Jacques Darriet

Synthesis, crystal structure and magnetic properties of A3A′RuO6 (A = Ca, Sr; A′ = Li, Na)

The compounds A{sub 3}A{prime}RuO{sub 6} (A = Ca, Sr; A{prime} = Li, Na) have been synthesized by solid state reactions and studied by x-ray powder diffraction and magnetometry. They all adopt the K{sub 4}CdCl{sub 6} (Sr{sub 4}PtO{sub 6}) structure with the cations A{prime} and Ru occupying the trigonal prismatic and octahedral sites respectively. Infinite chains of octahedra and trigonal prisms sharing faces run parallel to the c axis, the chains being separated by the cations A (Ca, Sr). The validity of this description of the structure is discussed in the light of magnetic susceptibility data which suggest that these phases all transform to a magnetically ordered state in the temperature range 70 < T/K < 120. Preliminary results of powder neutron diffraction experiments confirm the presence of long-range magnetic order in the low temperature phase.

Preparation, thermal stability and crystal structure of a newruthenium(V) oxide containing peroxide ions:Ba5Ru2O9(O2). Structuralrelationships to thehexagonal-type perovskite

The compound Ba n 5 nRu n 2 nO n 11 n has been nsynthesized by solid state reactions at 980 °C and 1 atm oxygen npressure. The crystal structure has been determined on a single crystal n(R n 1 n=0.027, wR n 2 n=0.058). The nsymmetry is hexagonal (P6 n 3 n /mmc) with the ncell parameters a=5.9470(5) A, c=18.0428(10) nA, Z=2. Perpendicular to the c-axis, the structure nis built up by the periodic stacking of three hexagonal close packed n[BaO n 3 n ] layers separated by a layer of composition n[Ba n 2 nO n 2 n ] containing n(O n 2 n) n 2- n peroxide ions. The detailed formula is nthen Ba n 5 nRu n 2 nO n 9 n(O n 2 n) if we nemphasize the formation of peroxide ions in the phase. The ruthenium(v) nions occupy the octahedral sites formed between the [BaO n 3 n ] nlayers (hexagonal perovskite slabs) and then constitute isolated n[Ru n 2 nO n 9 n ] pairs. nBa n 5 nRu n 2 nO n 9 n(O n 2 n) belongs to a new nfamily of compounds, generically formulated as n[A n 2 n(O n 2 n)][A n n nB n n n-1 nO n 3n n] wheren represents the nnumber of [AO n 3 n ] successive layers in the hexagonal perovskite nslabs. Each member of the series is an intergrowth of n[A n 2 n(O n 2 n)] and hexagonal perovskite layers; nn=3 corresponds to nBa n 5 nRu n 2 nO n 9 n(O n 2 n). The nisostructural phase has been observed when ruthenium is replaced by nniobium. The n=4 member and the intergrowth between the nn=3 and n=4 members of this family has been observed in nthe Ba–Nb(Ta)–S ternary systems. The former proposed formula nfor the ternary barium sulfides is discussed in the light of the framework nof this new family.

Modulated structure ofBa6ZnIr4O15; a comparison withBa6CuIr4O15 andSrMn1-xCoxO3-y

The new phase Ba n 6 nCuIr n 4 nO n 15 n has been nshown by powder X-ray diffraction to be a member of the perovskite-related nstructural family A n 3n+3 nA′ n n n nB n n+3 nO n 6n+9 nwithn=1; space ngroup R32, a=10.1196(3), c=13.4097(4) A. nThe trigonal-prismatic A′ sites and the octahedral B sites are both noccupied by a disordered distribution of Ir and Cu. A combination of nelectron microscopy and X-ray diffraction has shown that nBa n 6 nZnIr n 4 nO n 15 n has a related incommensurate nstructure with a≈10.1, c≈4.4 A and a nmodulation vector q n= n a n*+0.35 n c n*. Alternatively, nBa n 6 nZnIr n 4 nO n 15 n can be considered as a ncomposite of trigonal (cell 1) and rhombohedral (cell 2) substructures nwith a n 1 n=a n 2 n=10.1228(9) nc n 1 n=4.4099(6), c n 2 n=2.6982(2) nA. It is shown that the same structural formalism can be used to ndescribe the incommensurate hexagonal phase of nSrMn n 1-x nCo n x nO n n3-y n.

Incommensurate phases in the Ba-Mn-Pd-O system

Polycrystalline samples of bulk composition Ba n 5 nMn n 3 nPdO n 12 n, Ba n 6 nMn n 4 nPdO n 15 n, and Ba n 7 nMn n 5 nPdO n 18 n have been synthesised and characterised by X-ray diffraction, electron diffraction and magnetometry. These compounds adopt a 2H-related crystal structure consisting of [001] chains of MnO n 6 n and PdO n 6 n polyhedra, with Ba n 2+ n cations separating the chains. The three compounds, each of which is commensurate in the xy plane but incommensurate along the z axis, differ in the ratio of octahedra to trigonal prisms in the chains (3:1, 4:1 and 5:1 respectively). The lattice parameters of the four dimensional trigonal unit cell are typically a≈10.02 A, c n 1 n≈4.3 A, c n 2 n≈2.61 A. The homogeneity and crystallinity of the samples increases with the ratio of octahedra to trigonal prisms and is greater in the xy plane than parallel to z. The magnetic susceptibility of all three phases deviates from the Curie-Weiss law below 100 K, but no magnetic phase transition is apparent above 5 K.

Nitric oxide decomposition over BaRuxBi1−xO3 (X=0.33, 0.50, 0.67, 1.00) perovskite-like catalysts

The catalytic activity of Ba2Ru0.67Bi1.33O6, Ba2RuBiO6, Ba3Ru2BiO9 and BaRuO3 towards NO-decomposition prior to and after treatment with NH3 has been studied. About 100% degree of conversion has been achieved at 400°C for samples with the highest percentage of hexagonal stacking in their crystal structures. It has been shown that the active sites are units of two or three face-sharing [RuO6] octahedra in which a strong Ru−Ru interaction takes placevia the common faces.

Commensurate and Incommensurate Oxide Structures Related to 2H Perovskite

The magnetic structures of Ca 3 LiRuO 6 and Ca 3 NaRuO 6 have been deduced from powder neutron diffraction data. In each case successive Ru 5+ cations in the [001] chains of the Sr 4 PtO 6 -like structure are antiferromagnetically coupled, and nearest-neighbour Ru 5+ cations in adjacent chains are also antiferromagnetically coupled.

Crystal structure and magnetic properties of Ba10(MnFeF11–xClx)3FxCl2 –x(x= 0.85). Structural relationships with the apatite-type structure

The crystal structure of the chlorofluoride Ba10(MnFeF11–xClx)3FxCl2 –x(x= 0.85) has been determined from single-crystal X-ray data with the following parameters: a= 11.075(2)A and c= 8.173(2)A, and the trigonal P31m space group. The residual values (R values) are R1 = 0.0275 (based on Fo) and wR2 = 0.0584 (based on Fo2) for 1384 unique reflections and 103 parameters. The structure can be described in terms of infinite chains of face-sharing MBa6(M = F or Cl) octahedra, located at the origin of the unit cell and parallel to [001]. Two other Ba atoms occupy the (1/3,2/3,0) and (2/3,1/3, ca. 0.5) positions. This network is similar to that encountered in the apatite-type structure [Ca5(PO4)3(F,OH,Cl)]. The structure is highly disordered with the Mn and Fe atoms forming either isolated dimeric [MnFeF11 –xClx] groups of corner-sharing octahedra (85%) or as two isolated units (15%). This result has been confirmed by the magnetic properties and a theoretical model is proposed to fit the thermal variation of the magnetic susceptibility. The homologous phase with Fe2+ and Cr3+ has been prepared. The corresponding unit-cell parameters are: a= 11.033(2)A and c= 8.180(2)A. The structural relationships with the apatite-type structure and related phases are discussed.

Crystal structure of a new lithium iron vanadate

Iron vanadates and phosphates have been widely explored [1-2] as possible electrode material for Li-ion batteries. In the goal of finding new materials, our approach was to consider existing materials and to investigate the flexibility of their network for possible substitutions. Among the different materials containing iron and vanadium, Cu3Fe4(XO4)6 (X = P, V) are isostructural to Fe7(PO4)6. Lafontaine et al. [3] discussed the structural relationships between β-Cu3Fe4(VO4)6 and several other vanadates, phosphates and molybdates of general formula AxBy(VO4)6. The interesting network flexibility was then demonstrated with the existence of four different crystallographic sites, which can be partially occupied depending on the x+y value : x+y = 7 for β-Cu3Fe4(VO4)6) and x+y = 8 for NaCuFe2(VO4)3. The LixFey(VO4)6 phase was then prepared considering the substitution of Li+ and Fe3+ for Cu2+ ions in βCu3Fe4(VO4)6 and the existence of an extra site to accommodate the charge compensation (7 ≤ x+y ≤ 8). As expected, a new lithium iron vanadate, isotructural to mineral Howardevansite was then obtained. Single crystal diffraction data were collected at room temperature on Enraf-Nonius CAD-4 diffractometer. Structure was refined with JANA-2006 program package. Mössbauer and magnetic measurements were also used to check the oxidation state of iron ions, to support the obtained crystal structure and to consider any possible structural/magnetic transitions. All the results will be presented and discussed in this presentation.

Structural, thermal, magnetic and electrical studies of the iron oxophosphate Rb{sub 7}Fe{sub 7}(PO{sub 4}){sub 8}O{sub 2}.2H{sub 2}O

Abstract A new iron oxophosphate of composition Rb 7 Fe 7 (PO 4 ) 8 O 2 ·2H 2 O has been synthesized and studied by X-ray diffraction, TG and DTA analysis, magnetic susceptibility, neutron diffraction, Mossbauer spectroscopy and ionic conductivity. This compound crystallizes in the monoclinic system with the P 2 1 / c space group and the unit cell parameters a xa0=xa08.224(8)xa0A, b xa0=xa022.162(6)xa0A, c xa0=xa09.962(6)xa0A and β xa0=xa0109.41(8)°. Its structure is built up from Fe 7 O 32 clusters of edge- and corner-sharing FeO 5 and FeO 6 polyhedra. Neighboring clusters are connected by the phosphate tetrahedra to form a three-dimensional framework. The Rb + cations and the water molecules are occupying intersecting tunnels parallel to a and c . The presence of water molecules was confirmed by TG and DTA analysis. The magnetic susceptibility measurements have shown the existence of antiferromagnetic ordering below 22xa0K with a weak ferromagnetic component. Additionally, these measurements show evidence for a strong magnetic frustration characterized by | θ / T N |xa0≈xa012. Powder neutron diffraction study confirms the presence of a long range antiferromagnetic order coupled to a weak ferromagnetic component along the b -axis. The strongly reduced magnetic moments extracted from the refinement support the existence of a magnetically frustrated ground state. The Mossbauer spectroscopy results confirmed the presence of only Fe 3+ ions in both five and six coordination. The ionic conductivity measurements led to activation energy of 0.81xa0eV, a value that agrees with the obtained for other rubidium phosphates.

Structural considerations and high-resolution electron-microscopy observations on La_{n}Ti_{n-\delta}O_{3n}(n\geq4\delta)

Abstract Crystals of the compounds LanTin-δO3n are shown, by means of electron diffraction and high resolution electron microscopy, to form a homologous series of hexagonal or rhombohedral structures. The structures can be described as resulting from the perovskite structure by the periodic introduction of intrinsic stacking faults in a cubic stacking of close packed LaO3 layers. The titanium ions, which have mixed valencies, occupy the octahedral interstices between the close packed LaO3 layers, whereas the interstices in the fault planes remain vacant, making up for the nonstoichiometry. The corner-sharing TiO6 octahedra are found to be cooperatively tilted about the normal to the layer planes. The cut and projection method is used to analyze the electron diffraction patterns and to determine the stacking sequences from geometrical features of the diffraction patterns. Six different stacking sequences are confirmed by direct imaging. High resolution images support the tilt scheme of the TiO6 octahedra. The formation mechanism of the regular block sequences is discussed and the defects which allow this mechanism to operate are modeled and visualized.