Fluorite Block Research Articles

Internal strain-driven bond manipulation and band engineering in Bi2-x Sb x YO4Cl photocatalysts with triple fluorite layers.

In extended solid-state materials, the manipulation of chemical bonds through redox reactions often leads to the emergence of interesting properties, such as unconventional superconductivity, which can be achieved by adjusting the Fermi level through, e.g., intercalation and pressure. Here, we demonstrate that the internal 'biaxial strain' in tri-layered fluorite oxychloride photocatalysts can regulate bond formation and cleavage without redox processes. We achieve this by synthesizing the isovalent solid solution Bi2-x Sb x YO4Cl, which undergoes a structural phase transition from the ideal Bi2YO4Cl structure to the Sb2YO4Cl structure with (Bi,Sb)4O8 rings. Initially, substitution of smaller Sb induces expected lattice contraction, but further substitution beyond x > 0.6 triggers an unusual lattice expansion before the phase transition at x = 1.5. Detailed analysis reveals structural instability at high x values, characterized by Sb-O underbonding, which is attributed to tensile strain exerted from the inner Y sublayer to the outer (Bi,Sb)O sublayer within the triple fluorite block - a concept well-recognized in thin film studies. This concept also explains the formation of zigzag Bi-O chains in Bi2MO4Cl (M = Bi, La). The Sb substitution in Bi2-x Sb x YO4Cl elevates the valence band maximum, resulting in a minimized bandgap of 2.1 eV around x = 0.6, which is significantly smaller than those typically observed in oxychlorides, allowing the absorption of a wider range of light wavelengths. Given the predominance of materials with a double fluorite layer in previous studies, our findings highlight the potential of compounds endowed with triple or thicker fluorite layers as a novel platform for band engineering that utilizes biaxial strain from the inner layer(s) to finely control their electronic structures.

Bi12O17Cl2 with a Sextuple BiO Layer Composed of Rock‐Salt and Fluorite Units and its Structural Conversion through Fluorination to Enhance Photocatalytic Activity

AbstractLayered bismuth oxyhalides with bilayered (Bi2O2) fluorite (FL) slabs are promising visible‐light photocatalysts because of their excellent stability and the ability to adjust band levels depending on the layers combined. It is interesting to manipulate the Bi2O2 slab itself, but only trilayered FL blocks (e.g., Bi3O4) are reported so far. Here, a structurally uncharacterized Bi12O17Cl2, which is extensively studied as a photocatalyst for a variety of reactions, has a sextuple Bi6O8.5 block separated by Cl is shown. Unlike double and triple layered cases, the inner region of the Bi6O8.5 block contains 1D rock‐salt (RS) units in the FL matrix along the a‐axis, causing in‐plane corrugation. A topochemical reaction involving anion‐exchange gives Bi12O17–0.5xFxCl2 (x ≤ 6) with alternate FL and RS slabs along the c‐axis. The elimination of the structural corrugation increases higher photo‐conductivity and improves photocatalytic activity against acetic acid decomposition under visible light irradiation. This study paves new opportunities of controlling the properties of layered bismuth oxyhalides by the thickness of Bi–O block, FL/RS configuration, and structural modulation.

The role of Tb-doping on the structural and functional properties of Bi4\u2212xTbxTi3O12 ferroelectric phases with the Aurivillius type structure

Synthesis, crystal structure, dielectric, and magnetic properties of the Aurivillius phase Bi4−xTbxTi3O12 (x = 0.0, 0.4, 0.6, 0.8) are reported. The samples were synthesized using standard solid state reaction technique. The thermal stability of the obtained solid solutions was investigated. For x ≤ 0.8, the samples crystallized in an orthorhombic symmetry. All the samples showed finite second harmonic generation response indicating a non-centrosymmetric structure. The structural data could be refined using the polar orthorhombic space group B2cb. The orthorhombicity decreases with an increase in the Tb3+ concentration. The orthorhombic distortions in these compositions are related to the Bi3+-based perovskite sublattice. Our results indicate that the non-lone pair Tb3+ cations preferentially occupy the perovskite sublattice initially, but with an increase in the doping concentration they can partially substitute the Bi3+ ions in the fluorite block. Temperature dependent dielectric measurements revealed a decrease in the ferroelectric Curie temperature TC with an increase in x from TC = 904 K (for x = 0) to 877 K (for x = 0.4). Further increase in x led to a cross-over to a relaxor-type behavior. Magnetic measurements showed that the samples are paramagnetic down to 5 K.

Pressure Effect on the Structural Transition and Suppression of the High-Spin State in the Triple-LayerT′−La4Ni3O8

We report a comprehensive high-pressure study on the triple-layer T'-La4Ni3O8 with a suite of experimental probes, including structure determination, magnetic, and transport properties up to 50 GPa. Consistent with a recent ab inito calculation, application of hydrostatic pressure suppresses an insulator-metal spin-state transition at P(c)≈6 GPa. However, a low-spin metallic phase does not emerge after the high-spin state is suppressed to the lowest temperature. For P>20 GPa, the ambient T' structure transforms gradually to a T(†)-type structure, which involves a structural reconstruction from fluorite La-O2-La blocks under low pressures to rock-salt LaO-LaO blocks under high pressures. Absence of the metallic phase under pressure has been discussed in terms of local displacements of O2- ions in the fluorite block under pressure before a global T(†) phase is established.

Nd 6Li(BO 3) 3O 4F 2: A novel neodymium oxyborate fluoride containing two-dimensional Nd layers

Crystal of a new neodymium oxyborate fluoride Nd 6Li(BO 3) 3O 4F 2 was grown by the flux method. Its structure, determined by single crystal x-ray diffraction, belongs to the space group C2/c with cell parameters of a = 12.0629(2) Å, b = 6.94650(10) Å, c = 16.0528(3) Å, β = 104.5360(10)°. In the structure, Nd atoms coordinate to oxygen or fluorine atoms to yeild 7 or 8 coordinated Nd(O,F)n polyhedra. Those polyhedra are edge-shared to form a double layer of (Nd 12O 23F 4) 14− fluorite blocks. The blocks are linked by oxygen atoms of planar BO 3 groups in the c direction into a 3-dimensional network. Another novel element in the structure is that Li coordinates to 6 oxygen atoms from three BO 3 groups forming a propeller like arrangement, and theoretical calculation shows that such arrangement should give 3/4 that of BO 3 contribution to second harmonic effect. The crystal shows deep violet color with typical Nd 3+ optical absorption and a UV transmission cut-off of 260 nm.

Neutron-diffraction study of the crystal structure of the superconductor TiSr 2(Eu 0.8Ce 0.2) 2Cu 2O z

TiSr 2(RE 1− x Ce x ) 2Cu 2O z superconductors (T c ≈20 K) have a 1222-structure with fluorite blocks. Neutron diffraction has been performed on the Eu-containing compound TiSr 2(Eu 0.8Ce 0.2) 2Cu 2O z , with natural Eu, and its crystal structure was refined. The neutron powder diffraction experiment was carried out on the VEGA diffractometer at KENS. The sample was contained in a thin flat cell of 0.5 mm thickness in order to reduce the large absorption effects of natural Eu. The absorption correction for the flat sample was taken into account in the Rietveld refinement. The results show that the O(1) atom in the Ti-layer is shifted from the 4 c-site to the split site (8 j), and that the concentration is deficient ( g≈0.19).

Oxygen stoichiometry in Co-1212, Co-1222 and Co-1232 of the homologous series Co-12 s2 of “category-B” layered copper oxides

The results of a systematic study on oxygen stoichiometry are reported for the first three members of the novel CoSr 2(Y,Ce) s Cu 2O 5+2 s± δ or Co-12 s2 homologous series, i.e. Co-1212, Co-1222 and Co-1232 phases, with a SrO–CoO 1± δ –SrO–CuO 2–(Y,Ce)–[O 2–(Y,Ce)] s−1 –CuO 2 layered structure. The oxygen content was precisely determined by two independent chemical techniques: coulometric Cu +/Cu 2+ titration and iodometric titration. Furthermore, oxygen stability/tunability was investigated by means of oxygenative and reductive annealings carried out in a thermobalance. It was revealed that all the three phases are rather stoichiometric in oxygen content ( δ≈0) and stable against both oxygenative and reductive annealings. The present results provide us with an indication that the nominally oxygen-stoichiometric B–[O 2–B] s−1 “fluorite block”, the characteristic structural element for all layered copper oxides of “category-B” [H. Yamauchi, M. Karppinen, Superlatt. Microstructr. 21A (1997) 128], is non-tunable in terms of the oxygen content not only in the Co-12 s2 ( s>1) phases but also in other category-B phases.

Electronic Structures of (Pb2Cu)Sr2EuxCen-xCu2O2n+6(n= 2,3): Effect of Fluorite Blocks between Adjacent CuO2Layers

The electronic structures of (Pb 2 Cu)Sr 2 Eu x Ce n - x Cu 2 O 2 n +6 ( n = 2, 3) compounds which have fluorite blocks between two adjacent CuO 2 layers have been studied by using ab-initio method. It is found that the anisotropy is enhanced by inserting the fluorite blocks. The Fermi velocity perpendicular to the CuO 2 layers decreases as the thickness of fluorite blocks increases. The Eu substitution is found to affect both the atomic positions and electronic structures. The distance between apical oxygen and copper becomes shorter by the Eu substitution. The energy bands derived from oxygens in the fluorite blocks approach Fermi energy as the content of Eu substitution increases.

Effects of Dy substitution for Ce on transport properties of (Pb 2Cu)Sr 2Dy xCe n− x− δCu 2O 2 n+6 ( n=5,6) epitaxial films

The crystal structure, in-plane resistivity, and thermopower for (Pb 2Cu)Sr 2Dy x Ce n− x− δ Cu 2O 2 n+6 (Pb-32 n2 phase, n=5 or 6) have been studied as a function of Dy content, x, in the fluorite block to ascertain the limit of hole doping. The samples were grown by sequential deposition using the molecular beam epitaxy technique. For n=5, a single phase sample was obtained in the range of 0.9< x<3.2. The hole density was estimated from the thermopower. With increasing x from 1.0 to 1.6, the hole density slightly increased. For the sample with x>1.6, resistivity and thermopower abruptly increased beyond the value for x=1.0, suggesting the electronic state change without changes in crystallinity. The ab-initio electronic structure calculations suggest that the substitution of Ln 3+ for Ce 4+ in the fluorite block has two effects detrimental to conducting properties and superconductivity: (a) creation of vacancy or the hole trap at the oxygen site in the fluorite block, and (b) decrease of the distance between Cu and apical oxygen. These effects may explain the experimental results. They are also considered to be the reasons for the absence of superconductivity in the layered cuprates having a multiple fluorite-type block with n⩾3.

Design and Optical Properties of Rare Earth-Based Mixed-Anions (O, S, F) Compounds

ABSTRACTRare earth-based mixed-anions (O, S, F) compounds have been prepared by solid state route. The structures have been determined on the basis of powder diffraction (X-ray/neutrons) refinement and/or single crystal analysis. They have been described as a stacking of [Sx] 2x- layers and fluorite type blocks. The evaluation of the rare earth environment (bondlength, number of neighbors) can be directly related to the bandgap and to the UV-visible absorption in the La-based compounds.

The crystal structures of BiTeO3I, NdTeO3X (X=Cl, Br) and Bi5TeO8.5I2: some crystal chemistry peculiarities of layered Bi(Ln)Te oxyhalides

Four new layered oxyhalides related to the Sillen family have been prepared and characterized by Rietveld refinement of powder X-ray and neutron diffraction data. BiTeO3I and NdTeO3Br both adopt tetragonal symmetry, space group P4/nmm (for BiTeO3I, a=4.10811(8), c=27.988(1) Å; NdTeO3Br, a=4.06603(7), c=26.922(1) Å, at 25°C). The structures are composed of triple and double fluorite-related mixed metal oxide layers separated by single and double halogen layers, in the sequence MTe2O5XXMTe2O5XM2O2X, which may be represented by the symbol X13X13X22, where the subscript signifies the number of halogen layers and the superscript the number of metal sublayers within the fluorite block, by analogy with Sillen’s notation. The double fluorite layers are occupied exclusively by Bi, whereas there is an ordered arrangement of Bi/Te within the triple fluorite layers, with Te exclusively occupying the outer sublayers of the block. NdTeO3Cl adopts an orthorhombically distorted form of this structure type, space group Pmmn, a=4.08096(8), b=4.03441(8), c=25.7582(7) Å at 25°C. Bi5TeO8.5I2 adopts a distorted, non-centrosymmetric version of the simpler X13 structure type, space group Cmm2, a=5.6878(3), b=5.7230(3), c=9.7260(6) Å, consisting of single halogen layers sandwiched between triple fluorite layers, in which there is partial ordering of the Bi/Te cations.

Transport properties of separated pairs ofCuO2planes in(Pb2Cu)Sr2DyxCen−x−δCu2O2n+6(n=3,5)epitaxial films

The in-plane resistivity and thermopower for $\mathrm{Pb}\ensuremath{-}32n2$ phase, ${(\mathrm{P}\mathrm{b}}_{2}{\mathrm{C}\mathrm{u})\mathrm{S}\mathrm{r}}_{2}{\mathrm{Ln}}_{x}{\mathrm{Ce}}_{n\ensuremath{-}x\ensuremath{-}\ensuremath{\delta}}{\mathrm{Cu}}_{2}{\mathrm{O}}_{2n+6}$ $(\mathrm{L}\mathrm{n}=\mathrm{D}\mathrm{y}$ or Eu, $n=3$ or 5), have been studied. The distance, d, between a pair of ${\mathrm{CuO}}_{2}$ planes can be arbitrarily chosen in the $\mathrm{Pb}\ensuremath{-}32n2$ family. So, one can expect the control of electronic anisotropy using this family. The samples were grown by sequential deposition using the molecular beam epitaxy technique. The hole concentration per [CuO], p, was estimated from the thermopower at room temperature. The highest value of p for the Pb-3252 film sample is 0.089, which is within the range for the occurrence of superconductivity in layered cuprates, although resistivity of the Pb-3252 sample shows weak localization behavior. The increase in hole concentration by the substitution of ${\mathrm{Dy}}^{3+}$ for ${\mathrm{Ce}}^{4+}$ is lower than expected from the simple calculation of the valence. We discuss possible reasons for these findings and for the absence of superconductivity. The cation deficiency in the fluorite block is supposed to be one of the reasons for the low efficiency of carrier doping and localization behavior. Sheet conductance per ${\mathrm{CuO}}_{2}$ plane is similar for the Pb-3232 phase $(d=0.87\mathrm{nm})$ and Pb-3252 phase $(d=1.41\mathrm{nm}).$

Synthesis, structure, and XPS characterization of the stoichiometric phaseSr2CuO2F2

Thermogravimetric analysis has revealed that reduction of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{F}}_{2+\mathrm{\ensuremath{\delta}}}$ in ${10%\mathrm{}\mathrm{H}}_{2}{/90%\mathrm{}\mathrm{N}}_{2},$ occurs in two distinct steps on heating up to 930 \ifmmode^\circ\else\textdegree\fi{}C. Whereas reduction to give ${\mathrm{SrF}}_{2},$ SrO, and Cu is complete above 800 \ifmmode^\circ\else\textdegree\fi{}C, a stable intermediate forms in the region between 250 and 450 \ifmmode^\circ\else\textdegree\fi{}C. This has been identified as a new tetragonal $(I4/mmm)$ phase, ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{F}}_{2},$ with unit-cell dimensions $a=3.967(1)\AA{}$ and $c=12.816(2)\AA{}.$ The structure has been determined from powder x-ray-diffraction data and is related to ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ (${T}^{\ensuremath{'}}$-type). Madelung energy, bond valence sum calculations, and F $1s$-XPS data clearly indicate that the ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ions occupy sites within the fluorite block $({\mathrm{Sr}}_{2}{\mathrm{F}}_{2})$ insulating layers. This contrasts with the ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ ($T$-type) structure which is adopted by ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{F}}_{2+\mathrm{\ensuremath{\delta}}}.$

Molecular beam epitaxy growth of a layered cuprate having a multiple fluorite-type block for use in trilayer junctions

(Pb2Cu)Sr2(Eu,Ce)nCu2Oy: n=3–8 (Pb-32n2 phase) has been synthesized by sequential deposition using the molecular beam epitaxy technique. This layered cuprate having a multiple-stacked fluorite-type block is thought to be suitable for use in trilayer Josephson junctions, from the viewpoints of atomic arrangement at the superconductor-insulator interface and the need for an atomically flat thin insulator. The thickness of the fluorite block, which will act as an intra-cell barrier, has been controlled up to 2.2 nm (n=8). Pb-3272/Pb-3212 epitaxial superlattices have also been prepared. The two-dimensional growth with the growth unit of a unit cell is observed by in situ reflection high-energy electron diffraction.

WTh 8Zr 18F 4O 53: A New Superstructure of Fluorite Type, Associating Anion-Excess and Anion-Deficient Blocks

The crystal structure of a new complex oxyfluoride WTh 8Zr 18F 4O 53, a member of a wide series MTh 8Zr 18(F, O) 57, was solved on the basis of single-crystal data obtained on a four-circle diffractometer CAD-4 ( R = 4.2%). The space group is Fm3̄ m ( Z = 4) and the lattice derives from fluorite-type one by the relationship a = 1578.5 pm = 3 a F. This structure is characterized by a fee cationic array with full cationic ordering and by an ordered distribution of two kinds of structural units: (i) anion-excess fluorite units containing "cuboctahedral" clusters, similar to those observed in anion-excess fluorite phases, e.g., β-U 4O 9 and KY 3F 10 (these units are composed of six ZrO 8 square antiprisms, sharing edges with eight ThFO 9 polyhedra, about a cuboctahedral anionic cavity occupied by an extra F anion); (ii) Anion-deficient fluorite units, based on the association, about a single WFO 5 octahedron, of six ZrO 7 and six ZrO 8 polyhedra, in a similar way as in "stabilized" zirconia and Ln n O 2n-2 rare earth oxides (e.g., Pr 7O 12). Then, WTh 8Zr 18F 4O 53 is the first example of a fluorite superstructure composed of an ordered intergrowth of anion-excess Th 8Zr 6F 3O 30 and anion-deficient WZr 12FO 23 fluorite blocks. WTh 8Zr 18F 4O 53 is the anion-excess fluorite superstructure the poorest in anions ever characterized: MX 2.11 . The possibility, demonstrated for the first time, to associate in a fluorite structure anion-excess and onion-deficient parts constitutes a conceptual advance in the knowledge of nonstoichiometry.

Perovskite stacking in the structures of the high temperature cuprate superconductors

Most of the known cuprate superconductors belong structurally to one family and are closely related to each other. Ignoring odd structural details and considering the idealized structures only, most of them adopt a Ruddlesden-Popper (R-P) or an oxygen-deficient R-P structure with or without intergrowth layers which are composed of either a rock-salt type atomic arrangement, chains, bare cations, or a mixture of them. The rock-salt layers found in R-P structures are simply the result of the stacking of perovskite unit cells. Sometimes a fluorite type structural unit can be found, but the structures containing fluorite blocks may also be considered to be of the R-P structure type with minor readjustment of anions in the rock-salt layers to stabilize the structure. A general formula can be used to describe the basic structures of all the cuprate superconductors, even those having fluorite units.

The formation of strontium analogues of YBa 2Cu 3O 7−x

The existence of materials of the type (Ln,Ce) 2Sr 2Cu 3O 8+x (Ln=lanthanide, Y), analogies of the unstable YSr 2Cu 3O 7−x 2 with yttrium replaced by a fluorite block, (Ln,Ce) 2O 2, have yet to confirmed by detailed structural studies. Reported here are the results of an investigation into their formation and structure which indicate the partial substitution of copper on the “Cu1” sites by cerium to be necessary to stabilise the YBCO-type strucure.

Synthesis and structural characterisation of YBCO related phases containing cobalt

The synthesis of (Nd 1.5Ce 0.5)Sr 2CoCu 2O 91 a fluorite-type block material containing Co 3+, was recently reported in an impure form with its structure related to YBCO with Y 3+ replaced by a fluorite-type block of composition (Nd,Ce) 2O 2, Ba 2+ by Sr 2+ and the copper on the Cu1 sites wholly by Co 3+. This material was reported to be tetragonal, in contrast to its non fluorite block analogue YSr 2CoCu 2O 7, which exhibits long range directional ordering of chains of corner linked CoO 4 tetrahedral, producing orthorhombic symmetry. In this study the presence of a high level of impurities was confirmed and attributed to the formation of the perovskite impurity NdCoO 3 and subsequently a single phase product was synthesised when neodymium was replaced by yttrium, forming (Y 1.5Ce 0.5)Sr 2CoCu 2O 9. Neutron diffraction indicates the presence of long range directional order of CoO 4 tetrahedra chains. No superconductivity was observed.

Synthesis and superconducting properties of the fluorite block system [Y/Ce] 2Sr 2−xBa xCu 3O 9−z substituted by oxy-anions (BO 3−3, SO 2−4, PO 3−4)

The partial substitution of the oxy-anions BO 3− 3, SO 2− 4, and PO 3− 4 for Cu has been successfully achieved in [Y/Ce] 2Sr 2−xBa xCu 3O 9−z. The highest substitution level (80% replacement of the “chain” Cu sites) was observed for BO 3− 3 ions, and was accompanied by a significant reduction in the unit cell c axis length. For SO 2− 4 and PO 3− 4 groups, the substitution limit was lower (∼25% of the “chain” Cu sites). The samples were non-superconducting as prepared, but superconductivity (T c onset∼15–20K) could be induced by annealing in a high pressure oxygen atmosphere.

Homologous compound series containing multiple-MO 2-unit fluorite block, (Fe, Cu)Sr 2 (Y, Ce) nCu 2O 4+2n+z( n=1,2,3,·,and z∼1)

We successfully synthesized a new homologous compound series, (Fe,Cu)Sr 2(Y,Ce) nCu 2O 4+2n+z ( z∼1.0). The crystal structures of the compounds with n=1−3 were refined by the Rietveld analysis of X-ray powder diffraction data and the compounds with n≥4 were analyzed by a high-resolution transmission electron microscope eHRTEM). The compound with n=1, (Fe,Cu)Sr 2YCu 2O 6+ z has triple-perovskite (tetragonal YBa 2Cu 3O 6+ z -type) structure and those with n≥2 consist o triple-perovskite blocks, (Fe,Cu)Sr 2(Y,Ce)Cu 2O 6+ z , and fluorite blocks, [(Y,Ce)O 2] n−1 . The number, ( n−1), corresponds to the thickness of the fluorite block. The compound with n=1 exhibited superconductivity and those with n≥2 were semiconductive. The crystal structures and superconducting properties of these homologous series compounds are discussed from a crystal chemical point of view.