso) have been observed and determined for RbCl, RbClO4, Rb2SO4, and RbNO3 from variable-temperature (VT) 87Rb magic-angle spinning (MAS) NMR spectra for the temperature range from about −100 to +165 °C. VT 87Rb multiple-quantum MAS NMR spectra of the trigonal form for RbNO3 have assisted the retrival of CQ, ηQ, and δiso for the three Rb sites of this phase. The results demonstrate that CQ(87Rb) and δiso(87Rb) are highly temperature-dependent for all samples over the studied temperature range. Linear correlations of δiso with temperature, corresponding to temperature coefficients in the range −0.014 to −0.036 ppm/°C (i.e., increasing shielding with increasing temperature), are observed for all samples. An exception is the Rb(2) site in Rb2SO4 for which a small linear decrease in shielding is observed with increasing temperature. For RbClO4 and Rb2SO4 the three CQ(87Rb) values...

87RbNMR spectra of hydrogen-bondedRb3D(SO4)2andRb3H(SO4)2crystals

The paraelectric and antiferroelectric phases of ${\mathrm{Rb}}_{3}{\mathrm{D}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ and ${\mathrm{Rb}}_{3}{\mathrm{H}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ were investigated using rubidium NMR. The angle dependence of the second order quadrupole shifted central ${}^{87}\mathrm{Rb}$ transitions was recorded at 300 K. It was found to be in full agreement with the symmetry elements of a monoclinic $A2/a$ structure. The temperature dependence of the magnitude and asymmetry of the electrical field gradient (EFG) tensors at the Rb sites was determined using powder samples and revealed no differences between protonated and deuterated specimens. The antiferroelectric transition of ${\mathrm{Rb}}_{3}{\mathrm{D}(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ ${(T}_{N}=82\mathrm{K})$ is associated with a quadrupling of the unit cell. Below ${T}_{N}$ the magnitudes of the EFG at the Rb sites remain constant and smoothly tilt away from their high temperature orientations. The temperature dependence of the NMR line splittings associated with this tilt follows a power law implying an order parameter critical exponent of 0.21.

Anomalous Metallic State Coexisting with the Charge Density Wave in Rb3Cu8S6Studied by NMR and Spin Echo Double Resonance (SEDOR)

The metallic state below the charge density wave (CDW) transition temperature of Rb 3 Cu 8 S 6 has been studied by NMR and spin echo double resonance (SEDOR). This compound is known to show metallic characteristics below T c2 =55 K, although the CDW already sets in at T c1 =120 K. It has been found that some of the Cu-NMR lines start to spread out below T c1 , and remain broadened even below T c2 . In order to gain further insight into the state below T c2 , we applied SEDOR to the broadened Cu-lines below T c2 . A double-horned shape characteristic of CDW states was found in one of the SEDOR spectra, which is considered to be a manifestation of the presence of the CDW even below T c2 . The SEDOR reduction is attributed to a kind of motion in the CDW chains. On the other hand, the spin-lattice relaxation rate at the Rb sites was found to decrease rapidly below T c2 , implying that the Rb sites are not inert, but play important roles in this metallic state. As a possible mechanism for this anomalous metallic state, the spinless (charged)-soliton conduction on the CDW chains is discussed.