Uniform AMoO4:Ln (A=Sr, Ba; Ln=Eu, Tb) submicron particles: Solvothermal synthesis and luminescent properties

Rare-earth ions (Eu³⁺, Tb³⁺) doped AMoO₄ (A=Sr, Ba) particles with uniform morphologies were successfully prepared through a facile solvothermal process using ethylene glycol (EG) as protecting agent. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL) spectra and the kinetic decays were performed to characterize these samples. The XRD results reveal that all the doped samples are of high purity and crystallinity and assigned to the tetragonal scheelite-type structure of the AMoO₄ phase. It has been shown that the as-synthesized SrMoO₄:Ln and BaMoO₄:Ln samples show respective uniform peanut-like and oval morphologies with narrow size distribution. The possible growth process of the AMoO₄:Ln has been investigated in detail. The EG/H₂O volume ratio, reaction temperature and time have obvious effect on the morphologies and sizes of the as-synthesized products. Upon excitation by ultraviolet radiation, the AMoO₄:Eu³⁺ phosphors show the characteristic ⁵D₀–⁷F_1–4 emission lines of Eu³⁺, while the AMoO₄:Tb³⁺ phosphors exhibit the characteristic ⁵D₄–⁷F_3–6 emission lines of Tb³⁺. These phosphors exhibit potential applications in the fields of fluorescent lamps and light emitting diodes (LEDs).     

Formation, structure and magnetism of the metastable defect fluorite phases AVO3.5+x (A=In, Sc)

We report the preparation and stability of ScVO_3.5+x and the novel phase InVO_3.5+x. AVO_3.5+x (A=Sc, In) defect fluorite structures are formed as metastable intermediates during the topotactic oxidation of AVO₃ bixbyites. The oxidation pathway has been studied in detail by means of thermogravimetric/differential thermal analysis and in-situ powder X-ray diffraction. The oxidation of the bixbyite phase follows a topotactic pathway at temperatures between 300 and 400 °C in air/carbon dioxide. The range of accessible oxygen stoichiometries for the AVO_3.5+x structures following this pathway are 0.00x0.22. Rietveld refinements against powder X-ray and neutron data revealed that InVO_3.54 and ScVO_3.70 crystallize in the defect fluorite structure in space group Fm-3 m (227) with a=4.9863(5) and 4.9697(3)Å, respectively with A³⁺/V⁴⁺ disorder on the (4a) cation site. Powder neutron diffraction experiments indicate clustering of oxide defects in all samples. Bulk magnetic measurements showed the presence of V⁴⁺ and the absence of magnetic ordering at low temperatures. Powder neutron diffraction experiments confirmed the absence of a long range ordered magnetic ground state.     

A study of Co/Mo/Al2O3 hydrodesulphurization catalysts and related model compounds byxps and x-ray absorption spectroscopy (xanes andexafs )

A detailed investigation of sulphided Co/Mo/Al₂O₃ catalysts, their oxide precursors and several model oxides and sulphides of cobalt and molybdenum has been carried out using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy (xanes andexafs). Octahedrally coordinated Co(II) and Mo(IV) are shown to be present in a sulphidic environment on the surfaces of these catalysts. The surface species contain an excess of sulphur, probably involving disulphide linkages. The surface compositions of the catalysts examined conform to the general formula Co¹¹ Mo _2n ^IV (2n + 3)S ₂ ²⁻ (2n -2)S²⁻.     

Direct hydrothermal synthesis of metal intercalated hexagonal molybdates, M + x Mo6? x /3O18? x (OH)x.yH2O (M = Li, Rb, Cs, NH4)

Here we report direct hydrothermal synthesis of a few hexagonal molybdates with composition, M _x ^/+ Mo _6−x ^{Emphasis>/3/6+} O_18−x (OH) _x .yH₂O (M = Li, Rb, Cs, NH₄). The molybdates crystallize in the space groupP6 ₃ /m with a r∼ 10.5 andc r∼ 3.7 ? . Unlike previous studies, our work suggests that hexagonal molybdates could be stabilized in the presence of monovalent cations with varying ionic size (smaller lithium to larger cesium) under hydrothermal condition. The phases showed exceptional thermal stability till 550°C. Dedicated to Professor C N R Rao on his 70th birthday     

Phonon modes of A(Co1/2Mn1/2) O3 (A=La, Nd, Dy, Ho, Yb)

Phonon energies in cobaltite/manganites A(Co_1/2Mn_1/2)O₃, where A is a lanthanide, have been determined by far-infrared spectroscopy. The phonon energies systematically shift and split and new modes appear as the mass of the lanthanide is increased through the series A=La, Nd, Dy, Ho, Yb. The behavior of the phonon modes correlates with the magnetic properties of this series of compounds, in particular with the appearance of metamagnetism for the compounds with smaller ions on the A site.     

Ionic mobility and structure of fluorozirconates Rb2? x (NH4) x ZrF6 ( x > 1.5) by NMR, x-ray structure analysis, and impedance spectroscopy

The ionic mobility in the temperature interval 180 to 480 K, structure, and electrophysical properties of rubidium-ammonium hexafluorozirconates Rb_2−x (NH₄) _x ZrF₆ (1.5 ≤ x ≤ 2.0) are studied by methods of the ¹⁹F, ¹H NMR spectroscopy, x-ray structure analysis, differential thermal analysis, and impedance spectroscopy. Correlations between the composition of the cationic sublattice, the character of ionic motions, and the phase transition temperature (of the type order-disorder) are established in these compounds. The salient feature of the high-temperature modifications of these fluorozirconates with x ≥ 1.5 is the translation diffusion of ions inside the fluoride and ammonium sublattices and the ¹⁹F NMR spectra are characterized by monoaxial anisotropy of the magnetic shielding tensor of the fluorine nuclei. Fluorozirconates with x > 1.5 are shown to belong with the structural type (NH₄)₂ZrF₆. The rubidium cations isomorphically replace the ammonium cations. The electrophysical characteristics of the compounds are examined in the temperature interval 300 to 480 K. It is established that the electroconductivity of these compounds increases with x. Original Russian Text ? V.Ya. Kavun, A.V. Gerasimenko, A.B. Slobodyuk, N.A. Didenko, N.F. Uvarov, V.I. Sergienko, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 5, pp. 563–570. Based on the paper delivered at the 8th Meeting “Fundamental Problems of Solid-State Ionics”, Chernogolovka (Russia), 2006.     

Raman study of cation distribution in the scheelite-like double molybdates and tungstates

Compounds M⁺ M³⁺ (TO₄)₂ have been investigated (M⁺, M³⁺ are alkaline and rare earth metals, respectively; T=Mo, W). The way of cation distribution (statistical or orderec) is given by the ratio of their ionic radii. The boundary between the distributions has been determinea: r _ion (M⁺)/r _ion (M³⁺)=1.3–1.32. In molybdates the cations are partly ordered even when M⁺ and M³⁺ have close dimensions because they interact with each other. The compounds KLa(MoO₄)₂ and KN l(MoO₄)₂ fall within the transition range and have, therefore, a complex polymorphic composition. The phase transition from the high-to low-temperature form of KLa(MoO₄)₂ is identical to the corresponding transition in KEu(MoO₄)₂ known from X-ray studies.Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 34, No. 4, pp. 42–58, July–August, 1993.Translated by L. Smolina     

Synthesis and electrical properties of Li1+ x M x Ti2– x (PO4)3 (where M =Sc, Al, Fe, Y; x =0.3) superionic ceramics

Compounds of the system Li_{1+ x} M _x Ti_{2– x} (PO₄)₃ (where M=Sc, Al, Fe, Y; x=0.3) were synthesized by a solid-state reaction and studied by X-ray diffraction. The ceramic samples were sintered and investigated by complex impedance spectroscopy in the frequency range 10⁶–1.2×10⁹ Hz in the temperature range 300–600 K. Two relaxation dispersions related to the fast Li⁺ ion transport in bulk and grain boundaries were found. The activation energies of the bulk conductivity and relaxation frequency were obtained from the slops of Arrhenius plots. The values of the activation energies of the bulk ionic conductivity and relaxation frequency were found to be very similar in all the materials investigated. That can be attributed to the fact that the temperature dependences of the bulk conductivity are caused only by the mobility of the fast Li⁺ ions, while the number of charge carriers remains constant with temperature. Electronic Publication     

Assembly of ϵ-Keggin Polyoxometalate from Molecular Crystal to Zeolitic Octahedral Metal Oxide

Zeolitic octahedral metal oxides are inorganic crystalline microporous materials with adsorption and redox properties. New ϵ-Keggin nickel molybdate–based zeolitic octahedral metal oxides have been synthesized. ³¹P NMR spectroscopy shows that reduction of Mo^VI-based molybdates forms an ϵ-Keggin polyoxometalate that immediately transfers to the solid phase. Investigation of the formation process indicates that a low Ni concentration, insoluble reducing agent, and long synthesis time are the critical factors for obtaining the zeolite octahedral metal oxides rather than the ϵ-Keggin polyoxometalate molecule. The synthesized zeolitic nickel molybdate with Na⁺ is used as the adsorbent, which effectively separates C₂ hydrocarbon mixtures.     

Synthesis, structural characterization and Li ion conductivity of a new vanado-molybdate phase, LiMg3VMo2O12

A new vanado-molybdate LiMg₃VMo₂O₁₂ has been synthesized, the crystal structure determined an ionic conductivity measured. The solid solution Li_2−zMg_2+zV_zMo_3−zO₁₂ was investigated and the structures of the z=0.5 and 1.0 compositions were refined by Rietveld analysis of powder X-ray (XRD) and powder neutron diffraction (ND) data. The structures were refined in the orthorhombic space group Pnma with a∼5.10, b∼10.4 and c∼17.6 Å, and are isostructural with the previously reported double molybdates Li₂M₂(MoO₄)₃ (M=M²⁺, z=0). The structures comprise of two unique (Li/Mg)O₆ octahedra, (Li/Mg)O₆ trigonal prisms and two unique (Mo/V)O₄ tetrahedra. A well-defined 1:3 ratio of Li⁺:Mg²⁺ is observed in octahedral chains for LiMg₃VMo₂O₁₂. Li⁺ preferentially occupies trigonal prisms and Mg²⁺ favours octahedral sheets. Excess V⁵⁺ adjacent to the octahedral sheets may indicate short-range order. Ionic conductivity measured by impedance spectroscopy (IS) and differential scanning calorimetry (DSC) measurements show the presence of a phase transition, at 500-600 °C, depending on x. A decrease in activation energy for Li⁺ ion conductivity occurs at the phase transition and the high temperature structure is a good Li⁺ ion conductor, with σ=1×10⁻³-4×10⁻² S cm⁻¹ and E_a=0.6 to 0.8 eV.     

Unusual valences and fast electron exchange in MoFe2O4

The distribution of d electrons over the cations in MoFe₂O₄, which is represented by the formal valence assignment, is shown to be complicated by the equilibrium reactionsFe²⁺_B+Fe³⁺_A+Mo³⁺Fe³⁺_B+Fe²⁺_A+Mo⁴⁺We have used thermal treatment to confirm that the Mo are primarily on octahedral sites; Fe_A[Mo_BFe_B]O₄. K-shell absorption and Mössbauer data at T = 423 K > T_c demonstrate that the iron has an average valence near 2.5+ with fast electron transfer (τ_h < 10⁻⁸ sec) on both octahedral and tetrahedral sites. Paramagnetic susceptibility data give a Curie constant C_M = 7.95 ± 0.2 emu/mole and a Weiss constant θ_p = −445 K; magnetometer measurements confirm a compensation point near 160 K. Transport data give a surprisingly high electronic conductivity, but also give an activated mobility similar to that found in AlFe₂O₄ and CrFe₂O₄ where mixed Fe^3+/2+ valences on both A and B sites have been demonstrated. However, a positive Seebeck coefficient and a preexponential factor one order of magnitude higher in MoFe₂O₄ point to involvement of a fraction of the Mo atoms in electronic transport, which would be consistent with the observation of a τ_h < 10⁻⁸ sec on the A sites of a spinel. An energy diagram consistent with these data and other information about the relative redox potentials of these ions in oxides are proposed for this system.     

Syntheses and characterization of zero-dimensional molybdoantimonites, A2(Mo4Sb2O18) (A=Y, La, Nd, Sm, Gd and Dy)

Six new isostructural A₂(Mo₄Sb₂O₁₈) (A=Y, La, Nd, Sm, Gd and Dy) compounds have been synthesized by solid-state reactions and characterized by single crystal X-ray diffraction and spectroscopic techniques. They crystallize in C2/c space group with 4 formula units and contain A³⁺ cations and discrete centrosymmetric anionic (Mo₄Sb₂O₁₈)⁶⁻ aggregates, made of tetrahedral MoO₄ and disphenoidal SbO₄ moieties. They exhibit characteristic Sb³⁺ photoluminescence.     

Theoretical Study of the Interactions between Isolated DNA Bases and Various Groups IA and IIA Metal Ions by Ab Initio Calculations

Interactions of the DNA bases adenine (A), guanine (G), cytosine (C), and thymine (T) with various metal ions (M) of groups IA and IIA of the periodic table of the elements were studied at the HF, MP2, and DFT levels of theory. The structures and thermodynamic stabilities of these species were studied at the gas phase. The calculations uphold that there exist two active sites in G and one in A, C, and T. The calculations also show that the O² atom in T is a more active site for metal ion bindings than that in C. The stability energies for G … M complexes are larger than those for A … M complexes and the stability energies for T … M complexes are larger than those for C … M complexes. As z/r ratio for the metal ion increases, the interaction energy for the complex increases systematically. Thermodynamic quantities such as ΔH, ΔG, ΔS, and ln K were determined for each complexation reaction, [Base+M ⁿ⁺ →(Base … M) ⁿ⁺]. A, G, and C complexation reactions except for C … Rb⁺ are exothermic. The situation is quite different for T complexation reactions and all except for T … Be²⁺ and T … Mg²⁺ are endothermic.     

A2VNb6Cl18 (A=Rb, In, Tl): synthesis, crystal structure and magnetic properties of a new series of quaternary niobium chloride cluster compounds

A new series of quaternary niobium chloride cluster compounds corresponding to the general formula, A₂VNb₆Cl₁₈ (A=Rb, Tl or In), has been prepared in sealed quartz tubes from a mixture containing NbCl₅, Nb, VCl₃ and RbCl, or In or Tl metal by solid state reactions at 750°C. The structure of Tl₂VNb₆Cl₁₈ was determined using single-crystal X-ray diffraction: Crystal data: rhombohedral, (No. 148), a=9.1122(17), c=25.178(7) Å, V=1810.5(7) ų and Z=3. The full-matrix least-squares refinement against F² converged to R₁=0.0515, wR₂=0.1104 (all data). The structure consists of discrete octahedral cluster units, [Nb₆Clⁱ₁₂Cl^a₆]⁴⁻ linked by V²⁺ and A⁺ cations, located in a 6-coordinated octahedral and 12-coordinated anticubeoctahedral chloride environment, respectively. The intra-cluster bond lengths indicate 16 valence electrons per cluster. Magnetic susceptibility studies show paramagnetic behavior with a magnetic moment of 3.37 μ_B per formula unit. Electrical resistivity measurements indicate a semiconducting behavior.     

[Bis(trispivalatodimolybdenum (II))-μ-bis(4′-carboxylato-2,2′:6′,2″-terpyridine) Ruthenium (II)] (2+) Tetrafluoroborate. Preparation,Electronic Structure and Physical Properties

The homoleptic compound Ru(II)(L)₂ where L = 4′-carboxylato-2,2′:6′,2″-terpyridine was employed as a bridge to link two [Mo₂(O₂CBu ^t )₃]⁺ units in the formation of the title complex: [Mo₂(O₂CBu ^t )₃]₂-μ-Ru(II)L₂] (2+) [BF₄⁻]₂, which has been characterized by ¹H-NMR, UV–vis and emission spectroscopy, MALDI-TOF-MS and cyclic voltammetry. The electronic structure of the complex has been investigated by density functional theory employing Turbomole on the model complex cation [Mo₂(O₂CH)₃]₂-μ-(Ru(II)L₂)²⁺. The intense blue color of the cation arises from M₂ δ to bridge/terpyridine charge transfer. This paper is dedicated to Prof. F. A. Cotton in memoriam.     

Zur Kristallchemie der Oxometallate des Thoriums

The crystal chemistry of the valence stable Thorium (Th⁴⁺) shows in oxovanadates, ‐niobates, ‐molybdates, ‐tantalates etc. typical cationic property. It depends on the big size of the Th⁴⁺ ion in the order of r(Th⁴⁺) = 0.92 Å (C.N. = 6) up to r(Th⁴⁺) = 1.34 Å (C.N. = 12), comparable with the radii of alkaline and alkaline earth metals: r(Na⁺) = 1.02 Å (C.N. = 6) or r(K⁺) = 1.5 Å (C.N. = 8); r(Ca²⁺/Sr²⁺) = 1.1 Å/1.26 Å (C.N. = 8). There exists only one typical compound that may be of the oxothorate type with the composition NaK₃Th₂O₆. Na⁺ and K⁺ can be declared to be components of the cationic part of the crystal structure. In contrary, thorium and oxygen are common components of the anionic part of the crystal structure. From the point of view of the shared polyhedra (exclusively octahedra), NaK₃Th₂O₆ belongs to the thorium‐oxometallates too. The report on oxometallates of thorium shows compounds with one, two, four and five Th⁴⁺ atoms in combination with V⁵⁺, Nb⁵⁺, Ta⁵⁺ and Mo⁶⁺. W⁶⁺ is completely missing. Some of the Tantalates belong to the Jahnberg compounds, showing planar nets of corner shared pentagonal bipyramids. ThMO₄ and cationic mixed compounds, Th_1–xA_xMO₄, crystallize with the Scheelite (respectively Huttonite) structure and at least Th⁴⁺ is part of ordered and disordered perowskites showing more or less deviations from the cubic symmetry.     

Synthesis, structure and X-ray excited luminescence of Ce-doped AREP2O7-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu)

The crystal structures of five new alkali rare earth diphosphates were obtained by Rietveld refinement of powder X-ray diffraction (XRD) profiles, including four alkali lutetium diphosphates ALuP₂O₇ (A=Na, K, Rb, Cs) and the low temperature phase of KYP₂O₇. The scintillation properties of Ce³⁺-doped AREP₂O₇ (A=Na, K, Rb, Cs; RE=Y, Lu) powder samples were studied under static and pulsed X-ray excitations, and featured outstanding scintillation properties with light yields 1–2 times of that of Bi₄(GeO₄)₃ and relatively short decay time of 20–28 ns. Considering the suitable emission wavelength range, large light yield, short decay time, and non-hygroscopic nature, Ce³⁺-doped AREP₂O₇-type alkali rare earth diphosphates are potential candidates for high-counting-rate scintillation applications.     

XANES and EXAFS of copper compounds: Studies of copper carboxylates with metal-metal bonds and of the complex formed by Pseudomonas aeruginosa

X-ray absorpion near edge structure (xanes) of copper compounds with copper in 1⁺, 2⁺ and 3⁺ states has been studied. Extended x-ray absorption fine structure (exafs) has been employed to determine bond distances and coordination numbers in several model copper compounds. Employing bothxanes andexafs, the structure of the copper complex formed by the micro-organismPseudomonas aeruginosa has been shown to be square-planar with the Cu-O distance close to that in cupric glucuronates and cupric acetylacetonate.exafs has been shown to be useful for studying metal-metal bonds in copper carboxylates. Communication No. 228 from the Solid State and Structural Chemistry Unit     

15-Hydroxybenzomonothia-15-crown-5 with the sulfur atom linked with the benzene ring and the derived sulfoxide: synthesis,structure, and complexation with the metal cations

Novel 15-hydroxybenzomonothia-15-crown-5 containing the sulfur atom linked with the benzene ring and its S-oxide were synthesized. The stability constants for the complexes of the obtained benzocrown ethers and a reference 15-hydroxybenzo-15-crown-5 with Na, Ca, Ag^I, Cd, Hg^II, and Pb^II perchlorates were determined by ¹H NMR titration. In MeCN-d₃, the benzothiacrown ether demonstrates a high selectivity towards the thio- and oxothiophilic Hg²⁺ (logK ₁ = 7.1) and Pb²⁺ ions (logK ₁ = 7.4). In MeCN-d₃-D₂O mixtures, the stabilities of the most of complexes decrease sharply due to competitive hydration of the metal cations except for the “soft” Ag⁺ and Hg²⁺ ions having low affinity for the “hard” oxygen atoms and, on the contrary, very high affinity for the “soft” S^II atoms. This results in the change in selectivity of complexation: at the water content in solution of 20%, the benzothiacrown ether binds preferably the Hg²⁺ (logK ₁ = 5.0) and Ag⁺ ions (logK ₁ = 2.7). In MeCN-d₃, the benzothiacrown-derived sulfoxide is a weak and non-selective complexing agent towards all the metal cations under study; the reference 15-hydroxybenzo-15-crown-5 forms more stable complexes with the oxophilic sodium, calcium, and lead(ii) cations. The conformational features of the benzocrown ethers and their metal complexes established by NMR spectroscopy and X-ray diffraction are discussed. The found characteristics of the complexing ability of benzomonothia-15-crown-5 where the sulfur atom is in conjugation with the benzene ring reveal that the macrocyclic ligands with such a structure are promising as high-selective and efficient complexing agents for the “soft” mercury(ii) and silver(i) cations in acetonitrile-water mixtures.     

Kinetic and mechanistic studies on the electron-transfer reactions between diaquabisethylenediaminecobalt(III) and ethylenediaminetetraacetatocobaltate(III) with promazine in acidic aqueous media

The kinetics of the electron-transfer reactions between promazine (ptz) and [Co(en)₂(H₂O)₂]³⁺ in CF₃SO₃H solution ([Co^III] = (2–6) × 10⁻³ m, [ptz] = 2.5 × 10⁻⁴ m, [H⁺] = 0.02 − 0.05 m, I = 0.1 m (H⁺, K⁺, CF₃SO ₃ ⁻ ), T = 288–308 K) and [Co(edta)]⁻ in aqueous HCl ([Co^III] = (1 − 4) × 10⁻³ m, [ptz] = 1 × 10⁻⁴ m, [H⁺] = 0.1 − 0.5 m, I = 1.0 m (H⁺, Na⁺, Cl⁻), T = 313 − 333 K) were studied under the condition of excess Co^III using u.v.–vis. spectroscopy. The reactions produce a Co^II species and a stable cationic radical. A linear dependence of the pseudo-first-order rate constant (k _obs) on [Co^III] with a non-zero intercept was established for both redox processes. The rate of reaction with the [Co(en)₂(H₂O)₂]³⁺ ion was found to be independent of [H⁺]. In the case of the [Co(edta)]⁻ ion, the k _obs dependence on [H⁺] was linear and the increasing [H⁺] accelerates the rate of the outer-sphere electron-transfer reaction. The activation parameters were calculated as follows: ΔH ^≠ = 105 ± 4 kJ mol⁻¹, ΔS ^≠ = 93 ± 11 J K⁻¹mol⁻¹ for [Co(en)₂(H₂O)₂]³⁺; ΔH ^≠ = 67 ± 9 kJ mol⁻¹, ΔS ^≠ = − 54 ± 28 J K⁻¹mol⁻¹ for [Co(edta)]⁻.