Split Atom Research Articles

An unusual member of the solid solution series between cristobalite and potassium ferrate(III) obtained from hydroflux

Abstract Dark octahedral crystals of K1−x [Fe1−x Si x O2] with x ≈ 0.2 were synthesized under ultra-alkaline conditions in a KOH hydroflux at 200 °C. The compound is a member of the solid solution series between SiO2 and K[FeO2]. Due to its SiO2 content, K0.8[Fe0.8Si0.2O2] is much less sensitive to moisture than K[FeO2]. The crystal structure is a stuffed cristobalite with a charged framework of vertex-sharing [MO4/2] tetrahedra (M = Fe3+, Si4+) and potassium counter ions in the large voids of the framework. It has the pseudo-symmetry and metrics of the cubic space group F d 3 ‾ m $Fd\overline{3}m$ , but adopts the tetragonal space group I41 md with four formula units in the cell. Unlike other tectosilicates of this type, the [MO4/2]− tetrahedra are not regular but distorted to disphenoids. The O atom can be modeled by a huge disk-shaped ellipsoid or, better, by split atom positions forming a six-membered ring with a diameter of 1.1 Å. The M–O distances range from 1.66(2) to 1.96(2) Å, the M–O–M angles are 135(2)°, 137(1)°, and 151(1)°.

Optimization of boron dispersion on fibrous-silica-nickel catalyst for enhanced CO2 hydrogenation to methane

There are numerous reports regarding boron-containing catalysts for hydrogen-related reactions from CO2 including dry reforming of methane and methanation. Besides enhancing the productivity, boron also improved nickel activity and stability. However, the detailed mechanistic study, particularly in explaining the starring role of boron in the enhanced reactions, is still lacking. Thus, herein we loaded boron on fibrous-silica-nickel and investigated their physicochemical properties and mechanistic route by means of in-situ FTIR for enhanced CO2 methanation. It was found that the appropriate dispersion of boron surrounds the nickel particles is an important factor to improve the adsorption of CO2 before interacting with split hydrogen atom from the nickel sides to form intermediates which are subsequently dehydrated, and then serial hydrogenation gave the final product of methane. Boron also accelerated the methanation and restricted coke formation. A hybrid approach on optimization via a face-centered central composite design and a response surface methodology showed that reaction using H2/CO2 ratio of 6, GHSV of 10,500 mL g−1 h−1, at 500 °C gave the highest percentage of CH4 of 84.3%. To indicate the error, the predicted values were compared to the experimental values, yielding an accurately minimal error ranging from 0 to 11%. As a result, the empirical models generated for CO2 hydrogenation to methane were reasonably accurate, with all actual values for the confirmation runs fitting within the 94% prediction interval.

Tracing the Jahn‐Teller Distortion of C60n− in Isostructural Fullerides [M(NH3)7]C60 ⋅ NH3, M=Ba, Sr, K

AbstractSingle crystals of new [Sr(NH3)7]C60 ⋅ NH3 (compound 1) and [K(NH3)7]C60 ⋅ NH3 (compound 2) were prepared by extracting mixtures of potassium and C60 with liquid ammonia, where for the synthesis of 1 solid strontium chloride was added to the extraction mixture. The procedure applied relies on self‐contained continuous evaporation and condensation of ammonia at subcritical conditions in a special, sealed glass vessel, at room temperature (RT). According to single crystal structure analyses on data collected at 123 K, both, 1 and 2, are isostructural with [Ba(NH3)7]C60 ⋅ NH3 (compound 3). The structure determinations suffer either from systematic twinning (3), or positional disorder for 1 and 2. Crystals of 3, grown and handled throughout at low temperature (123 K), developed large enough twin domains such that an inversion twin type refinement in space group R3m converged properly. For 1 and 2, the crystals of which were grown at RT and subsequently cooled for data collection, refinements of the same structure model were only possible in R‐3m while applying split atom positions. For 1 and 3 the accuracy achieved was sufficient to extract a particular bonding distortion pattern for (C60)2−, which is lifting the degeneracy of the HOMO according to the Jahn‐Teller theorem. Independent quantum chemical calculations are lending convincing support to the experimental findings.

Incommensurate phase of the stuffed tridymite derivative BaSrFe4O8

We demonstrate a new modulation for a high-temperature polymorph of BaSrFe4O8 via synchrotron powder X-ray diffraction and transmission electron microscopy (TEM) experiments. Although the powder X-ray diffraction pattern was successfully analyzed using a structure model of space group Pnna, an unusually large isotropic atomic displacement parameter (Uiso) was observed for the O1 atom. A split atom model that was applied to the O1 site and the adjacent Ba/Sr site, both of which are on the two-fold axis along the c-axis, indicated that positional disorder occurred in these atoms along the c-axis. The electron diffraction pattern and high-resolution TEM observations combined with the structure refinements revealed an incommensurate structure with a modulation vector q=0.4270(1)a* and a displacement vector perpendicular to the a-axis, which gave rise to the large Uiso value of the O1 atom. The modulation observed in this study may be a signature of structural instability, which has commonly been discussed in the stuffed tridymite-type oxides.

Generalized Bloch states of a two-level atom in the field of counterpropagating waves: application to the Kapitza–Dirac diffraction problem

The Bloch-state approach to atomic translational states is extended out of adiabatic following and Raman–Nath approximations, taking into account the possible transfer of population from the initially populated internal energy level. The presented theory suggests a notion of potential energy corresponding to the initially populated internal state, and makes a generalization in the treatment of energy spectrum band-gap structure. Applying the theory to off-resonant Kapitza–Dirac diffraction and two-pulse standing wave atom interferometer (for the study of non-coherent processes in the cold atomic gases) we discuss the ability of the present approach for large-angle split multipath atom interferometry.

Rochelle salt - a structural reinvestigation with improved tools. I. The high-temperature para-electric phase at 308 K.

The crystal structure of the high-temperature paraelectric phase of Rochelle salt (K(+)·Na(+)·C4H4O6 (2-)·4H2O) at 308 K has been reinvestigated using synchrotron X-ray diffraction with refinement parameters R(int) = 0.0123, final (shift/e.s.d.)max = 0.019, R 1(all) = 0.0371 and wR 2(all) = 0.0608. The application of a new gas-flow sample cell designed to control both temperature and relative humidity permitted collection of data of excellent quality and enabled unrestrained refinement of all parameters, including those of the isotropic hydrogen atoms. A precise description of the structure has ensued. One K atom is disordered between two symmetry-equivalent sites; three O atoms in three of the four water molecules exhibit very strong anisotropy. Refining one O atom as a split atom was successful, yielding small improvements in the bonding parameters of several H atoms. The H atoms of all water molecules behave as single pairs. Their final U values are of moderate magnitude indicating that these atoms do not participate in the anisotropy of the parent O atoms. It is suggested that the three water O atoms are in part statically disordered, while the bonded H atoms are not. Except for the split K atom and the three water O atoms there is no evidence of general disorder in the structure.

Electron density, d-p hybridization and dipole moment of PbTiO3 and BaTiO3

Present experiment first reveals electron density (ED) distributions and split atomic displacements of ferroelectrics PbTiO3 (PTO) and BaTiO3 (BTO) by high-resolution x-ray single crystal diffraction study under high pressures using maximum entropy method (MEM). Diffraction measurements were carried out using a four-circle diffractometer installed at BL-10A Photon Factory, KEK, Japan. *** ED observed by FMEM(h) illustrates the d-p pai-bond resulting from the hybridization of Ti 3d and O 2p orbitals. The split atom distribution is generated from the dynamical disorder (static time and space average) due to the thermal vibration of atoms in the [001] direction of these crystal structures. The split distributions of Ti and O are clearly disclosed in the tetragonal ferroelectric PTO and BTO. The ferroelectric pseudo-particle occupies double-well potential. The feature disappears in the cubic paraelectric phase and electron density is more localized with increasing pressure in the cubic phase. *** Effective charges of atoms in PTO and BTO are evaluated from the least-squares refinement based on diffraction intensities. Using the pseudopotential model, inner-core electrons are frozen with bonding effects, but only valence electron clouds are deformed due to coordination and thermal vibration of atoms. The pressure dependence of the dipole moment (l) is calculated by the effective charge (q) and atomic deformation Δr. The dipole moment of ferroelectric materials is derived from the spontaneous polarization Ps (= ρ l = ρNΔr). Pressure dependences of dipole moments of PTO and BTO are first experimentally determined by observed effective charge q and Δr from their structure refinements. The dipole moment of PTO is much larger than BTO, which implies the difference in the dielectricity of these ferroelectrics. The result is consistent with the dielectric measurement.

Engineering fast and stable splitting of matter waves

When attempting to split coherent cold atom clouds or a Bose-Einstein condensate (BEC) by bifurcation of the trap into a double well, slow adiabatic following is unstable with respect to any slight asymmetry, and the wave "collapses" to the lower well, whereas a generic fast chopping splits the wave but it also excites it. Shortcuts to adiabaticity engineered to speed up the adiabatic process through non-adiabatic transients, provide instead quiet and robust fast splitting. The non-linearity of the BEC makes the proposed shortcut even more stable.

Crystal structure of bis-(1-(2-(diethylamino)ethyl)-3-methylimidazolin-2- ylidene) di-silver(I) bis-(triflimide), Ag2[C10H19N3]2[C2F6NO4S2]2, C24H38Ag2F12N8O8S4

C24H38Ag2F12N8O8S4, triclinic,P1¯ (no. 2), a = 9.4438(4) Å, b = 9.8561(3) Å, c = 12.0065(4) Å,α = 101.606(2)°, β = 93.366(2)°, γ = 92.789(2)°,V = 1090.7 Å3, Z = 1, Rgt(F) = 0.0347, wRref(F2) = 0.0837, T = 233 K.

New ternary rare-earth metal boride carbides R15B 4C 14 ( R=Y, Gd–Lu) containing BC 2 units: Crystal and electronic structures, magnetic properties

The ternary rare-earth boride carbides R 15B 4C 14 ( R=Y, Gd–Lu) were prepared from the elements by arc-melting followed by annealing in silica tubes at 1270 K for 1 month. The crystal structures of Tb 15B 4C 14 and Er 15B 4C 14 were determined from single crystal X-ray diffraction data. They crystallize in a new structure type in space group P4/ mnc (Tb 15B 4C 14: a=8.1251(5) Å, c=15.861(1) Å, Z=2, R 1=0.041 (w R 2=0.088) for 1023 reflections with I o>2 σ( I o); Er 15B 4C 14: a=7.932(1) Å, c=15.685(2) Å, Z=2, R 1=0.037 (w R 2=0.094) for 1022 reflections with I o>2 σ( I o)). The crystal structure contains discrete carbon atoms and bent CBC units in octahedra and distorted bicapped square antiprisms, respectively. In both structures the same type of disorder exists. One R atom position needs to be refined as split atom position with a ratio 9:1 indicative of a 10% substitution of the neighboring C 4− by C 2 4−. The actual composition has then to be described as R 15B 4C 14.2. The isoelectronic substitution does not change the electron partition of R 15B 4C 14 which can be written as ( R 3+) 15(C 4−) 6(CBC 5−) 4•e −. The electronic structure was studied with the extended Hückel method. The investigated compounds Tb 15B 4C 14, Dy 15B 4C 14 and Er 15B 4C 14 are hard ferromagnets with Curie temperatures T C=145, 120 and 50 K, respectively. The coercive field B C=3.15 T for Dy 15B 4C 14 is quite remarkable.

Diffuse and doubly split atom occupation in hexagonal LiBH4

A theoretical study has been performed to explain problems in the structural analysis of LiBH4 and its recently discovered superionic conductance. First-principles molecular dynamics simulations for the high temperature (hexagonal) phase show doubly split and diffuse occupation in the c-direction at Li and B sites, respectively. Li hopping within the split sites and libration of H atoms are also found. These dynamics are supported by the Rietveld analysis showing atomic displacement ellipsoids for Li and B atoms.

On the Thermoelectric Potential of Inverse Clathrates

AbstractIn the context of a general survey on the thermoelectric potential of cationic clathrates, formation, crystal chemistry and physical properties were investigated for novel inverse clathrates deriving from Sn19.3Cu4.7P22I8. Substitution of Cu by Zn and Sn by Ni was attempted to bring down electrical resistivity and lower thermal conductivity. Materials were synthesized by mechanical alloying using a ball mill and hot pressing. Structural investigations for all specimens confirm isotypism with the cubic primitive clathrate type I structure (lattice parameters a = ˜1.1 nm and space group type Pm-3n). Studies of transport properties evidence holes as the majority charge carriers. Thermal expansion data, measured in a capacitance dilatometer from 4 to 300 K on Sn19.3Cu1.7Zn3P19.92.1I8, compare well with literature data available for Sn24P19.62.4Br8and for an anionic type I clathrate Ba8Zn8Ge38. From the rather complex crystal structure including split atom sites and lattice defects thermal conductivity in inverse clathrates is generally low. Following Zintl rules rather closely inverse clathrates tend to be semiconductors with attractive Seebeck coefficients. Thus for thermoelectric applications the main activity will have to focus on achieving low electrical resistivity in a compromise with still sufficiently high Seebeck coefficients.

Determination of the crystal structure and charge density of (Ba 0.5Sr 0.5)(Co 0.8Fe 0.2)O 2.33 by Rietveld refinement and maximum entropy method analysis

The crystal structure and charge density of (Ba 0.5Sr 0.5)(Co 0.8Fe 0.2)O 2.33, a promising cathode material for solid oxide fuel cells with lower operating temperatures, were investigated by the Rietveld refinement method and the maximum entropy method (MEM) using neutron and synchrotron X-ray diffraction data. The crystal structure of (Ba 0.5Sr 0.5)(Co 0.8Fe 0.2)O 2.33 was refined by using the orthorhombic Pnma space group and the split atom model to cation sites. The occupancy of O1 ( 4 c ) and O2 ( 8 d ) sites are 0.59 and 0.87, respectively. From the MEM analysis, we found that (Ba 0.5Sr 0.5)(Co 0.8Fe 0.2)O 2.33 possessed both covalent and ionic bonds.

A neutron diffraction study of macroscopically entangled proton states in the high temperature phaseof the KHCO3 crystal at 340 K

We utilize single-crystal neutron diffraction to study theC 2/m structure of potassiumhydrogen carbonate (KHCO3) and macroscopic quantum entanglement above the phase transition atTc = 318 K. Whereas split atom sites could be due to disorder, the diffraction pattern at 340 Kevidences macroscopic proton states identical to those previously observed belowTc by Fillaux et al (2006 J. Phys.: Condens. Matter 18 3229). We propose a theoreticalframework for decoherence-free proton states and the calculated differential cross-sectionaccords with observations. The structural transition occurs from one orderedP 21/a structure(T<Tc) to anotherordered C 2/m structure. There is no breakdown of the quantum regime. It is suggested that the crystal isa macroscopic quantum object which can be represented by a state vector. Ramanspectroscopy and quasi-elastic neutron scattering suggest that the state vector is a superposition of the state vectors for twoP 21/a-like structuressymmetric with respect to (a,c) planes.

Thermal expansion and thermally induced variations of the crystal structure of AlPO4 low cristobalite

The crystal structure of AlPO 4 low cristobalite was refined at 25, 75, 125, 150, 175 and 200 °C from X-ray powder data with the Rietveld method confirming the orthorhombic symmetry C222 1 with very small deviation from tetragonal symmetry. Upon heating, the atoms are shifted by 0.022(3) A on the average towards their average positions in high temperature cristobalite. The volume gap at the displacive α-β transformation at 202 °C is slightly larger (4.6 %) than that of silica cristobalite (4.2 %). The high temperature modification was refined at 200 °C in space group F43 m using a split atom model for oxygen.

Disordered and Displacive Models for the Structure of the Normal Phase in {N(CH3)4}2MnCl4

The crystal structure of the normal phase in {N(CH 3 ) 4 } 2 MnCl 4 has been analyzed for two models: a displacive model and a disordered model. In the former model, the constituent tetrahedra of tetramethylammonium and tetrachloromanganate satisfy a mirror symmetry themselves imposed by the crystal symmetry. In the latter model, each tetrahedron occupies two equivalent positions related by the mirror symmetry with an equal occupation probability: split atom method. A special attention has been paid to their thermal parameters; the former model exhibits an abnormal increase of the parameters near the transition temperature indicating a critical phenomenon. From the significant test for the R factor, the latter model seems to be more adequate than the former one. The disorderness of tetrahedra of tetramethylammonium is also confirmed by double peaks in Fourier synthesis map. On the other hand, MnCl 4 can be well represented both by the displacive and the disordered models.

Anharmonicity and disorder in simple and complex perovskites: a highenergy synchrotron and hot neutron diffraction study

We report a study of simple ABO3 type perovskites BaTiO3,PbTiO3, KNbO3, SrTiO3 and the relaxor perovskitesPbSc1/2Nb1/2O3 (PSN) in their cubic phase usinghard synchrotron radiation and hot neutrons. Gram-Charlier expansions of thethermal parameters have been performed and have revealed interesting featuresabout the probability density function and the one-particle potential of thedifferent atoms. This description is compared with other descriptions in termsof the split atom model and in terms of the rotator model. Structural trendsregarding the order-disorder versus displacive character of the phasetransitions have been obtained. It is concluded that SrTiO3 and BaTiO3 are quasi-harmonic systems whereas KNbO3 shows weakanharmonicity and PbTiO3 and PSN display strong anharmonic features.

The InSnCl3-Type Arrangement: I. A New ABX3 Structure Type with Close Cation–Cation Contacts

We report on the synthesis and structure of InSnCl3 and InSnBr3. Their structure represents a new ABX3 structure type which will only be found for compounds with large cations A and B having an ns2 electron configuration. The cations have high coordination numbers and therefore their coordination polyhedra are connected mainly by common edges and even faces. This leads to fairly short cation–cation distances. However, the electrostatic repulsion may be decreased by a shielding effect of the s-electrons. The distribution of the cations on different sites, which cannot be determined by routine X-ray investigations, is discussed on the basis of crystal chemical considerations. Low temperature diffraction measurements on InSnCl3 show that the high symmetry of the arrangement is mimicked by a polydomain configuration, which can be dealt with by a split atom refinement.

The hydrogen bond system in natrochalcite-type compounds ? an FTIR spectroscopic study of the H3O2 unit

FTIR spectroscopy was used to study the vibrational behaviour of the hydrogen bond system of the H3O2 unit, present with widely varying O-H...O distances in natrochalcite-type compounds, MeCu2(H3O2)(ZO4)2 (Me = Na, K; Z=S, Se). Absorption bands of powder and polarized single-crystal spectra of non-deuterated and partially deuterated natrochalcite phases are assigned to stretching and bending modes of clearly divided OH− and H2O groups. These results are indicative for a not centro-symmetric configuration of the H3O2 unit and confirm the presence of a very strong hydrogen bond with a split hydrogen atom position.

Syntheses and crystal structures of NH4Ag2(AsS2)3 and (NH4)5Ag16(AsS4)7 with a discussion of (NH4)Sx polyhedra

The atomic arrangements within the structures of NH4Ag2(AsS2)3 [a=9.557(2),b=7.414(2),c=16.29(1) A; β=91.30(5)°; space group P21/n;R(F)=0.042] and (NH4)5Ag16(AsS4)7 [a=64.49(6),b=6.471(2),c=12.806(4) A; β=95.47(5)°; space group Cc;R(F)=0.073] were determined from single crystal X-ray data. In these two compounds the coordination spheres of the Ag atoms are quite different. In NH4Ag2(AsS2)3, the Ag atoms exhibit a [2+2]- and a [3+1]-coordination to S atoms up to 3.3 A and with Ag atom neighbours at 2.93 A and 3.05 A respectively. In (NH4)5Ag16(AsS4)7, the Ag atoms are — with one exception- [4] coordinated (Ag-S<3.3 A) and the distances to further Ag atom neighbours are greater than 3.1 A. NH4Ag2(AsS2)3 represents an ordered cyclo-thioarsenate(III) with three-membered As3S6 rings, (NH4)5Ag16(AsS4)7 a neso-thioarsenate(V) with two split Ag atom positions. Both compounds were synthesized under moderate hydrothermal conditions.