Exfoliated Ferrierite-Related Unilamellar Nanosheets in Solution and Their Use for Preparation of Mixed Zeolite Hierarchical Structures.

Direct exfoliation of layered zeolites into solutions of monolayers has remained unresolved since the 1990s. Recently, zeolite MCM-56 with the MWW topology (layers denoted mww) has been exfoliated directly in high yield by soft-chemical treatment with tetrabutylammonium hydroxide (TBAOH). This has enabled preparation of zeolite-based hierarchical materials and intimate composites with other active species that are unimaginable via the conventional solid-state routes. The extension to other frameworks, which provides broader benefits, diversified activity, and functionality, is not routine and requires finding suitable synthesis formulations, viz. compositions and conditions, of the layered zeolites themselves. This article reports exfoliation and characterization of layers with ferrierite-related structure, denoted bifer, having rectangular lattice constants like those of the FER and CDO zeolites, and thickness of approximately 2 nm, which is twice that of the so-called fer layer. Several techniques were combined to prove the exfoliation, supported by simulations: AFM; in-plane, in situ, and powder X-ray diffraction; TEM; and SAED. The results confirmed (i) the structure and crystallinity of the layers without unequivocal differentiation between the FER and CDO topologies and (ii) uniform thickness in solution (monodispersity), ruling out significant multilayered particles and other impurities. The bifer layers are zeolitic with Brønsted acid sites, demonstrated catalytic activity in the alkylation of mesitylene with benzyl alcohol, and intralayer pores visible in TEM. The practical benefits are demonstrated by the preparation of unprecedented intimately mixed zeolite composites with the mww, with activity greater than the sum of the components despite high content of inert silica as pillars.

Compositely modulated structures of phosphor materials SrxLi2+xAl2-xO4:Eu2.

Composite crystals SrxLi2+xAl2-xO4:Eu2+ were synthesized and their structures were determined using single-crystal X-ray diffraction. The commensurate structure with a modulation wavevector q = 5c*/6 was analyzed in a conventional manner in 3D space, while a structure model in (3+1)-dimensional superspace was used for the other two crystals with modulation wavevectors slightly differing from 5c*/6. The superstructure of the commensurate phase was described using the space group P4/n and a common superspace group I4/m(00γ)00 was used for the (3+1)D structures of all three crystals. The whole structure of each crystal consists of two substructures. Basis vectors a and b are common, but c is different for the two substructures. The first substructure is a host framework constructed by (Li/Al)O4 tetrahedra sharing edges. A linear connection of cavities is seen to be channel-like, in which Sr ions locate as guest cations forming the second substructure. The crystal of q = 5c*/6 contains five Sr ions per six cavities in a channel. Sr ions are distributed at seven sites, some of which are partially occupied. Statistical disorder of local structure models for the location of Sr ions in the channel was assumed to explain the results. Such a partially disordered character was also seen in the incommensurate phases and properly embodied by a (3+1)D model containing an atomic domain of the Sr ion with occupational modulation. Plots of the occupation factor, interatomic distances and the bond valence sum at each metal site as functions of t (= x4 - q·r) are roughly identical in the three crystals, which are considered as members of the same series of composite crystals.

Surface-Modified Two-Dimensional Titanium Carbide Sheets for Intrinsic Vibrational Signal-Retained Surface-Enhanced Raman Scattering with Ultrahigh Uniformity.

Surface-enhanced Raman scattering (SERS) employing a non-noble substrate in comparison with conventional noble-metal ones offers advantages of low cost and rich selection of candidates; however, its application has been seriously hindered by its unsatisfactory detection sensitivity, poor uniformity, and undesirable modification of vibrational signals via changing the orientation and/or polarizability of probe molecules. Here, an unusually sensitive but nonselective enhancement was achieved by employing titanium carbide sheets modified with aluminum oxyanions in situ as active supports for Raman measurement. The analyte molecules adopted a conformation similar to what they adopt on a bare substrate, while closely interacting with the aluminum oxyanion surface, which leads to the rare observation of highly sensitive but nonselective enhancement with a detection limit close to the pM level. With the substrate surface roughness in the nanometer region, an outstanding uniformity with a relative standard deviation of less than 4.3% was achieved. In addition, the SERS effect on the modified titanium carbide sheets was shown to be applicable to a wide range of analyte molecules, including both organic dyes and trace harmful compounds. The success of the work demonstrates the feasibility of surface tuning to improve the SERS effect, and it introduces a new window for two-dimensional materials in SERS applications.

Incommensurately Modulated Crystal Structure and Photoluminescence Properties of Eu2O3- and P2O5-Doped Ca2SiO4 Phosphor.

We prepared four types of Eu2O3- and P2O5-doped Ca2SiO4 phosphors with different phase compositions but identical chemical composition, the chemical formula of which was (Ca1.950Eu3+0.013☐0.037)(Si0.940P0.060)O4 (☐ denotes vacancies in Ca sites). One of the phosphors was composed exclusively of the incommensurate (IC) phase with superspace group Pnma(0ß0)00s and basic unit-cell dimensions of a = 0.68004(2) nm, b = 0.54481(2) nm, and c = 0.93956(3) nm (Z = 4). The crystal structure was made up of four types of ß-Ca2SiO4-related layers with an interlayer. The incommensurate modulation with wavelength of 4.110 × b was induced by the long-range stacking order of these layers. When increasing the relative amount of the IC-phase with respect to the coexisting ß-phase, the red light emission intensity, under excitation at 394 nm, steadily decreased to reach the minimum, at which the specimen was composed exclusively of the IC-phase. The coordination environments of Eu3+ ion in the crystal structures of ß- and IC-phases might be closely related to the photoluminescence intensities of the phosphors.

Crystal Structure and Photoluminescence Properties of an Incommensurate Phase in EuO- and P2O5-Doped Ca2SiO4.

We have for the first time clarified the incommensurately modulated crystal structure as well as the photoluminescence properties of Eu2+-activated Ca2SiO4 solid solution, the chemical formula of which is (Ca1.88Eu2+0.01□0.11)(Si0.78P0.22)O4, where □ denotes vacancies in Ca sites with the replacement of Si4+ by P5+. The emission spectrum upon the 335 nm excitation showed a relatively broad band centered at ca. 490 nm and a full width at half-maximum of ca. 80 nm. The crystal structure was made up of the four types of ß-Ca2SiO4-like layers with one type of interlayer. The incommensurate modulation with superspace group Pnma(0 ß 0)00 s was induced by the long-range stacking order of these layers. The modulation wavevector was 0.27404(2) × b*, with the basic unit-cell dimensions being a = 0.68355(2) nm, b = 0.54227(2) nm, and c = 0.93840(3) nm ( Z = 4). The basic structure contained two nonequivalent Ca sites. One site was fully occupied by Ca2+ and free from Eu2+ in the overall incommensurate structure. The occupational modulation at the other site was so significant that the sum of site occupation factors for Ca2+ and Eu2+ as low as 0.5 was seen at the interlayer. This site was too large for accommodation of Ca2+ but was suitable for Eu2+. Thus, the Eu2+ ions would exclusively concentrate at the relevant site, which would cause the emission peak of the incommensurate phase to be shifted to the shorter wavelength ranges as compared with those of the other commensurate phases such as ß and α'L.

Thermoelectric Properties of Variants of Cu4Mn2Te4 with Spinel-Related Structure.

Thermoelectric properties of Cu4Mn2Te4, which is antiferromagnetic with a Néel temperature TN = 50 K and crystallizes in a spinel-related structure, have been investigated comprehensively here. The phase transition occurring at temperatures 463 and 723 K is studied by high-temperature X-ray diffraction (XRD) and differential scanning calorimetry (DSC), and its effect on thermoelectric properties is examined. Hypothetically Cu4Mn2Te4 is semiconducting according to the formula (Cu+)4(Mn2+)2(Te2-)4, while experimentally it shows p-type metallic conduction behavior, exhibiting electrical conductivity σ = 2500 Ω-1 cm-1 and Seebeck coefficient α = 20 µV K-1 at 325 K. Herein, we show that the carrier concentration and thus the thermoelectric transport properties could be further optimized through adding electron donors such as excess Mn. Discussions are made on the physical parameters contributing to the low thermal conductivity, including Debye temperature, speed of sound, and the Grüneisen parameter. As a result of simultaneously boosted power factor and reduced thermal conductivity, a moderately high zT = 0.65 at 680 K is obtained in an excess Mn\In co-added sample, amounting to 5 times that of the pristine Cu4Mn2Te4. This value ( zT = 0.65) is the best result ever reported for spinel and spinel-related chalcogenides.

Substitutional disorder in Sr2-yEuyB2-2xSi2+3xAl2-xN8+x (x ≃ 0.12, y ≃ 0.10).

A novel nitride, Sr2-yEuyB2-2xSi2+3xAl2-xN8+x (x ≃ 0.12, y ≃ 0.10) (distrontium europium diboron disilicon dialuminium octanitride), with the space group P62c, was synthesized from Sr3N2, EuN, Si3N4, AlN and BN under nitrogen gas pressure. The structure consists of a host framework with Sr/Eu atoms accommodated in the cavities. The host framework is constructed by the linkage of MN4 tetrahedra (M = Si, Al) and BN3 triangles, and contains substitutional disorder described by the alternative occupation of B2 or Si2N on the (0, 0, z) axis. The B2:Si2N ratio contained in an entire crystal is about 9:1.

A ferroelectric-like structural transition in a metal.

Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric (R3c) to non-centrosymmetric (R3c) transition in metallic LiOsO3 that is structurally equivalent to the ferroelectric transition of LiNbO3 (ref. 3). The transition involves a continuous shift in the mean position of Li(+) ions on cooling below 140 K. Its discovery realizes the scenario described in ref. 2, and establishes a new class of materials whose properties may differ from those of normal metals.

Boron carbide, B(13-x)C(2-y) (x = 0.12, y = 0.01).

Boron carbide phases exist over a widely varying compos-itional range B(12+x)C(3-x) (0.06 < x < 1.7). One idealized structure corresponds to the B(13)C(2) composition (space group R-3m) and contains one icosa-hedral B(12) unit and one linear C-B-C chain. The B(12) units are composed of crystallographically distinct B atoms B(P) (polar, B1) and B(Eq) (equatorial, B2). Boron icosa-hedra are inter-connected by C atoms via their B(Eq) atoms, forming layers parallel to (001), while the B(12) units of the adjacent layers are linked through inter-icosa-hedral B(P)-B(P) bonds. The unique B atom (B(C)) connects the two C atoms of adjacent layers, forming a C-B-C chain along [001]. Depending on the carbon concentration, the carbon and B(P) sites exhibit mixed B/C occupancies to varying degrees; besides, the B(C) site shows partial occupancy. The decrease in carbon content was reported to be realized via an increasing number of chainless unit cells. On the basis of X-ray single-crystal refinement, we have concluded that the unit cell of the given boron-rich crystal contains following structural units: [B(12)] and [B(11)C] icosa-hedra (about 96 and 4%, respectively) and C-B-C chains (87%). Besides, there is a fraction of unit cells (13%) with the B atom located against the triangular face of a neighboring icosa-hedron formed by B(Eq) (B2) thus rendering the formula B(0.87)(B(0.98)C(0.02))(12)(B(0.13)C(0.87))(2) for the current boron carbide crystal.

Structure of (Ga2O3)2(ZnO)13 and a unified description of the homologous series (Ga2O3)2(ZnO)(2n + 1).

The structure of (Ga(2)O(3))(2)(ZnO)(13) has been determined by a single-crystal X-ray diffraction technique. In the monoclinic structure of the space group C2/m with cell parameters a = 19.66 (4), b = 3.2487 (5), c = 27.31 (2) Å, and ß = 105.9 (1)°, a unit cell is constructed by combining the halves of the unit cell of Ga(2)O(3)(ZnO)(6) and Ga(2)O(3)(ZnO)(7) in the homologous series Ga(2)O(3)(ZnO)(m). The homologous series (Ga(2)O(3))(2)(ZnO)(2n + 1) is derived and a unified description for structures in the series is presented using the (3+1)-dimensional superspace formalism. The phases are treated as compositely modulated structures consisting of two subsystems. One is constructed by metal ions and another is by O ions. In the (3 + 1)-dimensional model, displacive modulations of ions are described by the asymmetric zigzag function with large amplitudes, which was replaced by a combination of the sawtooth function in refinements. Similarities and differences between the two homologous series (Ga(2)O(3))(2)(ZnO)(2n + 1) and Ga(2)O(3)(ZnO)(m) are clarified in (3 + 1)-dimensional superspace. The validity of the (3 + 1)-dimensional model is confirmed by the refinements of (Ga(2)O(3))(2)(ZnO)(13), while a few complex phenomena in the real structure are taken into account by modifying the model.

Undulating layers in a new rhodate network: structure of Bi(1.4)CuORh5O10.

A new rhodate, Bi(1.4)CuRh(5)O(11), with an hitherto unknown channel structure containing undulating layers of RhO(6) octahedra sharing corners and edges has been discovered and its structure refined from single crystal X-ray diffraction data. The channels contain Bi(3+), Cu(2+), and some O strongly bound to Cu. The Cu coordination is distorted square planar. Mixed Rh(3+)/Rh(4+) valency leads to significant electrical conductivity.

Structural study of a series of layered rare-earth hydroxide sulfates.

We report structure analysis of a new family of rare-earth hydroxides Ln(2)(OH)(4)SO(4)·2H(2)O (Ln = Pr, Nd, Sm, Eu, Gd, Tb) from synchrotron X-ray and electron diffraction data. Rietveld profile analysis revealed that all members were isostructural and crystallized in a face-centered monoclinic system A2/m (No. 12), in which the monoclinic angles were approximately equal to the right angle, varying from 90.387(1)° for Pr sample to 90.0718(3)° for Tb sample. The structure consisted of LnO(9) polyhedra connected by µ(3)-hydroxyl groups and µ(2)-water molecules, forming a corrugated two-dimensional layer, which was pillared by bidentated sulfate ions. This series of compounds had a supercell a' = 2a, b' = 2b because of the local orientation ordering of SO(4)(2-). Structural features along the series, such as unit-cell parameters and average Ln-O distances, represented a progressive contraction associated with the shrinking radius of the lanthanide cations from Pr to Tb.

A bona fide two-dimensional percolation model: an insight into the optimum photoactivator concentration in La2/3-x Eu x Ta2O7 nanosheets.

La-Eu solid solution nanosheets La2/3-x Eu x Ta2O7 have been synthesized, and their photoluminescence properties have been investigated. La2/3-x Eu x Ta2O7 nanosheets were prepared from layered perovskite compounds Li2La2/3-x Eu x Ta2O7 as the precursors by soft chemical exfoliation reactions. Both the precursors and the exfoliated nanosheets exhibit a decrease in intralayer lattice parameters as the Eu contents increase. However, there is a discontinuity in this trend between the nominal Eu content ranges x≤ 0.3 and x ≥ 0.4. This discontinuity is attributed to the difference in degree of TaO6 octahedra tilting for the La- and Eu-rich phases. La2/3-x Eu x Ta2O7 nanosheets exhibit red emission, characteristic of the f-f transitions in Eu3+ photoactivators. The photoluminescence emission can be obtained from both host and direct photoactivator excitation. However, photoluminescence emission through host excitation is much more dominant than that through direct photoactivator excitation, and this behavior is consistent with that of all the other rare-earth photoactivated nanosheets reported previously. The absolute photoluminescence quantum efficiency of the La2/3-x Eu x Ta2O7 nanosheets increases as the experimentally determined Eu contents increase up to x=0.45 and decrease above it. This result is in good agreement with the optimum photoactivator concentration expected from the percolation theory. These solid solution La2/3-x Eu x Ta2O7 nanosheets are excellent models for validating the theory of optimum photoactivator concentration in the truly two-dimensional photoactivator matrix.

Superspace description of the homologous series Ga2O3(ZnO)m.

A unified description for the structures of the homologous series Ga(2)O(3)(ZnO)(m), gallium zinc oxide, is presented using the superspace formalism. The structures were treated as a compositely modulated structure consisting of two subsystems. One is constructed with metal ions and the other with O ions. The ideal model is given, in which the displacive modulations of ions are well described by the zigzag function with large amplitudes. Alternative settings are also proposed which are analogous to the so-called modular structures. The validity of the model has been confirmed by refinements for phases with m = 6 and m = 9 in the homologous series. A few complex phenomena in real structures are taken into account by modifying the ideal model.

Eu3Si(15-x)Al(1 + x)OxN(23-x) (x approximately 5/3) as a commensurate composite crystal.

A new Eu-SiAlON crystal, Eu3Si(15-x)Al(1 + x)O(x)N(23-x) (x approximately 5/3), was found and the structure was determined by an X-ray diffraction technique using a twinned sample. The structure consists of a host framework, which is constructed by the connection of MX4 tetrahedra (M: Si or Al; X: O or N), and Eu ions as the guest ions. The structure is considered to be a commensurate composite crystal. The basic vectors are a1 = a/3, b and c for the first substructure, and a2 = a/5, b and c for the second substructure. The first substructure consists of part of the host framework and the Eu ions, while the remainder of the host structure is taken as the second substructure. Possible phases belonging to the series are proposed using the composite crystal model in (3 + 1)-dimensional superspace. Chemical composition, possible space groups, cell parameters, and the basic model for those phases are presented.

Structure of Ga2O3(ZnO)6: a member of the homologous series Ga2O3(ZnO)m.

The structure of Ga(2)O(3)(ZnO)(6) was determined using single-crystal X-ray diffraction techniques in the space group Cmcm. The metal ion sublattice resembles some of the Zn ions in the wurtzite ZnO structure. The oxygen ion sublattice in Ga(2)O(3)(ZnO)(6) also resembles some of the O ions in ZnO. Structural relationships between Ga(2)O(3)(ZnO)(6) and ZnO are discussed, illustrating the process for obtaining the centrosymmetric Ga(2)O(3)(ZnO)(6) structure from the noncentrosymmetric ZnO. Structures of phases in the homologous series Ga(2)O(3)(ZnO)(m) are predicted on the basis of the structural data for Ga(2)O(3)(ZnO)(6). The structures of even m are constructed by simply extending the structure units seen in Ga(2)O(3)(ZnO)(6), while those of odd m consist of structure units which are of different types from those used for even m.

Mg(4)Sb(2)O(9) of the ilmenite structure type.

Single crystals of the title compound, tetra-magnesium dianti-monate(V), were obtained by the slow cooling method with K(2)CO(3). The structure is isotypic with ilmenite, which is constructed by the alternate stacking of layers consisting of metal-oxygen coordination octa-hedra. In each layer, the octa-hedra are connected by sharing edges so as to make holes. One of the two non-equivalent metal sites is fully occupied by Mg (3 symmetry), while the second metal site (3 symmetry) is disordered and occupied by Mg and Sb with occupation factors of 1/3 and 2/3, respectively.

Probability density analyses of guest ions in hollandite A(x)Mg(x/2)Ti(8 - x/2)O(16) (A = K, Rb).

Probability density functions (PDFs) of mobile ions in one-dimensional ionic conductors of hollandite A(x)Mg(x/2)Ti(8 - x/2)O(16) (A = K, Rb) were examined by single-crystal X-ray diffraction. A conventional structure model was modified by imposing an additional constraint condition, which is based on microscopic description for the possible displacement of mobile ions adjacent to a vacancy in the tunnel. Joint PDFs and one-particle potentials for mobile ions were obtained from the structure models applying harmonic and anharmonic atomic displacement parameters (ADPs). Potential curves of the ion hopping between neighboring cavities were calculated from the joint PDF of the specific ions of the process. Energy barriers of the ion hopping were estimated at 52-60 meV from anharmonic ADP models of K-hollandite, while the values varied from 140 to 250 meV for Rb-hollandite.

Superspace description of the homologous series Ga4Tim-4O2m-2 with the crystallographic shear structure based on that of rutile.

A generalized approach is proposed for the structure description of the homologous series Ga(4)Ti(m-4)O(2m-2) with crystallographic shear (CS) structures based on that of rutile. A (3 + 1)-dimensional model for an ideal CS structure is built up in connection with the CS operation in three-dimensional space. Its structural parameters are determined by m, the shear plane and the shear vector. The Rietveld fitting of the X-ray diffraction profile of Ga(4)Ti(13)O(32) (m = 17) was successfully carried out using the ideal CS structure as an initial model, where modulation functions of atomic positions are inherently discontinuous and sawtooth-like. The deviation of a real structure from the ideal one was described by a few additional Fourier terms. This method was confirmed to be efficient for reducing the number of structural parameters required in the refinement in comparison to the conventional three-dimensional description.