Molybdate Groups Research Articles

Structural role of molybdenum in nuclear glasses: an EXAFS study

The Mo environment has been investigated in inactive nuclear glasses using extended X-ray absorption spectroscopy (XAS). Mo is present in a tetrahedron coordinated to oxygen in the form of molybdate groups [MoO 4] 2− (d(Mo–O)=1.78 Å). This surrounding is not affected by the presence of noble metal phases in the nuclear glass. Relying on the XAS results, on the bond-valence model and on molecular dynamics simulations of a simplified borosilicate model glass, we show that these groups are not directly linked to the borosilicate network but rather located within alkali and alkaline-earth rich domains in the glass. This specific location in the glass network is a way to understand the low solubility of Mo in glasses melted under oxidizing conditions. It also explains the possible phase separation of a yellow phase enriched in alkali molybdates in molten nuclear glasses or the nucleation of calcium molybdates during thermal aging of these glasses. Boron coordination changes in the molten and the glassy states may explain the difference in the composition of the crystalline molybdates, as they exert a direct influence on the activity of alkalis in borosilicate glasses and melts.

Structures of Novel R2Mo5O18 and R6Mo12O45 (R: Eu and Gd) Prepared by Thermal Decomposition of Polyoxomolybdate Precursor [R2(H2O)12Mo8O27] ×nH2O.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structures of novel R(2)Mo(5)O(18) and R(6)Mo(12)O(45) (R = Eu and Gd) prepared by thermal decomposition of polyoxomolybdate precursor [R2(H2O)(12)Mo(8)O(27)].nH2O.

Single crystals of R(2)Mo(5)O(18) and R(6)Mo(12)O(45) (R = Eu and Gd), which are novel compounds in the R(2)O(3)-MoO(3) system, have been obtained by thermal decomposition of [R(2)(H(2)O)(12)Mo(8)O(27)].nH(2)O in air at 750 degrees C for 2 h. TG-DTA and X-ray diffractometry showed that R(2)Mo(5)O(18) crystallizes in a melt of the dehydrated precursor (R(2)Mo(8)O(27)), and R(2)Mo(5)O(18) is transformed to R(6)Mo(12)O(45) in the solid state, both occurring with the loss of MoO(3). R(2)Mo(5)O(18) species crystallize isostructurallyas orthorhombic, Pbcn, Z = 4, with lattice constants of a = 19.2612(7) and 19.246(1) A, b = 9.4618(3) and 9.4414(5) A, c = 9.3779(3) and 9.3446(4) A for R = Eu and Gd, respectively. R(6)Mo(12)O(45) crystallize isostructurally as triclinic P1, Z = 1, with lattice constants of a = 9.3867(4) and 9.3409(3) A, b = 10.9408(5) and 10.8826(5) A, c = 11.4817(5) and 11.4377(5) A, alpha = 104.194(2) degrees and 104.170(1) degrees, beta = 109.567(3) degrees and 109.288(4) degrees, gamma = 108.998(2) degrees and 109.266(2) degrees for R = Eu and Gd, respectively. Both structures consist of [RO(8)] square-antiprisms and [MoO(n)] polyhedra. In R(2)Mo(5)O(18), an [RO(8)] polyhedron is attached by only molybdate groups, being isolated from adjacent [RO(8)] groups. The 12 nearest R atoms surrounding an R atom with R...R distances of 6.0735(4)-7.0389(4) A form an approximate cuboctahedron. All the [RO(8)] square-antiprisms in R(6)Mo(12)O(45) are connected to each other by face-sharing to form dimeric [R(2)O(13)] and [R(2)O(12)] groups. The latter unusual [R(2)O(12)] group is achieved by sharing a square-face via four bridging O atoms with a very short R...R separation (3.4741(7) and 3.4502(6) A for R = Eu and Gd, respectively).

Influence of Hydrolysis Mechanisms on Molybdate Sorption Isotherms Using Chitosan

Molybdate sorption using chitosan sorbents has proved to be strictly controlled by the pH of the solution. Sorption isotherms exhibit a sigmoid trend, which has been correlated to the appearance of polynuclear hydrolyzed species, the most favorable species for sorption on chitosan. Sorption capacity exceeds 7 mmol·g−1, which corresponds to a molar ratio between Mo and the amine group significantly higher than 1. The formation of complexes in a pendant fashion and/or the ion-exchange mechanism of polynuclear metal ions are suspected to occur between polynuclear molybdate species and protonated amine groups, though several amine groups can interact with the same polynuclear molybdate group.

Local Structure of Simplified Waste Glass: Complementarity of XAS and MD Calculations

ABSTRACTX-ray Absorption Spectroscopy (XAS) together with x-ray diffraction methods were used in inactive analogues of the French nuclear waste glass to analyze Pd-Te precipitates and the peculiarity of the sites occupied by some structural probe elements such as Mo, Zn, Zr and Si in the glassy part of the material. The influence of the precipitates on the structure of the glassy matrix around these structural probes was investigated by comparing noble metal bearing compositions together with noble metal free glasses.In noble metal bearing glasses, Pd is associated to Te as (Pd90Te10) precipitates. Pd K-edge EXAFS shows the presence of Te in the Pd coordination shell and is in accordance with a non-metallic character of the Pd-Te bond. Mo K-edge EXAFS shows that Mo occurs as molybdate groups non connected to the glassy matrix, which may be related to the occasional separation of Mo-rich phases in some compositions. Zr is located in an octahedron with d(Zr-O)=2.08Å 0.01Å whereas Zn is 4-fold coordinated with d(Zn-0)=-1.95 ű 0.01Å. Beyond this well defined first coordination shell, XAS detects some degree of medium range order which gives insight on the bonding of the site to the polymeric borosilicate network. The interatomic Zn-Si and Zr-Si EXAFS-determined distances agree with a ZnO4and Si04tetrahedra sharing corners geometry and Zr-Si octahedra sharing corner with Si04tetrahedra. A third shell of neighbors around Zr was evidenced in the noble metal bearing glasses and in a four oxide glass, indicating that Zr is an element useful for detecting subtle changes in the local structure of complex borosilicate glasses. Si K XANES shows modifications in the connection between Si04tetrahedra when noble metals are present. To obtain accurate and precise structural interpretations, a direct comparison between EXAFS data and Molecular Dynamic (M.D.) calculations on simplified nuclear glass comprising six oxides has been performed. MD calculations show that the distribution of the Zr-O distances is harmonic in the k-range accessible with EXAFS in glasses.

Metamagnetism in the Cr2V4-xMoxO13+0.5x solid solutions.

Magnetic investigations were carried out in the temperature range 4.2--300 K, in stationary magnetic fields up to 14 T and in the pulsed magnetic fields up to 38 T on polycrystalline solid solutions of ${\mathrm{Cr}}_{2}$${\mathrm{V}}_{4\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mo}}_{\mathit{x}}$${\mathrm{O}}_{13+0.5\mathit{x}}$ with x=0.0, 0.22, 0.29, 0.35, 0.42, and 0.5. Antiferromagnetic ordering with a N\'eel temperature ${\mathit{T}}_{\mathit{N}}$\ensuremath{\approxeq}13 K and the paramagnetic Curie-Weiss temperature ${\mathit{FTHETA}}_{\mathrm{CW}}$\ensuremath{\approxeq}-22 K was established for all samples. Additionally two metamagnetic thresholds were observed for samples with x=0.0, 0.29, and 0.5 in stationary magnetic fields up to 14 T. All samples have low effective magnetic moments which are far from the saturation expected for the spin-only ${2\mathrm{C}\mathrm{r}}^{3+}$ moment of 6${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. The metamagnetic state is explained in terms of a large anisotropy, an asymmetric cation-anion-cation superexchange interaction, and an additional superexchange interaction involving the nonmagnetic tetrahedral vanadate and molybdate groups.

Luminescence and nonlinear optical properties of bariumfluoromolybdate (Ba2MoO3F4)

Abstract The luminescence and nonlinear optical properties of Ba2MoO3F4 are reported and discussed. The fluoromolybdate luminesces at low temperatures only. The luminescence properties are very similar to those of molybdate groups in the MgWO4 lattice. Ba2MoO3F4 has an effective nonlinear optical tensor coefficient which is eight times larger than that of α-quartz.

Thermal, physical, electrical and XPS studies of the Li 2O:P 2O 5: MoO 3 glass system

The glass-forming region in the Li 2O:P 2O 5:MoO 3 system has been determined. Three series of glasses with 40, 50 and 60 mol% Li 2O have been studied. Maxima in glass transition temperature were observe around x = [MoO 3]/([MoO 3]+[P 2 O 5]) = 0.4. This was attributed to a convolution of the increasing cross-link density and decreasing mean bond strength with increasing x. DC conductivity, σ, was found to increase with x for a given Li 2O content, with the composition 0.5Li 2O:0.5(0.6P 2O 5:0.4MoOin3) having the highest σ at 25 °C. Linear correlation between the activation energy for conduction, Eact, and logarithm of the pre-exponential factor, σ 0, in the Arrhenius equation was attributed to strong entropy-enthalpy interaction. Binding energies of the Li 1s, P 2p, Mo3 d 5 2 and O1s core-levels have been determined from the respective XPS spectra. Upon deconvolution, the XPS O1s spectrum was found to consist of two peaks. Phosphate groups in the glasses were found to be preferentially modified by Li 2O compared with molybdate groups, consistent with the order of acidity, P 2O 5>MoO 3, of glass-forming oxides in melts. The presence of Li 2O is also believed to have promoted the formation of POMo units at the expense of POP and MoOMo units.

Structure and electrical properties of Cu + ion-conducting oxyhalide glasses in the system CuI—CuPO 3—Cu 2MoO 4

Cu + ion-conducting glasses were prepared in the pseudoternary system CuI—CuPO 3—Cu 2MoO 4. Infrared spectra of the glasses with a constant CuI content suggested that non-bridging oxygens were more preferentially formed in the phosphate groups than in the molybdate groups. This preference order for non-bridging oxygen formation was consistent with the order of acidity, P 2O 5>MoO 3, of the glass-forming oxides in melts. These glasses showed high ionic conductivities of 10 -2 to 10°Sm -1 at room temperature. For the glasses with a constant CuI content, the conductivity at 298 K of the glasses decreased non-linearly with an increase in the composition parameter x, which denotes the ratio of the number of phosphorus atoms to the total number of phosphorus and molybdenum atoms included in the glasses, x=[ rmP]/([ rmP] +[ rmMo]). The activation energy for conduction inceased non-linearly with an increase in x. These non-linear changes in the conductivity and in the activation energy were correlated with the structural change during melting of these glasses.

Some studies of the optical properties of molybdenum-phosphate glasses

A series of binary MoO3-P2O5 and ternary MoO3-In2O3-P2O5 glasses was prepared and their optical properties were investigated. The optical absorption edge of the glasses was measured for specimens in the form of thin blown films. It was found that the fundamental absorption edge of these glasses usually occurs in the ultraviolet region. The linear variation of (αħω)1/2 with ħω where α is the absorption coefficient and ħω is the incident photon energy, is taken as evidence of non-direct interband transitions. The linear dependence of optical gap with molybdenum content indicates that the molybdenum content controls the absorption phenomena in this glassy system. The infrared spectra of all the glasses appeared to be almost the same, indicating that the infrared bands arise primarily from the vibrations of the phosphate and molybdate groups. The structure of molybdenum phosphate glasses is discussed in terms of the formation of mixed phosphate and MoO4 tetrahedra.

Ferromagnetic exchange in α-manganese molybdate and the structural consequences

Powder neutron diffraction patterns of alpha -MnMnO4 have been recorded between 2 and 127 K in 0.48 K intervals. A subtle structural transition from C2/m to P2/m (or lower) symmetry is observed at Ts=79.1(2) K and antiferromagnetic order occurs below TN=10.7(2) K. The magnetic structure consists of collinear Mn2+ moments of 4.71(3) mu B close to the (101) direction of the a*b*2c magnetic supercell. The moment reduction due to covalency is compared to that in other oxo salts of d5 cations. Ferromagnetic alignment of the four spins within Mn4O16 clusters of edge-sharing MnO6 octahedra is observed and weaker antiferromagnetic interactions between clusters are mediated by molybdate groups. A potential exchange mechanism involving spin transfer from three Mn2+ ions onto one O2- is proposed to explain the unusual ferromagnetic interaction. Refinements of the crystal structure using each of the 260 diffraction profiles reveal an exchange-strictive structural distortion around TN that is consistent with the ferromagnetic interactions within the clusters.

Luminescence properties of alkali europium double tungstates and molybdates AEu M2O 8

The luminescence properties of AEuW 2O 8 and AEuMo 2O 8 ( A + = alkali metal ion) are reported. These properties depend on the crystal structure type. Vibronic coupling between the electronic transitions of the Eu 3+ ion and the vibrational transitions of the tungstate or molybdate group is observed. The concentration quenching of the Eu 3+ luminescence is weak. The analysis of the Eu 3+ decay curves points to energy migration and shows the two-dimensionality of the Eu 3+ sublattice in KEuMo 2O 8 and the one-dimensionality of the Eu 3+ sublattice in KEuW 2O 8 and RbEuW 2O 8.

Crystallochemical classification scheme for molybdates

The authors propose a classification scheme for molybdates of the type R/sub c/A/sub d/ (MoO4)/sub y/ based on analysis of crystallochemical formulas of the structural groups (A/sub d/(MoO3)/sub y/), in the composition of which there are high-valence cations of the complexing agent A and molybdate groups coordinated to them. It has been determined that the 84 studied molybdates are representatives of 32 crystallochemical groups of complexes, in which there are 13 different types of coordination between the tetrahedral MoO4S anions and the A atoms. They have considered the effect of the MoO4:A ratio and the nature of the A cations on the structure and dimensionality of the structural groups. On the basis of the proposed crystallochemical formulas, they have analyzed the interrelationship between the number of bridge oxygen atoms in the AO/sub n/ polyhedra and the method for joining the polyhedra; and they also discuss the reasons for differences in crystal structure between the considered molybdates and sulfates which are analogous in stoichiometric composition.

Synthesis and structure of a mixed-valence tetramolybdate containing a binucleating diazene ligand

Reaction of MoO2Cl2(dmf)2(dmf = dimethylformamide) with 1,4-dihydrazinophthalazine (dhphH6) gives [Et3NH]2[Mo4O11dhphH2], which is shown by an X-ray crystal structure analysis to be a binuclear, oxo-bridged, formally MoIV, diazene complex with bridging molybdate groups; upon acidification the complex reversibly converts into a MoVI derivative of the fully protonated, parent hydrazine.

Luminescence in trilanthanumtrichlorotungstate (La 3WO 6Cl 3)

The luminescence properties of La 3WO 6Cl 3 are reported and discussed. The tungstate group occurs as a trigonal prismatic WO 6− 6 complex. The blue luminescence is, for the greater part, quenched at room temperature. No energy migration occurs in this lattice. The decay times are discussed in terms of a simple molecular-orbital (MO) scheme. The luminescence of the following activating ions was studied: Mo 6+, Bi 3+, Eu 3+, Sm 3+, Ce 3+, and Tb 3+. The molybdate group produces a red emission with low efficiency. The Bi 3+ ion induces a narrow band emission with small Stokes shift. This is interpreted using a Bi 3+O 2−W 6+ charge-transfer state. Except for Ce 3+, the rare earth activators show luminescence, but the total transfer efficiency from tungstate to the rare-earth ions is low. This is not due to the one-step tungstate-rare-earth transfer (which is efficient), but to the localized nature of the tungstate excitation. The Eu 3+ charge-transfer band is at very low energies.

Luminescence in lanthanum chlorotungstate (LaWO 4Cl)

The luminescence properties of LaWO 4Cl are reported and discussed. The tungstate group occurs as a trigonal bipyramid. Although there is some analogy with the luminescence of the tetrahedral tungstate group, the small Stokes shift reveals the presence of five oxygen ions. The luminescence of the following ions was studied: Mo 6+, Eu 3+, Sm 3+, and Tb 3+. The molybdate group shows red emission with a larger Stokes shift than the tungstate group. Energy transfer from the tungstate group to the rare earth ions occurs over nearest-neighbor distances, except for Tb 3+. Because of a low-lying f- d transition, the transfer efficiency is much higher for this ion.

Some observations on the luminescence of rare-earth molybdates with β′-Gd2(MoO4)3structure

The luminescence properties of rare-earth activated and unactivated Gd2(MoO4)3 and those of Eu2(MoO4)3 and Tb2(MoO4)3 are reported. Gd2(MoO4)3 shows two different molybdate emissions which are assigned to intrinsic and defect molybdate groups. Energy transfer from the molybdate group to rare-earth activators is not efficient in the diluted samples Gd2(MoO4)3–RE. Time-resolved spectroscopy and decay-curve measurements were not able to demonstrate energy migration in Eu2(MoO4)3. Only one-step transfer processes to centres in the surface were observed. The absence of migration is explained by a structural property, viz. the occurrence of rare-earth sites in pairs. For Tb2(MoO4)3 we were able to demonstrate energy migration through the Tb3+ sublattice. The nature of the processes involved is complicated and a detailed analysis was not possible.

Some new observations on the luminescence of PbMoO 4 and PbWO 4

We report on the luminescence of doped and undoped PbMoO 4 and PbWO 4 single crystals; a red emission is found for the first time for PbMoO 4. The degree of polarization of both the emissions of PbWO 4 and of the shorter-wavelength emission of PbMoO 4 is measured. The shorter-wavelength emissions are assigned to the 3 T 1 1 A 1 transition in the tetrahedral MoO 4 2−(WO 4 2−) group. It is assumed that the 3 T 1 level is split due to spin-orbit coupling. The longer-wavelength emissions are assigned to a transition in a molybdate (tungstate) group lacking an oxygen ion, i.e., a MoO 3(WO 3) group.

Tetrahedral oxyanions as ligands

The vibrational spectra of a number of normal and deuteriated pentammine and tetrammine cobalt(III) complexes involving MO4n– ligands have been measured (M = P, S, Cr, or Mo), and from the observed splitting of the MO4n– vibrations it is confirmed that the chromate and molybdate groups, like sulphate, can function as mono- or bidentate ligands in cobaltammines.

On the Eu3+ Fluorescence in Mixed Metal Oxides. III. Energy Transfer in Eu3+-Activated Tungstates and Molybdates of the Type Ln2WO6 and Ln2MoO6

The fluorescence of unactivated and rare-earth-activated materials of the type Ln2WO6, Ln2MoO6, and Y2W1−xMoxO6 (0≤x≤0.1) has been studied. From the experiments on unactivated materials it follows that energy transfer from tungstate to tungstate or mobyldate group takes place, whereas there is no transfer from molybdate to molybdate or tungstate group. Arguments in favor of exchange-regulated energy transfer from tungstate or molybdate group to rare-earth ion are put forward. The efficiencies of the rare-earth-activated tungstates and molybdates can be qualitatively understood assuming a strong dependence of the energy transfer on the angle tungsten (molybdenum)—oxygen—rare earth. In Y2W1−xMoxO6:Eu the molybdate group acts as a killer of the fluorescence under short-wave uv excitation and as a sensitizer under long-wave uv excitation.