Structural determination of some MPS3 layered phases (M = Mn,Fe, Co,Ni and Cd)

The structures of most of the MPS₃ phases (M = Mn, Fe, Co, Ni, Cd) were determined from single crystal analysis. They show the expected CdCl₂ structural type, some weak disorder on the cationic sites being detected on NiPS₃ and CoPS₃. Fair stoichiometry is inferred for the phases since no interslab cation could be seen in the Van der Waals' gap of the structures. Polytypism may occur in the case of NiPS₃.     

Transformation des composes Ba2M2+F6 (M2+ = Mn,Fe, Co,Ni, Zn,Mg) sous haute pression et haute temperature

New quenchable high pressure phases have been found for the Ba₂M²⁺F₆ compounds (M²⁺ = Mn, Fe, Co, Ni, Zn, Mg). They are isotypic with K₂GeF₆. The synthesis has been performed by subjecting a mixture of 2BaF₂ + MF₂ to 40 kbar and 800°C in a belt-type apparatus. The cation coordination is the same in the high pressure phase as in the low pressure one whereas the fluorine-baryum packing changes from cubic to hexagonal. The previously unknown compounds Ba₂MnF₆ and Ba₂MgF₆ with the normal-pressure structure have been synthesized at 10 kbar and 700°C.     

Nouvelles pyrochlores Cd2M2F6S (M = Mn,Fe, Co,Ni, Cu,Zn)

New pyrochlores of formula Cd₂M₂F₆S have been prepared and studied (X-rays and magnetic measurements). All these compounds, except Cd₂Cu₂F₆S are cubic. Their structure is explained as formed by two sublattices (Cd₂S) and (M₂F₆).     

Flux growth of crystals of MNb2O6 (M = Ni,Co, Mn,Fe)

The growth of crystals of MNb₂O₆ (M = Ni, Co, Mn, Fe) from fluxed melts is described. With M = Ni and Co, platinum crucibles were used, but with M = Mn and Fe, growth was carried out under nitrogen in molybdenum and iron crucibles respectively.     

Chemical Thermodynamics of the Compounds AIIUO2(SO4)2·nH2O (AII = Mn, Fe, Co, Ni, Cu, Zn)

The standard enthalpies of formation of anhydrous 3d transition metal uranyl sulfates and their pentahydrates at 298.15 K were determined by reaction calorimetry. The heat capacities of the compounds A^IIUO₂(SO⁴)₂·5H₂O (A^II = Mn, Fe, Co, Ni, Cu, Zn) and CuUO₂(SO₄)₂ at 7– 300 K were determined by adiabatic vacuum calorimetry, and the thermodynamic functions of these compounds were calculated. The standard entropies and Gibbs energies of formation of these compounds at 298.15 K were determined. The thermodynamics of dehydration of A^IIUO₂(SO₄)₂·5H₂O and of the precipitate-solution equilibria involving these compounds were studied. __________ Translated from Radiokhimiya, Vol. 47, No. 2, 2005, pp. 110–122. Original Russian Text Copyright ? 2005 by Karyakin, Knyazev, Gavrilova.     

Layer-like cation ordering in a perovskite-type compound Ca3(CaTa2)O9 with monoclinic symmetry

A structural scheme to fit in a monoclinic cell of the ordered perovskites Ca₃(XZ₂)O₉ is proposed based on a layer-like ordering of X and Z ions. According to this scheme, the structure of compound Ca₃(CaTa₂)O₉ was analyzed with X-ray powder method. The structure is found to involve an alternate stacking of one Ca-layer and two Ta-layers in the octahedral cation sites, and has a base-centered monoclinic Bravais lattice with the constants a=9.808 A,b=5.533 A,c=7.061 A, and ß=89°1′. The space group is $\text{C}\text{2}\text{m-C}{}_{\mathrm{2h}}{}^3$. The unit cell contains six triclinic perovskite units with the lattice constants a′=b′=4.000 A, c′=3.996 A, α′=ß′=89°32′, and γ′=87°32′.     

Structural characterization of mixed conducting perovskites La(Ga,M)O3−δ (M=Mn, Fe, Co, Ni)

Comparative analysis of the structure refinement results of perovskite-like LaGa_0.5M_0.5O_3−δ (M=Mn, Fe, Co, Ni) and data on other LaGaO₃-based phases, heavily doped with transition metal cations, shows that on doping the structural changes in these oxides follow common trends for the perovskite-type systems. The maximum ionic conductivity, observed in various perovskites when the tolerance factor values are approximately 0.96-0.97, was found to correlate with the transition from orthorhombic to rhombohedral structure and maximum lattice distortion. The perovskite unit cell distortion near the orthorhombic-rhombohedral phase boundary may hence play a positive role in the ionic transport processes.     

Stability of the perovskite phase LaBO3 (B = V,Cr, Mn,Fe, Co,Ni) in reducing atmosphere I. Experimental results

The chemical stability of perovskites LaBO₃ where B = V, Cr, Mn, Fe, Co, Ni was studied by thermogravimetry at 1000°C in gas mixtures of $\text{CO}{}_2\text{H}{}_2$, $\text{O}{}_2\text{CO}{}_2$ and $\text{O}{}_2\text{Ar}$ at 1 bar.The stability limits of the perovskite phases expressed in terms of -log Po₂★ (Po₂★ = critical oxygen partial pressure in bar) were for LaCrO₃ and LaVO₃ (greater than 21.1), LaFeO₃ (16.95), LaMnO₃ (15.05), LaCoO₃ (7.0) and for LaNiO₃ (～0.6). The changes in standard enthalpy ΔH° and entropy ΔS° of the following reactions were obtained.$\text{LaVO}{}_4\text{=}\text{LaVO}{}_3\text{+}\text{1}\text{20}{}_2\text{δ=328}\text{kJ}\text{mol}\text{°=135}\text{J}\text{mol·deg.}$,$\text{LaMnO}{}_3\text{=}\text{1}\text{2}\text{La}{}_2\text{O}{}_3\text{+}\text{MnO}$     

Ionic conductivity of solid lithium ion conductors with the spinel structure: Li2MCl4 (M = Mg,Mn, Fe,Cd)

The ionic conductivity of the polycrystalline samples of the lithium chloride spinel, Li₂MCl₄ (M = Mg, Mn, Fe, Cd) is measured by the help of ac techniques in the temperature range of 25–500°C. The highest conductivity of 0.3 Scm^?1 at 400°C is obtained in Li₂CdCl₄. In addition, the solid solution, Li_2?2δM_1+δCl₄, has been examined. It is found that Li_1.9Cd_1.05Cl₄ has higher conductivity than that of Li₂CdCl₄. The phase transitions of these compounds are also discussed.     

Monoclinic perovskite family of 1:1 ordered A2(B′B′')O6-compounds caused by g-type antiparallel displacements of a ions

Most of ternary perovskite-type oxides ABO₃ having small tolerance factors are known as distorted to be orthorhombic (P_bnm). While, the most ordered perovskite-type compounds A₂(B′B′')O₆, which have similarly small tolerance factors as the orthorhombic ABO₃, are distorted to be monoclinic. The structural reason is proposed as the antiparallel displacements of A ions in c-planes, which are assumed to be the driving force to deform ABO₃ into the orthorhombic, are forced to tilt cooperatively out of the c-planes by the cationic field due to the rock-salt arrangement of B′ and B′' ions in A₂(B′B′')O₆, resulting the monoclinic A₂(B′B′')O₆-compounds.     

Proprietes electriques,magnetiques et structurales des phases mxtis2 (M = Fe,Co, Ni)

The structures of M_xTiS₂ phases are reported (M = Fe, Co, Ni; x = .25, .33, .40, .50, .75). Electric and magnetic measurements were carried out from 7 to 400°K. Iron compounds seem to show a competition between ferromagnetism and antiferromagnetism. Cobalt derivatives are mainly antiferromagnetic. For nickel compounds, no paramagnetic moment can be observed, in agreement with an electronic delocalization. Direct and super exchange interactions are discussed, so are energy levels diagrams.     

Ternary and pseudoternary AB2X4 compounds (A = Zn,Cd; B = Ga,In; X = S,Se)

Multication compounds are considered to have interesting applications. Among the variety of possible A_mB_nX_p materials, however, the research work has been unevenly distributed, as the ABX₂ chalcopyrites have attracted the majority of the efforts. The related family of AB₂X₄ defect chalcopyrites, which is characterized by an ordered sublattice of vacancies and then by a further degree of freedom as to tailoring possibilities, has on the other hand received much less attention.Here we discuss the results obtained in the crystal growth and in the structural and dynamical (Raman) analysis of about sixty different ternary and pseudoternary AB₂X₄ compounds or phases.The reaction products can be grouped in three different structural families: cubic-spinel AB₂X₄ structures (CdIn₂S₄ like), AB₂OX₄ layered hexagonal structures (ZnIn₂S₄ like) and AB₂□X₄ defective tetragonal structures (CdGa₂S₄ like).Our attention has been primarily devoted to the defective phases, which have been obtained in the great majority of the starting compositions studied.     

Physical properties of the quarternary chalcogenides Cu2IBIICIVX4 (BII = Zn,Mn, Fe,Co; CIV = Si,Ge, Sn; X = S,Se)

X-ray, electrical and magnetic measurements have been made on several quarternary chalcogenides of the type Cu₂^IB^IIC^IVX₄ with B^II = Zn, Mn, Fe, Co; C^IV = Si, Ge, Sn; X = S, Se. The structures of these compounds result from a superstructure of zincblende or wurtzite. The resistivities have been found to be extrinsic. The magnetic measurements show that the susceptibilities of the compounds containing zinc are independent of temperature, whereas those of the transition metal compounds obey the Curie Weiss law above 77 K. The sulfur compounds have a negative Weiss constant whereas the selenide compounds show a positive Weiss constant.     

Magnetic Resonance in ZnGeP2 and (Zn,Mn)GeP2

Electron paramagnetic (EPR) and ferromagnetic resonance (FMR) have been studied in the system of (Zn,Mn)GeP₂ ferromagnetic layer grown on undoped ZnGeP₂ single crystal. Strong FMR signals are registered in the wide temperature range up to room temperature. EPR and photo-EPR of intrinsic defects are observed in ZnGeP₂ substrate. EPR spectra characteristic of Mn²⁺ ions on Zn²⁺ sites in the bulk appear after the growth of the ferromagnetic layer on ZnGeP₂ crystal indicating the efficient Mn-diffusion into the bulk crystal by the annealing treatments.     

Structural evolution of the double perovskites Sr2B′UO6 (B′ = Mn, Fe, Co, Ni, Zn) upon reduction: Magnetic behavior of the uranium cations

We describe the preparation of five perovskite oxides obtained upon reduction of Sr₂B′UO₆ (B′ = Mn, Fe, Co, Ni, Zn) with H₂/N₂ (5%/95%) at 900 °C during 8 h, and their structural characterization by X-ray powder diffraction (XRPD). During the reduction process there is a partial segregation of the elemental metal when B′ = Co, Ni, Fe, and the corresponding B′O oxide when B′ = Mn, Zn. Whereas the parent, oxygen stoichiometric double perovskites Sr₂B′UO₆ are long-range ordered concerning B′ and U cations. The crystal structures of the reduced phases, SrB′_0.5−xU_0.5+xO₃ with 0.37 < x < 0.27, correspond to simple, disordered perovskites; they are orthorhombic, space group Pnma (No. 62), with a full cationic disorder at the B site. Magnetic measurements performed on the phase with B′ = Zn, indicate uncompensated antiferromagnetic ordering of the U⁵⁺/U⁴⁺ sublattice below 30 K.     

Solubility of Compounds AI I(BVUO6)2·nH2O in Aqueous Solutions (AI I = Mn, Fe, Co, Ni, Cu, Zn, Cd; BV = P, As)

The solubility of uranophosphates and uranoarsenates of 3(4)d transition elements in dilute HCl solutions was determined. The equilibrium constants and Gibbs energies of the process, and also the Gibbs energies of formation of these compounds were calculated from the experimental data.     

Synthesis and photoluminescence of new phosphors M2(Mg, Zn)Si2O7:Mn (M = Ca, Sr, Ba)

New phosphors M₂(Mg, Zn)Si₂O₇:Mn²⁺ (M = Ca, Sr, Ba) were prepared by sol-gel process, and their luminescent properties in ultraviolet and vacuum ultraviolet region were investigated. The results showed that the (Ca, Sr, Ba)₂MgSi₂O₇:Mn²⁺ samples did not emit any visible light; the Sr₂ZnSi₂O₇:Mn²⁺ and Ca₂ZnSi₂O₇:Mn²⁺ samples showed green light. The Ba₂ZnSi₂O₇:Mn²⁺ sample mainly showed green light under 254 nm excitation and red light under 147 nm excitation. The different emission was due to the Mn²⁺ ions occupied different sites, which were excited selectively. Among the three phosphors Sr₂ZnSi₂O₇:Mn²⁺ showed the highest green emission intensity, and its decay time was shorter than that of Zn₂SiO₄:Mn²⁺ under 147 nm excitation.     

Dielectric Properties of Ba0.6Sr0.4TiO3-La(B0.5Ti0.5)O3 (B=Mg, Zn) Ceramics

Ba_0.6Sr_0.4TiO₃-La(B_0.5Ti_0.5)O₃ (B = Mg, Zn) ceramics were prepared by a solid-state reaction method, and their microwave dielectric characteristics and tunability were investigated. The ferroelectric-dielectric solid solutions with cubic perovskite structures were obtained for compositions of 10 to 60 mol% La(Mg_0.5Ti_0.5)O₃ and 10 to 50 mol% La(Zn_0.5Ti_0.5)O₃. With the increase of linear oxide dielectric content, the dielectric constant and tunability were decreased and Qf was increased. Ba_0.6Sr_0.4TiO₃-La(Mg_0.5Ti_0.5)O₃ has better dielectric properties than Ba_0.6Sr_0.4TiO₃-La(Zn_0.5Ti_0.5)O₃. 0.9Ba_0.6Sr_0.4TiO₃-0.1La(Mg_0.5Ti_0.5)O₃ has a dielectric constant ε = 338.2, Qf = 979 GHz and a tunability of was 3.7% at 100 kHz under 1.67 kV/mm. The Qf value of 0.5Ba_0.6Sr_0.4TiO₃- 0.5La(Mg_0.5Ti_0.5)O₃ reached 9367 GHz, but the tunable properties were lost.     

Synthesis and properties of A6B2(OH)16Cl2·4H2O (A = Mg, Ni, Zn, Co, Mn and B = Al, Fe) materials for environmental applications

Double layered hydroxide materials of composition A₆B₂(OH)₁₆Cl₂·4H₂O (A = Mg, Ni, Zn, Co, Mn and B = Al, Fe) were synthesized by chemical precipitation at 60 °C. Different levels of crystallinity and ordering degree were observed depending upon the chemical environment or the combination between divalent and trivalent cations. The results from high-resolution transmission electron microscopy revealed that nanostructured layered samples were obtained with interplanar spacing compatible with previous literature. Raman scattering was employed to investigate the complex band structure observed, particularly the lattice vibrations at lower frequencies, which is intimately correlated to the cationic radius of both divalent and trivalent ions. The results showed that strongly coordinated water and chloride ions besides highly structured hydroxide layers have a direct influence on the stability of the hydrotalcites. It was observed that transition and decomposition temperatures varied largely for different chemical compositions.