Magnetoelectric effect in samarium molybdate

This paper reports on the nonlinear magnetoelectric effect (MEE) in the orthorhombic ferroelectric ferroelastic β′ phase of samarium molybdate Sm₂(MoO₄)₃ observed in magnetic fields up to 20 T and temperatures from 4.4 to 0.43 K. The magnetic-field-induced electric polarization in Sm₂(MoO₄)₃ is an order of magnitude larger than that in isomorphic Gd₂(MoO₄)₃. This provides support for the magnetostriction mechanism proposed by us for the MEE in rare-earth molybdates. The polarization in Sm₂(MoO₄)₃ was found to fall off with time. The relaxation time constant τ increases with decreasing temperature from τ=10² s at T=4.4 K to τ≈10³ s at T=0.43 K.     

Anisotropy of the magnetoelectric effect in terbium molybdate

Anisotropy of the nonlinear magnetoelectric effect in a single-crystal, single-domain sample of the β′ metastable ferroelectric paramagnetic phase of terbium molybdate Tb₂(MoO₄)₃ was studied experimentally in dc magnetic fields of up to 6 T at temperatures of 4.2 and 1.8 K. It was shown that the existing models of the magnetoelectric effect cannot explain the experimental dependences of magnetic field-induced electric polarization on the direction of the applied magnetic field. A model of the magnetoelectric effect is proposed that qualitatively describes the observed angular dependence of the magnetic field-induced electric polarization.     

Effect of solid-phase amorphization on the spectral characteristics of europium-doped gadolinium molybdate

A method is proposed for detecting spectral characteristics of optically inactive molybdates of rare-earth elements by their doping with rare-earth ions whose luminescence lies in the transparency region of all structural modifications of the sample. Gadolinium molybdate is chosen as the object of investigations, while europium ions are used as an optically active and structurally sensitive admixture. It is shown that after the action of a high pressure under which gadolinium molybdate passes to the amorphous state, the spectral characteristics of Gd_1.99Eu_0.01(MoO₄)₃ (GMO:Eu) change radically; namely, considerable line broadening is observed in the luminescence spectra and the luminescence excitation spectra, while the long-wave threshold of optical absorption is shifted considerably (by approximately 1.1 eV) towards lower energies. It is found that by changing the structural state of GMO:Eu by solid-state amorphization followed by annealing, the spectral characteristics of the sample can be purposefully changed. This is extremely important for solving the urgent problem of designing high-efficiency light-emitting diodes producing “white” light.     

Evolution of the spectral response of amorphous europium molybdate under annealing

The phase transformations occurring in amorphous europium molybdate Eu₂(MoO₄)₃ during annealing at atmospheric pressure are studied using optical spectroscopy and x-ray diffractometry. It is established that the metastable β phase is formed at a temperature of ～550°C, whereas the transition to the stable equilibrium α phase takes place at higher temperatures T ≥ 700°C. The spectral characteristics of the α phase, which differ substantially from those of the amorphous state and the β phase, are measured for the first time.     

Electron Paramagnetic Resonance and Mössbauer Studies of Fe3+ Ions in the Fe2O3 [dbnd] MoO3 System

The electron paramagnetic resonance (EPR) and Mössbauer spectra of ironmolybdenum mixed oxides system have been investigated. Both EPR and Mössbauer measurements revealed the formation of ferric molybdate, Fe₂ (MoO₄)₃ and the existence of two different Fe³⁺ species. The EPR spectra of Fe₂ (MoO₄)₃, show two resonance lines of Fe³⁺ ions indicating that a strong exchange interaction is dominates in pure iron molybdate. Mösbauer measurements revealed that the isomer shift (I. S) increases with increasing the iron content. A maximum of quadrupole splitting (Q. S) is observed at the stoichiometric concentration corresponding to the formation of Fe₂ (MoO₄)₃. The effect of a catalytaic dehydration process on the redox behaviour of Fe³⁺ located in iron molybdate frame work is investigated. Results indicate that the catalytaic dehydration of 2-proppanol over this catalyst reduces Fe³⁺ ions to Fe²⁺ leading to the formation of the inactive phase FeMoO₄. the essential role of oxygen, in the alcohol gas feed, in regeneration the activity of the catalysts was demonstrated.     

Direct observation of MoO2 crystal growth from amorphous MoO3 film

The formation process of MoO₂ crystal from amorphous MoO₃ film has been imaged by in situ observation with a transmission electron microscope. Selective growth of flower-shaped MoO₂ crystals by heating above 673 K in vacuum was directly observed. Since the MoO₂ crystal has metallic conductivity of the order of indium oxide film containing tin (ITO film), the thin film growth of the MoO₂ phase has been discussed on the basis of a new substitute for ITO film.     

Low-temperature specific heat of crystalline and amorphous Eu2(MoO4)3

The specific heat C _total of crystalline and amorphous Eu₂(MoO₄)₃ is measured in the temperature interval 4.5–30 K. The amorphous state is obtained by applying pressure ∼7 GPa at room temperature. It is found that the specific heat of the crystal at T⩽7.5 K is described by a cubic function of temperature, while the specific heat of the amorphous sample has a strongly non-Debye character in the entire experimental temperature interval. The curve of C _total for amorphous europium molybdate is analyzed in a model of soft atomic potentials, and it is shown that it agrees well with universal low-temperature anomalies of the specific heat of classical glasses obtained by quenching from the liquid. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 8, 623–627 (25 October 1998)     

XPS study of ion-induced chemical effect on β-bismuth molybdate catalyst

Ar ion-induced chemical effect on MoO₃, Bi₂O₃, and β-Bi₂O₃·2MoO₃ has been studied by ESCA. It was found that the reduction behavior of Mo(VI) and Bi(III) in the complex oxide is quite different from that in the single oxides. A relative oxygen enrichment of the ion-bombarded and reduced surface was observed. It has been shown that the oxygen on the bombarded surface is in a special chemical state and incapable of reoxidizing the reduced low valent metal ion. But, exposing to air leads to the reoxidation of the reduced surface and leaves the lattice oxygen anion as the only surface oxygen species. The reduction behavior of β-Bi₂O₃·2MoO₃ under Ar ion bombardment seems to be similar to its redox behavior in butene catalytic oxidation. It was suggested that both of them should reflect the effect of crystal structure on the redox potential of the oxides.     

Anomalously high photovoltages in terbium molybdate

In terbium molybdate Tb₂(MoO₄)₃ at room temperature, a laser beam of wavelength λ ₁=4880 Å induces an electric polarization that lasts for several days after the irradiation is turned off. The photoinduced polarization has the same sign as the spontaneous polarization. At a laser power of 0.5 W and an exposure time of 4.5 h, the photoinduced polarization exceeds the spontaneous polarization by an order of magnitude. The corresponding electric field is 2.5×10⁶ V/cm. Under the influence of radiation with λ ₁=4880 Å, terbium molybdate luminesces in the green part of the spectrum near λ ₂≈5425 Å. The luminescence quantum yield is 20%.     

Photoinduced permittivity in lead molybdate

The effect of illumination on the dielectric properties of lead molybdate (PbMoO₄) single crystals was studied. Illumination with intrinsic light gives rise to a substantial enhancement of low-frequency permittivity (the photodielectric effect, PDE). The PDE is dependent on temperature and reaches a maximum near 400 K. The permittivity reaches the steady state a long time after the light is turned on (off). The relaxation time and activation energy of the process were determined. The relation of the PDE to the photoconductivity of the crystal is discussed. A possible mechanism of the PDE in PbMoO₄ crystals is proposed.     

Structural transformations in MoO<Subscript>x</Subscript> thin films grown by pulsed laser deposition

In this work, laser-induced crystallization in MoO_x thin films (1.8x2.1) is reported. This transformation involves a MoO_x oxidation and subsequently a crystallization process from amorphous MoO₃ to crystalline MoO₃. For comparison purposes crystallization is induced thermally, in an oven, as well. The crystallization kinetics is monitored by Raman spectroscopy; a threshold in the energy density necessary to induce the phase transformation is determined in the case of photo-crystallization. This threshold depends on the type of substrate on which the film is deposited. For the thin films deposited on glass substrates, the structural transformation is from amorphous MoO_x to the thermodynamically stable MoO₃ crystalline phase. For the thin films deposited on Si(100) the structural transformation is from amorphous MoO_x to a mixture of MoO₃ and the thermodynamically unstable MoO₃ crystalline phases. The structural transformations are also characterized by scanning electron microscopy and light-transmission experiments. PACS 81.15.Fg; 61.80.Ba; 78.30.-j     

Acoustic properties of biaxial crystal of double lead molybdate Pb2MoO5

The acoustic properties of crystalline double lead molybdate, which are promising for acoustooptics, have been investigated. All elasticity coefficients of Pb₂MoO₅ crystal, the phase velocities of acoustic waves, and the angles between the vectors of phase and group velocities of this material have been determined. The acoustic parameters of the crystal have been calculated based on experimental data from the Schaefer-Bergmann diagrams. The phase velocity of acoustic waves along some directions has also been determined by the standard (in acoustics) echo-pulse method.     

Optical spectroscopy of europium molybdate in the crystalline and amorphous States

Comparative spectroscopic studies of crystalline and amorphous samples of Eu₂(MoO₄)₃ were carried out. Amorphous samples were obtained through exposure of the β' crystal phase to a high pressure of ∼9 GPa. It was established that the transition to the amorphous state is accompanied by substantial changes both in the luminescence spectrum and the luminescence excitation spectrum. The long-wavelength absorption edge is estimated to shift by ∼0.8 eV, which is much more significant than in the case of amorphization of classical semiconductors.     

A potential Co36Fe36Si4B20Nb4 nanocrystalline alloy for high temperature soft magnetic applications

The paper addresses the structural, crystallization, soft magnetic and Curie temperature behaviour of Co₃₆Fe₃₆Si₄B₂₀Nb₄ alloy. The material, prepared in the form of ribbons by melt-spinning technique, was amorphous in the as-cast state. Differential scanning calorimetry (DSC) showed two stages of crystallization whereas thermal variation of electrical resistivity (TER) carried out to a higher range of temperature indicated three stages of crystallization. The first crystallization stage, which occurred at 845?K and 825?K in DSC and TER, respectively, was due to the formation of nanophase (CoFe)₂Si as evidenced by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The formation of these nanoparticles reduced the magnetocrystalline anisotropy, thereby revealing good soft magnetic properties in the samples annealed between 825?K and 875?K with coercivity less than 49.9?A?m^?1 (627?mOe) and susceptibility?～?0.72?×?10³. In this optimum nanocrystalline state, the material also exhibited a high Curie temperature above 1100?K, opening the scope of the present nanocrystalline alloy for high temperature applications.     

Evaluation of stoichiometric rare-earth molybdate and tungstate compounds as laser materials

Photoluminescence spectra, photoluminescence decay curves and Raman scattering spectra have been investigated for stoichiometric rare-earth molybdate and tungstate compounds. NaNd(MoO₄)₂ and NaNd(WO₄)₂ show emissions due to the transition ⁴F_3/2→⁴I_9/2 in Nd³⁺. A possibility of laser oscillation in NaNd(MoO₄)₂ is pointed from comparisons of the emission intensity and the decay time constant with NaNd(WO₄)₂ where laser oscillations have been reported. In NaLa(MoO₄)₂ and NaLa(WO₄)₂, observed emissions which are not related to La³⁺ are probably due to the transitions in MoO₄^2- and WO₄^2- molecular ions, respectively, in scheelite crystal. Raman spectra of these compounds are similar, probably related to the same crystal structure. LiEr(MoO₄)₂ shows the emissions due to transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 in Er³⁺, respectively, which are believed to be observed for the first time.     

Simultaneous calorimetric and quick‐EXAFS measurements to study the crystallization process in phase‐change materials

A Calvet‐type differential scanning calorimeter has been implemented on a synchrotron beamline devoted to X‐ray absorption spectroscopy. As a case study, the complex crystallization process in amorphous Ge₁₅Sb₈₅ phase‐change material is followed by simultaneous calorimetric and quick‐EXAFS measurements. A first crystallization at 514(1) K is related to the crystallization of an Sb‐rich phase accompanied by segregation of Ge atoms. Upon further heating, the as‐formed amorphous Ge regions crystallize at 604(1) K. A quantitative analysis of the latent heat allows a Ge₁₁Sb₈₉ stoichiometry to be proposed for the first crystallized phase.     

Electronic structure of β-RbNd(MoO4)2 by XPS and XES

β-RbNd(MoO₄)₂ microplates have been prepared by the multistage solid state synthesis method. The phase composition and micromorphology of the final product have been evaluated by XRD and SEM methods. The electronic structure of β-RbNd(MoO₄)₂ molybdate has been studied employing the X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). For the molybdate, the XPS core-level and valence-band spectra, as well as XES bands representing energy distribution of the Mo 4d- and O 2p-like states, have been measured. It has been established that the O 2p-like states contribute mainly to the upper portion of the valence band with also significant contributions throughout the whole valence-band region. The Mo 4d-like states contribute mainly to a lower valence band portion.     

Polymerization and photochromism of ammonium molybdate in porous glass

Modification of porous glass (PG) plates is carried out by impregnation with aqueous solutions of ammonium molybdate (NH₄)₂MoO₄ with subsequent removal of water at 120°C. A long-wavelength shift of absorption spectra upon accumulation of the salt in PG indicates polymerization of MoO₄^2- anions at low concentrations of the encapsulated salt. Photochromism manifests itself as the anionic forms in PG become larger. UV irradiation of the modified plates causes enhancement of continuous absorption in the visible range. The proposed mechanism of photoreduction of the polianions in PG involves the removal of oxygen atoms from the bridging–Mo–O–Mo–bonds and stabilization of the colored forms by means of conjugation of the electrons released from the 4d-levels of pentavalent molybdenum.     

A metastable hard magnetic phase in the crystallization process of the Fe75Si11B10Nb3Sn1 alloy

A very interesting characteristic of FeSiB based amorphous alloys is its soft magnetic behavior. Most of these alloys remains soft along the crystallization process up to the nucleation of the iron borides. Examples of this are the widely studied Finemet and the FeSiBSn. In this work the crystallization of Fe₇₆Si₁₁B₁₀Nb₃ and Fe₇₅Si₁₁B₁₀Nb₃Sn₁ is studied by means of X-ray diffraction, Mössbauer spectroscopy and coercive magnetic field measurements after one hour isothermal annealing at different temperatures. In the crystallization process of the latter alloy a hard magnetic phase appeared when the samples were annealed above 773 K. The soft magnetic behavior was recovered after annealing at 873 K. The hyperfine parameters as well as the X-ray diffraction patterns are reported.     

Influence of crystallization treatment on structure,magnetic properties and magnetocaloric effect of Gd71Ni29 melt-spun ribbons

The influence of crystallization treatment on the structure, magnetic properties and magnetocaloric effect of Gd₇₁Ni₂₉ melt-spun ribbons has been investigated in detail. Annealing of the melt-spun samples at 610 K for 30 min, a majority phase with a Fe₃C-type orthorhombic structure (space group, Pnma) and a minority phase with a CrB-type orthorhombic structure (space group, Cmcm) were obtained in the amorphous matrix. The amorphous melt-spun ribbons undergo a second-order ferromagnetic to paramagnetic phase transition at 122 K. For the annealed samples, two magnetic phase transitions caused by amorphous matrix and Gd₃Ni phases occur at 82 and 100 K, respectively. The maximum magnetic entropy change (–ΔS_M)^max is 9.0 J/(kgˑK) (5T) at 122 K for the melt-spun ribbons. The values of (–ΔS_M)^max in annealed ribbons are 1.0 and 5.7 J/(kgˑK), corresponding to the two adjacent magnetic transitions.