Marielle Huvé

From Bi4V2O11 to Bi4V2O10.66: the VV–VIV transformation in the aurivillius-type framework

Bi4V2O11, the parent compound of the BIMEVOX family, has been studied by TEM and HREM, at room temperature and at high temperatures. Under the experimental conditions, Bi4V2O11 undergoes a VV–VIV reduction, ultimately leading to the formation of Bi6V3O16(Bi4V2O10.66) with VIV/VV= 1/2. This transformation is confirmed by in situ X-ray diffraction studies of Bi4V2O11 under reducing atmospheres up to 330 °C. The symmetry of the α-Bi4V2O11 polymorph, stable at ambient temperature, is dependent on the chemical purity of V2O5 used for the synthesis.

From the mixed valent 6H-Ba3Ru5.5+2NaO9 to the 6H-Ba3(Ru1.69C0.31)(Na0.95Ru0.05)O8.69 oxycarbonate compound

Abstract Ba3Ru2NaO9 single crystals have been prepared by electrosynthesis in molten NaOH. It adopts a 6H perovskite crystal structure, a=5.8645(6) A, c=14.440(2) A, space group P63/mmc, Z=2, R1=1.84%, wR2=3.91%. The mean ruthenium valence is 5.5+ for a unique Ru crystallographic site suggesting itinerant electrons within Ru2O9 dimers. Previously to our study, a Ru(V)/Ru(VI) charge ordering has been evidenced at 210 K by both single crystal XRD and susceptibility measurements. The resistivity measurement versus temperature performed in the current work shows a brutal R increase at this temperature. Below this temperature, a residual magnetic moment is observed on magnetization plots but has not been observed at 2 K. An additional anomaly is also evidenced on the R versus T plot, at T2=50 K. It is related to a magnetic transition of questionable origin appearing around the same temperature. Attempts to prepare the title compound by solid state reaction lead to a new related-phase. In fact, combined X-ray and neutron diffraction data unambiguously show the presence of CO2−3 anions in the material. Thus, part of the Ru2O9 dimers are replaced by one RuO5 square pyramid and one CO3 group leading to the nominal Ba3(Ru1.69C0.31)(Na0.95Ru0.05)O8.69 formula. The carbonates typical vibration bands have been observed by infrared spectroscopy and clearly distinguished from possible BaCO3 impurity bands. Compared to the ideal 6H-Ba3Ru5.5+2NaO9, the oxycarbonate main characteristic is the ruthenium +5.28 mean valence. Numerically, such a valence can be obtained considering all the dimeric ruthenium with a +5 oxidation number and the RuO5 and (Na/Ru)O6 corner sharing octahedra to be +6. The structure has also been refined by Rietveld analysis of powder neutron diffraction data recorded at 20 K. No structural difference is observed at low temperature. The Ba/Na/Ru oxycarbonate shows sensitive modifications of its physical properties as compared to Ba3Ru2NaO9. Its conductivity obeys an Arrhenius law and no transitions is observed on cooling. The magnetic susceptibility shows a Curie–Weiss behavior until 120 K, afterward, a weak magnetic moment appears and may be due to the RuO5 magnetic interaction with the other magnetic moieties. Electron diffraction patterns show a superstructure phenomenon in the (a,b) plane for Ba3Ru2NaO9 while diffuse lines parallel to c ∗ are observed for the oxycarbonate. The HREM contrast has been satisfactorily simulated and explained on the basis of Ba contrast towards lighter species within the 6H blocks but does not allow to distinguish between both compounds.

A 116 K superconductor with “1223” structure: TlBaSrCa2Cu3O9−gd

Abstract A new superconducting thallium cuprate, TlBaSrCa 2 Cu 3 O 9−δ has been synthesized. The powder X-ray diffraction, electron diffraction and high resolution electron microscopy studies confirm that this oxide is isostructural with the “1223” TlBa 2 Ca 2 Cu 3 O 9−δ and exhibits a high regularity in the layer stacking along the c -axis. The critical temperature of the as-synthesized sample ( T c ∽ 103 K) is improved by reducing annealing ( T c ∽ 116 K). The influence of the post-annealings, at low temperature, on the physical properties ( T c and irreversibility line) is discussed.

Novel Tailormade Bi4MO4(PO4)2 Structural Type (M = Mg, Zn)

In the Bi(2)O(3)-MO-P(2)O(5) ternary system, the commonly observed sizable 1D ribbon-like units have been extended to their 2D infinite end member, leading to the novel tailormade Bi(4)MO(4)(PO(4))(2) compounds. It contains planar [Bi(2)O(2)](2+) derivatives, separated by two slabs of PO(4), which create channels hosting the M(2+) cations (M = Mg, Zn). For both compounds, supercell orderings occur comparatively to the predicted ideal crystal structure (V(Mg) = 2V(ideal) and V(Zn) = 8V(ideal)). In the Mg case a transition into the ideal lattice occurs above 450 °C. In spite of the conceptual assembly of 2D motifs, the final architecture is three-dimensional due to strong interbonds. Thus, our work gives new insights on the possibility for versatile organization of original secondary building units (SBUs) able to self-assemble into predicted structural edifices. Single-crystal and powder XRD versus temperature, high-temperature (31)P NMR, as well as transmission electron microscopy were used for structural characterization. Preliminary electric characterization is also reported.

A new class of mono-dimensional bismuth-based oxide anion conductors with a structure based on [Bi12O14]columns

Abstract Bi26Mo10O69, i.e. [Bi12O14]2[Bi2Mo10O41] is a pure oxide anion conductor. It can be considered as the first member of the bismuth-based mono-dimensional oxide anion conductors with a skeleton built upon [Bi12O14]∞ columns. In order to modulate its electrical performances, attempts of substitutions in bismuth sites and molybdenum sites were performed. Several dopants have been checked: (i) PbII, CaII, SrII, BaII for bismuth sites, (ii) PV, WVI for molybdenum sites. Six solid solutions, formulated Bi26−xMexMo10O69−0.5x with Me=Pb, Ca, Sr, Ba, Bi26Mo10−xPxO69−0.5x and Bi26Mo10−xWxO69, were characterized. The substitution ratio in bismuth site is systematically limited to two, indicating that the bismuth atoms located in the columns were not affected. In a general way, introduction of oxygen vacancy in the parent compound led to a small decrease of its performances. However the electrical performances of these materials were considerably enhanced by partial substitution for molybdenum with tungsten, giving rise to very attractive properties.

Inorganic polar blocks into controlled acentric assemblies.

We show here a strategy to predict the crystal structure, formulate, and prepare new noncentrosymmetric (NCS) bismuth-phosphate based compounds. It is based on the cooperative-arrangement of polar building units (BUs) which can be created at particular stoichiometric conditions. The arrangement of such BUs into NCS compounds arise from the shortest-periodicity of repartition of the cationic charges in NCS structures than in the plausible, but never observed centrosymetric polytypes. This work validates the possibilities for the prediction of an extended series of novel compounds, tuning the size of BUs within a variety of controlled edifices. Despite their closed chemical composition, all the predicted terms appeared strikingly stable at precise stoichiometries.

Ferroelectricity in La2Zr2O7 thin films with a frustrated pyrochlore-type structure

Thin films of lanthanum zirconate (La2Zr2O7) have been grown by a sol–gel route on (110)-oriented SrTiO3 substrates. Electrical measurements, locally performed by piezoresponse force microscopy, evidence unambiguously the ferroelectric state of the films at the nanoscale level. In the La2Zr2O7 bulk material, ferroelectricity is absent due to the centro-symmetric cubic pyrochlore structure. In thin films, the extensive study carried out by high resolution X-ray diffraction highlights a lowering of the cubic symmetry that may explain the emergence of ferroelectricity. This slight structural modification is interpreted as a geometrical frustration induced by the substrates strains during the film growth. In addition, pole figure experiments are used to give epitaxial relationships between the La2Zr2O7 film and the SrTiO3 substrate. Finally, high resolution transmission electron microscopy images obtained on a cross-section of the film are given.

Reversible Topochemical Exsolution of Iron in BaFe2+2(PO4)2

BaFe(2+) 2 (PO4 )2 was recently prepared and identified as the first 2D-Ising ferromagnetic oxide with an original reentrant structural transition driven by high-spin Fe(2+) ions arranged in honeycomb layers. Both long-term air exposure and moderate temperature (T>375 °C) leads to topochemical oxidation into iron-depleted compounds with mixed Fe(2+) /Fe(3+) valence. This process is unique, as the exsolution is effective even from single crystal with preservation of the initial crystallinity, and the structure of the deficient BaFe2-x (PO4 )2 (x<ca. 0.5) is fully ordered for x=2/7 and 1/3 with creation of novel original depleted triangular lattices. Under flowing H2 /Ar, Fe is reincorporated in the structure above 480 °C, as reproduced under the electron beam in a transmission microscope. After Fe exsolution, the insulating ferromagnetic compound turns into an antiferromagnetic semiconductor.

Multidimensional open-frameworks: combinations of one-dimensional channels and two-dimensional layers in novel BI/M oxo-chlorides.

Here we discuss the synthesis and characterization of three novel bismuth oxo-chlorides ([Bi6Na0.5O7.5][Na0.5Cl3]channel[Cl]layer; [Bi17PbO22][Cl6]channel[Cl3]layer; [Bi9(Pb0.2Mn0.8)O12][Cl3]channel [Cl2]layer) which all show an original multidimensional crystal structure. It is formed of two-dimensional (2D)-layered blocks separated by Cl(-) layers. The blocks are porous with triangular one-dimensional (1D)-Cl(-) channels with various section sizes. This multidimensional feature is unique in the field of Bi and Pb oxo-halides, while so far only 1D or 2D halides units have been reported. The stability of the framework is allowed by Bi(3+)/M(n+) aliovalent substitution to balance charge neutrality. The channel and tunnel walls are formed by edge-sharing O(Bi,M)4 oxocentered tetrahedra, while the triangular tunnel junctions are achieved by O(Bi,M)5 pyramids. The three compounds are rather stable, but only [Bi6Na0.5O7.5][Na0.5Cl3]tunnel[Cl]layer was obtain as a single-phase material so that its photoluminecence properties have been investigated. It shows an unusual red bright luminescence with a maximum at 14150 cm(-1) at low temperatures due to Bi(3+) transitions that are well explained by the Bi-Cl bonding scheme.

Labile degree of disorder in bismuth-oxophosphate compounds: Illustration through three new structural types

Here, we analyze the crystal structures of three new Bi/M oxophosphates, focusing on the ambiguity between order and disorder in different structural subunits. The three structures are original but systematically built on the assembly of O(Bi,M)4 tetrahedra into various 1D-oxocenterd units, separated by PO4 groups that create cationic channels. Two main subunits show versatile degrees of disorder, i.e., the cationic channels and some of the terminal O(Bi,M)4 entities. (a) In the compound [Bi2(Bi1.56K0.44)(dis)O3]K0.88(dis)(PO4)2, the K/K and K/Bi disorder is total on both nano- and micro-sized domains. (b) In the incommensurately modulated [Bi10(Bi∼0.5Cd∼0.5)8(dis)O16](Bi0.6Cd0.8)2(ord)(PO4)8, only the cationic channels show an ordered Bi/Cd arrangement which can be modified by minor stoichiometric changes between domains. (c) In [Bi18Zn10O21](ord)Zn5(ord)(PO4)14, both subunits are almost perfectly ordered (complex Bi/Zn sequence) into a 7-fold supercell, but this order strongly depends on the observation scale and is mainly lost in micronic-grains also due to slight compositional changes. However, the refined noncentrosymmetric organization is maintained (SHG tests) in the bulk. The relative stability of ordered versus disordered sites is discussed on the basis of the existence of two possible mixed sites and probably depends on the M chemical nature. Disorder was characterized by use of solid-state (31)P NMR probing for the first two cases. Finally, the observed disordered or long periodicities along the infinite dimension suggest the sketch of a periodic/rigid skeleton of O(Bi,M)4 units with counterions filling the interspace in more or less disordered arrangements.