Sur une nouvelle famille de composes MIIMIVLi2F8 de structure apparentee a celle de la scheelite

A new family of compounds of the type M^IIM^IVLi₂F₈ was prepared. These compounds are tetragonal and exhibit a structure related to that of CaWO₄.     

Etude du comportement de l'antimoine v et du strontium dans les oxydes a structure en cages du type A3M8O21

New compounds Ba₃(Sb₄Ti₄)O₂₁ and Ba_3?xSr_x(M₄Ti₄)O₂₁ (M = Nb, Ta, Sb) isostructural with the niobates A₃(Nb_8?xM_x)O₂₁ previously described, have been synthesized. The influence of the antimony and strontium ions on the stability of the A₃M₈O₂₁ structure has been discussed in terms of electrostatic repulsion forces, polarization and covalency. Atomic positions, determined from powder diffraction patterns, are given for Ba₃(Sb₄Ti₄)O₂ and Sr₃(M₄Ti₄)O₂₁ (M = Nb, Ta).     

Nouvelles phases multiples obtenues par intercroissance des reseaux “M6X4O26” et “M8O21”: Les oxydes K10(M8O21)2·M6X4O26

New oxides K₁₀(M₈O₂₁)₂·M₆X₄O₂₆ (M = Nb, Ta; X = Si, Ge) were synthesized. These compounds were investigated by electron diffraction and microscopy, the symmetry is hexagonal, space group P62m, with $\text{a}\text{? 9}\text{A}\text{?}$, $\text{c}\text{? 20}\text{A}\text{?}$. They show an intergrowth of two structural types: (M₈O₂₁) and (M₆X₄O₂₆) layers. Results of structural study by X-ray powder diffraction are discussed in terms of the stabilization of A₃M₈O₂₁ and A_6?xM₆Si₄O₂₆ structures.     

Insertion du lithium dans les oxydes de type A3M8O21: Les composes LiBa3M3Ti5O21

New compounds of formula LiBa₃M₃Ti₅O₂₁ (M = Nb, Ta, Sb) have been obtained by insertion of lithium in the lattice of Ba₃M₄Ti₄O₂₁ (M = Nb, Ta, Sb). They crystallize in the hexagonal system with a filled cage structure of A₃M₈O₂₁ type. Crystallographic studies by X-ray and neutron diffraction show a partial order between Ti^IV and M^V ions (Nb, Ta, Sb) in the octahedral sites of the cell (space group P6₃/mcm).     

Neutron diffraction studies of NiCoP4O12 and NiZnP4O12

The title compounds belong to a group of isostructural $\text{M}$₂P₄O₁₂ tetrametaphosphates with monoclinic ($\text{C}$2/$\text{c}$) symmetry. The two structures have been profile-refined by means of the Rietveld technique on the basis of neutron powder diffraction data. The three-dimensional framework is built up of tetrametaphosphate anions and two distinct metal-oxygen octahedra, (M1)O₆ and (M2)O₆. The latter octahedron is somewhat larger and more distorted than (M1)O₆. The results show that in both solid solutions nickel preferentially enters the M1 sites, a tendency also observed in olivine and sarcopside, which contain $\text{M}$O₆ octahedra similar in shape and symmetry to those in $\text{M}$₂P₄O₁₂.     

Stabilite relative des types structuraux Ca2TlTa5O15 et bronze quadratique de tungstene

The synthesis of new compounds with the general formulations LnNaM^ITa₅O₁₅ or M^II₂M^ITa₅O₁₅ M^II = Ca, Sr, Cd, M^I = K, Rb, Tl, Cs ; Ln = rare earth) leads to a discussion of factors which determine the structural type : either tungsten tetragonal bronze or Ca₂TlTa₅O₁₅ with orthorhombic symmetry. The orthorhombic phase consists of a covalent M^V₅O₁₅ framework which determines three types of cavities, one of type A, two of type B and two others of type C, the coordinations being 18, 12 and 9 respectively. Usually the C sites are unoccupied. Simultaneous presence of a large cation in the A site ($\text{r(VIII)}\text{> 1,58}\text{A}\text{?}$) and a relatively small one in the B site ($\text{1,08}\text{A}\text{?}\text{<}\text{r(VIII)}\text{< 1,19}\text{A}\text{?}$) is a favourable factor for the orthorhombic Ca₂TlTa₅O₁₅ structure. Piezoelectric properties of some phases are discussed in terms of the hardness and softness, as defined by Pearson, of the cations present in sites A and B.     

Syntheses of hollandite type Rb2Cr8O16, K2Cr2V6O16 and K2V8O16

Hollandite-type compounds, Rb₂Cr₈O₁₆, K₂Cr₂V₆O₁₆ and K₂V₈O₁₆, were synthesized under high P-T conditions up to 1200°C and 7GPa. The structural refinement using a single crystal of Rb₂Cr₈O₁₆ confirms that the structure is similar to that of K₂Cr₈O₁₆. Magnetic measurements indicate that Rb₂Cr₈O₁₆ is ferromagnetic below 295K, K₂Cr₂V₆O₁₆ paramagnetic down to 77K and K₂V₈O₁₆ has susceptibility anomaly at 175K. These compounds are all semiconductive and show discontinuities in temperature-resistivity curves at points corresponding to magnetic anomalies.     

Cu5Sb2O8SiO4, an open framework structure with copper uncommon coordination modes: CuO6 and CuO8

Cu₅Sb₂O₈SiO₄ is orthorhombic, space group Pcca, with a = 19.031 (2), b = 9.3944 (6), c = 9.602 (2) Å, and z = 8. Its crystal structure was determined using single crystal X-ray diffraction (R₁ = 0.0432 and wR₂ = 0.1146). The compound presents a parwelite-like structure. Its anionic three-dimensional framework is built up of corner-sharing SbO₆ octahedra and SiO₄ tetrahedra, delimiting interconnected channels wherein Cu²⁺ with different coordination modes, are located. The χ_M and χ_MT product versus T plots, showed the Cu₅Sb₂O₈SiO₄ material to exhibit an anti-ferromagnetic character with a Neel temperature of about 27 K.     

Dielectric properties of the spin-1/2 dimer compounds Ba3Cr2O8 and Sr3Cr2O8

We have investigated the dielectric and magnetic properties of the spin-1/2 compounds Ba₃Cr₂O₈ and Sr₃Cr₂O₈. For Ba₃Cr₂O₈, the real part dielectric constant ε′(T) exhibit a frequency-independent peak near the Jahn-Teller transition temperature T_JT = 70 K. However, no anomaly in the ε′(T) curve is observed near T_JT = 275 K for the isostructural compound Sr₃Cr₂O₈. The difference in the ε′(T) behaviors may be attributed to the different onset temperatures of the phonon mode splitting in the samples. Dielectric relaxation analysis revealed that extrinsic contribution due to Maxwell–Wagner effect dominated at high temperatures for both compounds. The magnetization data reveal the occurrence of antiferromagnetic interactions between the Cr⁵⁺ ions and the χ(T) curves can be described by the Bleaney–Bowers equation for both compounds.     

On the luminescence of Ti4+ In Mg5SnB2O10 and Mg3ZrB2O8

The following new compounds are reported: Mg₅TiB₂O₁₀ (ludwigite structure), Mg₅SnB₂O₁₀ (orthopinakiolite structure) and Mg₃ZrB₂O₈ (warwickite structure). The luminescences of the systems Mg₅(Sn,Tl)B₂O₁₀ and Mg₃(Zr,Ti)B₂O₈ were measured. The results are discussed in terms of delocalization in clusters of titanate octahedra. A general scheme for the luminescence of Ti⁴⁺-activated compositions is given. The composition Mg₅SnB₂O₁₀: Ti is a very efficient luminescent material for titanium concentrations below some 20 mole percent.     

Synthesis and crystal structure of Cu2NiO(B2O5) and Cu2MgO(B2O5)

A transport reaction synthesis technique has been used to prepare single crystals of two pyroborate compounds having the formulas Cu₂NiO(B₂O₅) and Cu₂MgO(B₂O₅). The two compounds are isostructural and crystallize in the monoclinic space group P2₁/c. Cu₂NiO(B₂O₅): a=3.2003(10), b=14.775(3), c=9.097(3), β=93.28(4), V=429.4(2) Å³, Z=4; and Cu₂MgO(B₂O₅): a=3.2401(6), b=14.790(2), c=9.147(2), β=94.88(2), V=436.7(2) Å³, Z=4. The structures of Cu₂NiO(B₂O₅) and Cu₂MgO(B₂O₅) were, respectively, refined from 804 and 1000 independent reflections to the final residuals R1=0.0366, wR2=0.0911 and R1=0.0231, wR2=0.0644. Both compounds exhibit a chevron-like structure built up of ribbons, made of edge-connected copper and nickel-oxygen polyhedra, running along the (1 0 0) direction. These ribbons are connected from one another via oxygen atoms and the cohesion of the three-dimensional network is ensured by [B₂O₅] entities. Cu in part occupies the position for Ni or Mg, so that the compounds actually are solid solution compounds. Ni or Mg atoms are octahedrally coordinated by oxygen, while the two pure Cu sites show [4] and [4+1] coordination, for Cu(1) and Cu(2), respectively. The ELNES B-K edge spectra for the two compounds support that the borate group present is [B₂O₅].     

Preparation,characterization, and some magnetic properties of Eu3VTa2O9 and Sr3VTa2O9 — New perovskites containing vanadium(II)

Two new materials of compositions Eu₃VTa₂O₉ and Sr₃VTa₂O₉ have been prepared and appear to contain vanadium(II), an unusual oxidation state. Powder X-ray data for both compounds can be indexed on a cubic perovskite cell with $\text{a}{}_0\text{= 4.001(3)}\text{A}\text{?}$. These cell dimensions are consistent with the ionic radius of V(II) and the cell dimensions of related Sr₃(Eu₃)M^IITa₂O₉ phases. A set of weak reflections found for both materials can be indexed on an hexagonal supercell of the Ba₃SrTa₂O₉-type (1). Further evidence for B-site ordering is provided by IR absorption spectra. Magnetic susceptibility data show the absence of a local moment on V(II). This behavior is compared with that for VO_x and VTa₂O₆, the only other V(II) oxides yet reported.     

Pressureless sintering of negative thermal expansion ZrW2O8/Zr2WP2O12 composites

Negative thermal expansion material ZrW₂O₈/Zr₂WP₂O₁₂ composite was prepared by liquid phase sintering. The apparent density of ZrW₂O₈ without any sintering additive was about 3.7 g/cm³, corresponding to about 73% of its theoretical density. However, the relative density of the samples, sintered with more than 5 mol% P₂O₅ was about 90%. The identified phases were mainly ZrW₂O₈ with small amounts of WO₃, ZrO₂ and Zr₂WP₂O₁₂ by XRD. The intensity of Zr₂WP₂O₁₂ peaks increased with increasing P₂O₅ content. It was surmised that the melting of ZrO₂-P₂O₅ resulted in liquid phase formation, which is then converted to Zr₂WP₂O₁₂ on the final stage of sintering. Therefore, Zr₂WP₂O₁₂ phase was observed at the gap between the ZrW₂O₈ grains and at the triple junctions. The ceramics sintered with 20 mol% P₂O₅ showed a negative thermal expansion coefficient of − 4.0 × 10^− 6 °C^− 1.     

Synthesis of new binary cobalt iron pyrophosphate CoFeP2O7

A new binary cobalt iron pyrophosphate, CoFeP₂O₇ was synthesized through solid phase reaction using cobalt carbonate, iron metal and phosphoric acid in the presence of water-methanol system with further calcinations at the temperature of 500 °C. The particle size obtained from X-ray line broadening is 36 ± 7 nm for the CoFeP₂O₇. FTIR spectrum of CoFeP₂O₇ is assigned based on the P₂O₇⁴⁻ ion in the structure. The XRD, UV-Vis near IR and FTIR results of the synthesized CoFeP₂O₇ appear to be very similar to that of M₂P₂O₇ (M = Fe and Co), which indicates the monoclinic phase with space group C2/m.     

Fluorite related ordered compounds in the ZrO2CaO and ZrO2Y2O3 systems

ZrO₂:CaO and ZrO₂:Y₂O₃ systems have been examined for possible ordered compounds. Fluorite related ordered compounds CaZr₄O₉ and Zr₃Y₄O₁₂ have been successfully prepared and identified. While it is believed that the former is monoclinic with $\text{a}\text{= 17.813 ± 0.005}\text{A}\text{?}$, $\text{b}\text{= 14.612 ± .004}\text{A}\text{?}$, $\text{c}\text{= 12.065 ± .003}\text{A}\text{?}$ and β = 119.5 ± .02° and isostructural with CaHf₄O₉, the latter appears to be isostructural with M₇O₁₂ type of compounds with $\text{a}\text{= 9.723 ± .001}\text{A}\text{?}$ and $\text{c}\text{= 9.090 ± 0.002}\text{A}\text{?}$, which is formed commonly in compounds containing tri and tetravalent cations and is characterized by a chain oxygen vacancy along a 〈111〉 of the original fluorite. No other fluorite related ordered compound could be detected in these systems.     

Obtention par trempe a partir de l'etat liquide d'une nouvelle phase dans le systeme Nd2O3.Al2O3. etude comparative avec Nd4Ga2O9

By using splat cooling devices associated with laboratory solar furnace a new phase is obtained in the neodymium rich part of the Al₂O₃Nd₂O₃ system. X ray and optical absorption studies and comparisons with Nd₄Ga₂O₉ show that the two phases are isostructural and monoclinic (space group P_21/C) with Al³⁺ in tetrahedral coordination. Optical examinations show that substitution of Al³⁺ by Si⁴⁺ induced a disorder on the neodymium site.     

Synthesis of Bi2Fe4O9 via PVA sol-gel route

Bi₂Fe₄O₉ powders were synthesized by a sol-gel process using polyvinyl alcohol (PVA) as a complexing agent. Differential scanning calorimetry (DSC), thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and Field emission scanning electron microscope (FSEM) techniques were used to characterize precursor and derived oxide powders. The effect of the ratios of positively charged valences to hydroxyl groups of PVA (Mⁿ⁺/-OH) on the formation of Bi₂Fe₄O₉ was investigated. XRD analysis showed that single-phase Bi₂Fe₄O₉ was obtained from the Mⁿ⁺/-OH = 2:1 and Mⁿ⁺/-OH = 1:1 precursors at the temperature of 700 °C. For the precursor with Mⁿ⁺/-OH = 4:1, pure Bi₂Fe₄O₉ formed at the temperature of 800 °C. Bi₂Fe₄O₉ powders clacined at 700 °C from Mⁿ⁺/-OH = 2:1 precursor shows weak ferromagnetism.     

Single step synthesis of Ce doped CaAl2O4 and CaAl4O7 systems

Both CaAl₂O₄ (CA2) and CaAl₄O₇ (CA4) oxide-systems possess monoclinic crystal structure. Herein, we have prepared CA2 and CA4 systems via single step combustion route. A good correlation is observed between calculated and the standard lattice parameters. Ce³⁺ ions were deliberately doped as extrinsic impurities in order to understand the crystal symmetry effects on the emission characteristics in the as-prepared matrices. Large red-shift was observed in CA4-emission spectrum despite of their same crystal structures. Possible reasons are discussed.     

Ferroelasticity in the LnNbO4-type rare earth niobates

The previously reported phase transitions for the isostructural rare earth niobates between 500°C and 850°C correspond to a point group transformation 4/mF2/m, which is purely ferroelastic. The correct room temperature point group for all LnNbO₄ compounds is 2/m. Crystal growth and domain wall behavior is discussed for LaNbO₄. The high temperature phase transition is described for YbNbO₄.     

Preparation, structure and infrared spectrum of NaEuP2O7

Crystals of NaEuP₂O₇ have been grown by the flux technique and characterized by X-ray diffraction. Single crystal structure of NaEuP₂O₇ has been solved, for the first time; it crystallizes in the monoclinic P2₁/n space group with lattice parameters : a = 5.238(2), b = 8.443(4), c = 12.486(6) Å, β = 91.404°(2), V = 552.0(4) ų, Z = 4. The crystal structure has been refined yielding a final R(F²) =0.049 and R_w(F²) = 0.138 for 1313 independent reflections (F_o² ≥ 2σ(F_o²)). In this structure, the PO₄ tetrahedral are linked by bridging oxygen to give P₂O₇ groups, these groups are connected to the EuO₈ polyhedron by sharing two oxygen corners to form three-dimensional framework in which channels are noticed and where the sodium ions are located. The frequencies of the vibrational modes of the crystal were obtained from measurements of the infrared spectra.