Fluorite-type Superstructure Research Articles

Electrical properties of murataite modules with complex and large-volume fluorite-type superstructures

We have synthesized murataite modules with oxygen-deficient fluorite-type superstructures by a solid-state reaction method and investigated their electrical properties. The multiples of the fluorite-type unit-cell were determined using X-ray diffraction (XRD) and transmission electron microscope-selected area electron diffraction (TEM-SAED). They revealed that well-crystalized 3×3×3 (M3) and 8×8×8 (M8) fluorite-type superstructures were successfully synthesized with minor impurities. The calculated lattice constants of the M3 and M8 were 1.4589 and 3.9269nm, respectively. Based on four-probe D.C. method and two-probe A.C impedance spectroscopy measurement, the electrical conductivity was measured as functions of temperature and oxygen partial pressure. The electrical conductivities of murataite modules increased with elevating temperature, which indicated typical semiconducting behavior. Impedance spectra suggested that either electronic or hole conduction must be superior to the ionic conduction in M3 phase. The M3 phase demonstrated good thermal stability under oxidizing and reducing atmosphere at 573–973K.

A new bismuth vanadate photocatalyst of Bi23V4O44.5 nanoplates with layered δ-Bi2O3 based fluorite-type superstructures

The work reports the large-scale synthesis and photocatalytic activity of Bi23V4O44.5 nanoplates. The structural refinement was conducted in the layered δ-Bi2O3 based superstructuLre. The detailed surface properties were characterized. Bi23V4O44.5 shows efficient absorption in visible region with a band-gap of 2.384eV from the hybridization of Bi-6s and O-2p orbitals. Particularly, Bi23V4O44.5 shows an efficient photodegradation for methylene blue (MB). The photocatalysis benefits from the layered B–O structure characteristic and the codoped defects of V4+/V5+ ions in the lattices. Bi23V4O44.5 nanoplates could be a potential photocatalyst.

Bi1−xNbxO1.5+x (x=0.0625, 0.12) fast ion conductors: Structures, stability and oxide ion migration pathways

A combined experimental and computational study of Bi1−xNbxO1.5+x (x=0.0625 and 0.12) has been carried out using laboratory X-ray, neutron and electron diffraction, impedance measurements and ab-initio molecular dynamics. We demonstrate that Bi0.9375Nb0.0625O1.5625, previously reported to adopt a cubic fluorite-type superstructure, can form two different polymorphs depending on the synthetic method: a metastable cubic phase is produced by quenching; while slower cooling yields a stable material with a tetragonal √2×√2×1 superstructure, which undergoes a reversible phase transition into the cubic form at ~680°C on subsequent reheating. Neutron diffraction reveals that the tetragonal superstructure arises mainly from ordering in the oxygen sublattice, with Bi and Nb remaining disordered, although structured diffuse scattering observed in the electron diffraction patterns suggests a degree of short-range ordering. Both materials are oxide ion conductors. On thermal cycling, Bi0.88Nb0.12O1.62 exhibits a decrease in conductivity of approximately an order of magnitude due to partial transformation into the tetragonal phase, but still exhibits conductivity comparable to yttria-stabilised zirconia (YSZ). Ab-initio molecular dynamics simulations performed on Bi0.9375Nb0.0625O1.5625 show that oxide ion diffusion occurs by O2− jumps between edge- and corner-sharing OM4 groups (M=Bi, Nb) via tetrahedral □M4 and octahedral □M6 vacancies.

Oxygen non-stoichiometry phenomena in Pr1-xZrxO2-y compounds (0.02 < x < 0.5).

New Pr1-xZrxO2-y oxides with x < 0.5 have been prepared by co-precipitation in basic medium and annealed under air at high temperatures (T≤ 1200 °C). Defined compositions with x = 0.02, 0.1, 0.2, 0.35, 0.40 and 0.5 have been characterized by XRD, Zr-K-edge EXAFS for the local structure, magnetic susceptibility measurements, and Pr LIII-edge XANES in order to identify the variation of the cell parameter and Zr local environment versus Zr content and Pr(n+) (4 < n < 3) oxidation states. The higher the Zr content, the lower the Pr valence state. The Zr amount stabilized in the distorted octahedral site is at the origin of the formation of defined compositions as discovered by Leroy Eyring et al. in the PrnO2n-2m series and the generation of oxygen vacancies stabilized in the fluorite-type network. TGA and TPR analyses help to follow the reduction properties under Ar/5% H2 and show high Pr reducible rates at low temperatures (T < 250 °C). The identification of the fluorite-type superstructure (SG: Ia3[combining macron]) of reduced compositions annealed at T = 900 °C under Ar/5% H2 shows the cationic and oxygen vacancy ordering. This feature plays a key role with Zr(4+) cations stabilized in flattened octahedral sites for the generation of oxygen vacancies and the stabilization of Pr(3+) in the reduced states.

Synthesis, crystal structures and ionic conductivities of Bi 14P 4O 31 and Bi 50V 4O 85. Two members of the series Bi 18−4mM 4mO 27+4m ( M=P, V) related to the fluorite-type structure

The two hitherto unknown compounds Bi 14P 4O 31 and Bi 50V 4O 85 were prepared by the direct solid-state reaction of Bi 2O 3 and (NH 4)H 2PO 4 or V 2O 5, respectively. Bi 14P 4O 31 crystallizes in a C-centred monoclinic symmetry ( C2/ c space group) with the unit-cell parameters: a = 19.2745 ( 2 ) Å , b = 11.3698 ( 1 ) Å , c = 52.4082 ( 2 ) Å and β = 93.63 ( 1 ) ° ( Z = 16 ). The symmetry of Bi 50V 4O 85 is also monoclinic ( I2/ m space group) with lattice parameters of a = 11.8123 ( 3 ) Å , b = 11.7425 ( 2 ) Å , c = 16.5396 ( 2 ) Å and β = 90.14 ( 1 ) ° ( Z = 2 ). Both structures correspond to a fluorite-type superstructure where the Bi and P or V atoms are ordered in the framework. An idealized structural model is proposed where the structures result of the stacking of mixed atomic layers of composition [Bi 14 M 4O 31] and [Bi 18O 27] respectively. This new family can be formulated Bi 18−4 mM 4 m O 27+4 m with M=P, V and where the parameter m ( 0 ⩽ m ⩽ 1 ) represents the ratio of the number of [Bi 14 M 4O 31] layers to the total number of layers in the sequence. Bi 14P 4O 31 corresponds to m = 1 when Bi 50V 8O 85 corresponds to m = 1 / 3 . In this last case, the structural sequence is simply one [Bi 14V 4O 31] layer to two [Bi 18O 27] layers. As predicted by the proposed structural building principle, Bi 14P 4O 31 is not a good ionic conductor. The conductivity at 650 °C is 4 orders of magnitude lower from those found in Bi 46 M 8O 89 ( M=P, V) ( m = 2 / 3 ) and Bi 50V 4O 85 ( m = 1 / 3 ).

Crystal structures of the ionic conductors Bi 46M8O 89 ( M=P, V) related to the fluorite-type structure

The crystal structures of the two oxides Bi 46 M 8O 89 ( M = P , V) have been solved from single crystals X-ray data at room temperature. Bi 46P 8O 89 crystallizes in the monoclinic symmetry (space group C 2 / m ) with the cell parameters a = 19.6073 ( 4 ) Å , b = 11.4181 ( 4 ) Å , c = 21.1119 ( 4 ) Å and β = 112.14 ( 3 ) ° . The symmetry of Bi 46V 8O 89 is also monoclinic but the space group is P 2 1 / c with the unit-cell parameters: a = 20.0100 ( 4 ) Å , b = 11.6445 ( 4 ) Å , c = 20.4136 ( 4 ) Å and β = 107.27 ( 3 ) ° . Both structures derive from an oxygen deficient fluorite-type structure where the Bi and M cations ( M = P , V) are ordered in the framework. The structures are characterised by isolated M O 4 tetrahedra ( M = P , V) which contradicts the previous results. The difference between the two structures is only due to a different order of the M atoms ( M = P , V) in the fluorite-type superstructure. It will be shown that some oxygen sites are partially occupied in both structures which can explain the ion conduction properties of these phases. A structural building principle will be proposed that can explain the large domain of solid solution related to the fluorite-type observed in both systems.

Alkaline earth/rare earth halides and related systems

Halides of di- and trivalent metals of the general formula BX 2 and MX 3 may react with each other forming a fair variety of complex halides in the solid state. Examples for ternary chlorides are: Ba 2ErCl 7 with isolated [ErCl 7] monocapped trigonal prisms, Ba 2Cl[ScCl 6] with isolated octahedra [ScCl 6] and solitary Cl −, Ba 8[{Sm 6Z}Cl 32] and Ba 9[{Sm 6Z}Cl 34] with Z being presumably an oxide ion residing in the large hole of the [Sm 6Cl 36] polyhedral clusters that occur in these fluorite type superstructures, and, finally, BaGdCl 5 with corrugated layers of edge- and vertex-connected [GdCl 8] square antiprisms. Two further lines may be followed: Firstly, with Nd 8[{Nd 6Z}Cl 32] and Nd 9[{Nd 6Z}Cl 34] isostructural with the above-mentioned chlorides with polyhedral clusters, other intra-rare-earth mixed valence halides may be investigated. “Sm 14Cl 33” is one example. Secondly, with the ionic radius of In 3+ in between those of Lu 3+ and Sc 3+, the BaCl 2/InCl 3 system comes into view. With ZnCl 2 as the solvent, Ba 6ZnIn 2Cl 20 and BaZnCl 4 (GaCl 2 type of structure) were obtained. Following this line further, SrZnCl 4 (scheelite type) was obtained as a model for EuZnCl 4, and, instead of BaCoCl 4, Ba 2CoCl 6 with edge-sharing bioctahedra according to Ba 2Cl 2[Co 2Cl 10].

Structural study of the trigonal Bi2.34U0.33La0.33O5 oxide ion conductor: Rietveld refinement of X-ray and neutron powder diffraction data

The structure of Bi2.34U0.33La0.33O5 has been investigated by Rietveld refinement of powder X-ray and neutron diffraction data. The compound is trigonal, space group P, Z = 1, and lattice parameters aH = 4.01395(8), cH = 9.5423(2) A. X-Ray diffraction data are in favour of a model in which U and La atoms are situated in different crystallographic positions: i.e. 1a and 2d of the P space group respectively, forming mixed Bi/U and Bi/La layers which alternate along the c axis. The structure of this bismuth-based mixed oxide can be described as built up of interlocked edge-sharing (Bi/U)O8 and (Bi/La)O8 polyhedra. The co-ordination polyhedron around the Bi/U atoms is a hexagonal bipyramid, the equatorial plane being a puckered hexagon. The Bi/La atoms are co-ordinated to eight oxygen atoms forming a slightly distorted cube. Electron diffraction studies showed that the structure can be described as a fluorite-type superstructure, the relation to the fluorite sub-cell is a*H ≈ ⅓[4]a*C, b*H ≈ ⅓[ 22]a*C, c*H ≈ ⅓[111]a*C. This compound is a good oxide ion conductor (σ300 °C = 2.5 × 10–5 S cm–1). On the basis of the anion vacancies distribution determined from neutron diffraction refinement, a pathway for the oxide ion conduction is proposed.

The Alkali-Poor Part of the Pseudoternary Triangle AX/BX2/MX3: Crystal Structures, Properties, and Potentials of (Alkali)/Alkaline-Earth/Rare-Earth Chloride Materials

Halides of mono-, di-, and trivalent metals of the general formulas AX, BX2, and MX3 may react with each other, forming a large variety of ternary and even quaternary phases in the solid state. Aside from the alkali-poor quaternary chlorides of the ABMX6 type, which are derivatives of the UCl3 type of structure, the alkali cation free pseudoternary chlorides that form in the systems BCl2/MCl3, especially with B = Ba, are reviewed. Examples of ternary chlorides are Ba2EuCl7, with isolated [EuCl7] monocapped trigonal prisms; Ba2Cl[ScCl6], with isolated octahedra [ScCl6] and lonesome Cl-; Ba8[{(Sm6)Z}Cl32], and Ba9[{(Sm6)Z}Cl34], with Z being presumably an oxide ion residing in the large hole of the [Sm6Cl36] polyhedral clusters that occur in these fluorite type superstructures; and, finally, BaGdCl5, with corrugated layers of edge-connected [GdCl8] square antiprisms. These halides may be cationic conductors (AgSrSmCl6), insulating mixed-valence halides such as NaNd2Cl6, or semiconductors such as Pr2Br5 with...

X-Ray Study of the Cation Distribution in the Ternary Oxide, 6 Bi2O3–V2O5

The structure of the ternary oxide 6 Bi2O3–V2O5has been investigated using single crystal X-ray diffraction data. It has a fluorite-type superstructure with a pseudo monoclinic cell,a= 20.023 Å,b= 11.668 Å,c= 20.472 Å, and β = 107.13°, (the relation to the fluorite subcell isa≃ 3/2[1, 1, 2]c,b≃ 3/2[1, 1, 0]c, andc≃ 1/2[5, 5, 2]c), but the real crystal symmetry isP1. The positions of metal atoms are refined toR= 0.12. The structure is characterized by six cation layers along thecaxis, each containing 18 metal atoms. The cation arrangement can be represented using the numbers of V atoms as 4–0–4–4–0–4. Along thebaxis, the crystal has square wave type displacements (≃0.6 Å). The origin of this large distortion is attributed to the Coulomb repulsion between V atoms in the nearest neighbor sites.

X-Ray Study of the Superstructure in the Bi2O3–V2O5System

The structure of a bismuth based oxide, Bi 2 O 3 :V 2 O 5 =9:1, has been studied by X-ray diffraction. This crystal has a fluorite type superstructure with 3×3×3 subcells. Vanadium atoms are located at every third layers along the [111] axis and occupy the third neighbor sites of the fcc lattice. The oxygen vacancies are concentrated at nearest neighbor sites of the vanadium atoms.

7424. Pure and mixed titanium, niobium and vanadium oxides as sputtered thick and thin films: crystallographic properties and phase transitions between 300 and 1800 K. I: X-ray diffraction investigations of thick films: M Gasgnier et al, Thin Solid Films, 187, 1990, 25–37

The two hitherto unknown compounds Bi14P4O31 and Bi50V4O85 were prepared by the direct solid-state reaction of Bi2O3 and (NH4)H2PO4 or V2O5, respectively. Bi14P4O31 crystallizes in a C-centred monoclinic symmetry (C2/c space group) with the unit-cell parameters: a=19.2745(2)Å, b=11.3698(1)Å, c=52.4082(2)Å and β=93.63(1)° (Z=16). The symmetry of Bi50V4O85 is also monoclinic (I2/m space group) with lattice parameters of a=11.8123(3)Å, b=11.7425(2)Å, c=16.5396(2)Å and β=90.14(1)° (Z=2). Both structures correspond to a fluorite-type superstructure where the Bi and P or V atoms are ordered in the framework. An idealized structural model is proposed where the structures result of the stacking of mixed atomic layers of composition [Bi14M4O31] and [Bi18O27] respectively. This new family can be formulated Bi18−4mM4mO27+4m with M=P, V and where the parameter m (0⩽m⩽1) represents the ratio of the number of [Bi14M4O31] layers to the total number of layers in the sequence. Bi14P4O31 corresponds to m=1 when Bi50V8O85 corresponds to m=1/3. In this last case, the structural sequence is simply one [Bi14V4O31] layer to two [Bi18O27] layers. As predicted by the proposed structural building principle, Bi14P4O31 is not a good ionic conductor. The conductivity at 650 °C is 4 orders of magnitude lower from those found in Bi46M8O89 (M=P, V) (m=2/3) and Bi50V4O85 (m=1/3).

Ternäre Nitride des Lithiums mit den Elementen Cr, Mo und W / Ternary Lithium Nitrides with Elements Cr, Mo and W

The ternary compounds were prepared by reaction of the transition metals with Li3N melt under nitrogen. The crystal structures of Li6MeN4 (P42/nmc, Z = 2; Me = Mo: a = 667.3(1) pm, c = 492.5(3) pm; Me = W: a = 667.9(1) pm, c = 492.7(1) pm) and Li15Cr2N9 (P4/ncc, Z = 4; a = 1023.3(5) pm, c = 938.9(7) pm) can be described as fluorite-type superstructures with the lithium and transition metal atoms in tetrahedral holes of the nearly fcc nitrogen arrangement and with an ordered distribution of defects within the cation substructure. In addition, the ternary system Li–Cr–N contains the compound Li6CrN4 with a crystal structure which is not quite clear at present, but which shows close relations to the structures of Li6MeN4 (Me = Mo, W). The previously reported compounds Li9MeN5 (Me = Cr, Mo, W) have not been observed during this study. The respective Cr compound in fact is a nitride-oxide (nitride-imide) of composition Li14Cr2N8(O,NH) with a crystal structure (P3̄, Z = 1, a = 579.9(1) pm, c = 826.3(6) pm) also showing a fluorite-type superstructure.