Growth of spinel crystals in vanadium slag and their characterization

Morphology of vanadium slags were investigated by scanning electron microscopy (SEM). The mineralogical phases were characterized by energy disperse X‐ray spectrometry (EDS) and powder X‐ray diffraction (XRD). The obtained results show that spinels and silicates are the major phases in the vanadium slag, and V is concentrated in Fe_xV_3‐xO₄ and Mg_x(V, Ti)_3‐xO₄. Both the spinel grain size and volume fraction in the slag with higher V₂O₃ content are much larger than that with lower V₂O₃ content. (Fe, Mn)₂SiO₄ and (Fe, Mn)SiO₃ have a higher proportion in the slag with lower V₂O₃ and higher SiO₂ content. The relation among cooling conditions and grain size and volume fraction of spinels is also discussed. It is found that low cooling rate and long holding time benefit spinel crystal growth, especially for the interval of 1200‐1250 °C. Both mean diameter and volume fraction of spinels could achieve or exceed the industry vanadium slag when holding more than 45 min. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X-ray and X-ray photoelectron spectra of vanadium oxides

X-ray (XS) and X-ray photoelectron (XPS) spectra are reported for vanadium oxides. Because of the multivalent character of vanadium in the oxide system high quality measurements can be used for chemical shift investigation. Both inner level and valence band spectroscopy give information on the electronic structure and their systematic change with increasing oxidation state. The experimental results are discussed favourable in terms of molecular orbital theory (MO-theory). The complete set of XS and XPS data reported here for V-oxides allows the identification of unknown vanadium oxidation states too.     

Structural phase transition in CuFe2O4 spinel

A structural transition with a reduction in symmetry of the high temperature cubic phase (sp. gr. Fd3m) to the tetragonal phase (sp. gr. I4₁/amd) and the appearance of a ferrimagnetic structure occur in CuFe₂O₄ copper ferrite at T ≈ 440°C. It is established by an experiment on a high-resolution neutron diffractometer that the temperature at which long-range magnetic order occurs is higher than that of tetragonal phase formation. When cooling CuFe₂O₄ spinel from 500°C, the equilibrium coexistence of both phases is observed in a fairly wide temperature range (～40°C). The composition studied is a completely inverse spinel in the cubic phase, and in the tetragonal phase the inversion parameter does not exceed few percent (x = 0.06 ± 0.04). At the same time, the phase formed upon cooling has a classical value of tetragonal distortion (γ ≈ 1.06). The character of temperature changes in the structural parameters during the transition from cubic to tetragonal phase indicates that this transition is based on the Jahn-Teller distortion of (Cu,Fe)O₆ octahedra rather than the mutual migration of copper and iron atoms.     

Modulated crystal structures of phases VII and V in (NH4)3H(SO4)2. Neutron Laue diffraction

A study of the crystal structure of the phases VII and V of (NH₄)₃H(SO₄)₂ by mean of neutron Laue diffraction was carried out at temperatures from 5 to 300 K. It is found that the crystal structures of phases VII and V are incommensurately modulated with different periods, and that the phase transition from phase VII to phase V is of first order.     

Structure parameters in mixed liquid crystals (I). Layer thickness of the smectic mixed phases B in the binary system of ethyl-4-(4-ethoxy-benzylidene-amino)-cinnamate and terephthylidene-bis-(4-n-butyl-aniline)

The thickness of the smectic layers in binary liquid crystalline mixed phases of the phase type B are measured in dependence on the concentration. Both components have different B-phase structures. In the part of the binary system with a smectic polymorphism BA the long molecular axes are perpendicular to the layer planes. If the polymorphism is changed by the appearance of a mixed phase of the type C, i.e. the smectic polymorphism BCA exists, there occurs a transition into the B-phase structures with tilted axes, the tilt angle of which is changed continuously with the concentration.     

Phase separation in high-T c superconductors, copper oxides, and related antiferromagnetic phases CuO and Y2BaCuO5

The acoustic properties and crystal structure of high-T _c superconducting cuprates and the related antiferromagnetic phases CuO and Y₂BaCuO₅ exhibit similar properties at a temperature of about 160 K and 240 K. These properties can be associated with the formation of inhomogeneous state of phase separation. Analysis of the magnetic properties of Y₂BaCuO₅ shows that these effects are of a nonmagnetic nature. The results of EXAFS data for the high-T _c superconducting compound Hg_0.8Tl_0.2Ba₂Ca₂Cu₃O_8.10 show that the phenomenon of phase separation is suppressed by superconductivity.     

Effect of magnetic field on lyotropic nematic in the vanadium pentoxide (V2O5)-water system

The optical textures of anisotropic phases of dispersed aqueous vanadium pentoxide solutions have been studied. It was established that the application of a magnetic field to these phases results in the formation of transient magnetohydrodynamic domains. The evaluation of the Freedericksz threshold field for the S-effect provided the determination of the elastic constant, K ₁ ≈ 3.6 × 10^?7 dyn. It is concluded that the studied anisotropic phases can be related to nematics.     

Effect of foreign phases on the growth of NaBi(WO4)2 crystals

Single crystals of NaBi(WO₄)₂ (NBW) have been grown by Czochralski technique, employing differently synthesized starting charge and different axial temperature gradients. The causes behind inherent problems related to compositional changes and associated lowering in crystallization temperature have been probed by analyzing XRD and DTA patterns of post growth residual charge. Presence of low melting phases viz. Na₂WO₄ and hitherto unreported compound Na₅Bi(WO₄)₄ is thought to be responsible for the observed decrease in crystallization temperature. This problem was tackled by segregating pure phase material through re‐crystallization, under high axial temperature gradient. The use of re‐crystallized charge enabled transformation of almost the entire charge into a single crystal of high transparency. The effect of starting charge synthesis and temperature gradient on the optical transmission characteristics of NBW crystal has also been investigated. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of cation ordering and magnetic phase transitions in ternary Fe-containing perovskite oxides by Mössbauer spectroscopy

Nuclear gamma-resonance (NGR) spectra were studied in ceramics of ternary perovskite-type oxides Pb(Fe_2/3W_1/3)O₃, Pb(Fe_1/2Ta_1/2)O₃, and Pb(Fe_1/2Nb_1/2)O₃ and in the Pb(Fe_1/2Sb_1/2)O₃ oxide synthesized under pressure. A singlet corresponding to compositional-ordering regions was observed in the NGR spectra for Pb(Fe_1/2Sb_1/2)O₃. Weak doublets associated with paramagnetic regions (in which the degree of cation ordering is higher than the volume average) were observed in the spectra for Pb(Fe_2/3W_1/3)O₃ and Pb(Fe_1/2Nb_1/2)O₃ at temperatures 30–70 K below the antiferromagnetic phase transition point.     

Semi-wet growth and characterization of multi-functional nano-engineered mixed metal oxides for industrial application

This review paper covers the low temperature wet growth of nano-engineered particles of ZnO-based mixed metal oxides, their growth mechanism, and characterization using X-ray diffraction, SEM, TEM and IR, UV–visible, and XPS spectral techniques. Main focus of this article is centered on low temperature semi-wet methods of synthesis that are suitable for large scale production of zinc oxide-based systems mixed with iron oxide, copper oxide, nickel oxide and cobalt oxide. These mixed metal oxides have broad industrial applications as catalyst, semiconductors, adsorbents, superconductors, electro-ceramics, and antifungal agents in addition to extensive applications in medicines. This paper discusses the low-cost and environment friendly synthesis of these mixed metal oxides, measurement of properties and applicability of these materials systems.     

Effects of mixed alkaline earth oxides additive on crystallization and structural changes in borosilicate glasses

The effects of mixed alkaline earth oxides on crystallization and structural changes in a multi-component borosilicate glass system are studied by using X-ray diffraction (XRD) and transmission electron microscope (TEM). It is found that the crystallization is decreased with increasing alkaline earth oxide content and there are also a series changes occurred in TEM images. This paper introduces the conception that alkaline earth ions tend to occupy their preferred sites regardless of glass systems. This conception is assisted to explain the TEM images to some extent by suggesting a simple structural model about non-bridging oxygen in glass network. The conception also indicates a ‘blocking effect’ existing in such a multi-component borosilicate glass system, which may be responsible for XRD results in chief. In addition, the structural model suggested by TEM results refers a new unit of Si–O–M²⁺–O–B, which helps in understanding the minimum exhibits in XRD results. Moreover, a dielectric test is taken to study glass properties in detail.     

Effect of mixed transition-metal ions in glasses. Part III: The P2O5–V2O5–MnO system

Electrical and optical properties of phosphate glasses containing vanadium and manganese ions in the xP₂O₅–[(100 − x)(V₂O₅ + MnO)] (PVM) system have been investigated. This is the last article of a III-part series devoted to the electronic properties of phosphate glasses containing a mixture of transition ions. The first article was devoted to the electrical conductivity of glasses having the general composition: xP₂O₅–[(100 − x)(V₂O₅ + Fe₂O₃)] (PVF). Competitive transport of small polarons on V and Fe ion sites was found to contribute to a mixed transition-ion effect (MTE) in PVF glasses. Several features of MTE were found to be similar to the well known mixed alkali effect, observed in glasses containing two alkali ions. In the second article, optical absorption and electronic conduction of xP₂O₅–[(100 − x)(Fe₂O₃ + MnO)] (PFM) glasses were reported. In the absence of competitive transport between the two transition ions (since Mn ions were determined not to contribute to dc conduction), MTE was not observed. The most important feature of PFM glasses was a sharp increase in resistivity at a critical concentration of iron ions, similar to ‘metal–insulator transition’ (MIT). In the present article, we report a resistivity transition in PVM glasses which is similar to that exhibited by the glasses of the PFM series. While Fe ions contributed the carriers in the PFM glasses, V ions serve the same purpose in the PVM compositions. As the concentration of vanadium ions, n_V, is decreased in the composition range 0.82 > n_V > 0.40, resistivity (ρ) increases marginally. For glasses with 0.2 < n_V < 0.40, resistivity and the activation energy for dc conduction (W) increase sharply with decreasing n_V, marking the incidence of an MIT-type transition. As in the PFM glasses, the observation of MIT coincides with the transformation of small polarons to small bipolarons, which is confirmed by the shifting of the small polaron optical absorption band to higher energies with decreasing V concentration.     

Studies on the hydrolysis of {Cu[Al(OPri)4]2}, a single source precursor for CuAl2O4 spinel

Controlled hydrolysis experiments of the heterobimetallic alkoxide {Cu[Al(OPrⁱ)₄]₂}, were investigated. The progress of hydrolysis was monitored by the FT-IR and NMR spectroscopy. A blue colored gel appeared after four days of hydrolysis and the hydrolysis experiments were continued up to ten days. The elemental analysis and FT-IR spectrum revealed the presence of isopropoxy group in the gel obtained even after ten days of hydrolysis. The presence of carbon was further confirmed from the presence of a weak signal at 3.351 ppm in the ¹³C CP-NMR spectrum. The thermal analysis of the hydrolyzed gels in air also suggested that they were not simple hydroxides of copper and aluminum. The gels on one time heat treatment at 900 °C in air yielded inverse spinel CuAl₂O₄ as shown by the PXRD patterns. CuO was observed along with CuAl₂O₄ in the case of the fired product of the gel obtained after five or six days of hydrolysis which was absent in the fired product of ten days hydrolyzed gel. The phase pure spinel was nanocrystalline in nature as revealed by the analysis of PXRD pattern. TEM images revealed porous structured nanocrystallites. The optical property of the spinel was evaluated by the diffuse reflectance spectroscopy. Raman spectrum showed five bands at 767, 698, 595, 450 and 356 cm^?1 conforming to the spinel structure.     

Electronic structure investigation of V1−xNbxO2 mixed oxide by XES and XPS

The mixed oxide system V_1−xNb_xO₂ (x ≦ 0.12) has been studied by X-ray and photo-emission spectroscopy as a function of the composition. The metal L valence band spectra (VL_III and NbLβ_2.15) with increasing Nb concentration change their width and relative band intensity substantially pointing to charge flow and hybridization effects. The XPS valence band for x ≧ 0.04 indicates a total d band shift of 0.4 eV relative to smaller Nb concentrations. The measured core level spectra at higher Nb doping x ≈︁ 0.10 support the expected existence of V³⁺-Nb⁵⁺ pairs. A comparison of different X-ray spectra and the XPS valence band spectrum for x = 0.08 exhibits a more stronger V3d localization relative to Nb4d, favouring charge transfer and bonding variations with Nb doping. Our spectroscopial data confirm the established ideas concerning the electronic structure of rutile-like compounds and its change at the phase transition point.     

Laser produced amorphous phases in NbSi and VSi thin films

The binary systems NbSi and VSi are investigated by laser induced melting and quenching of vapor deposited thin films. Glassy phases of various compositions are produced. The established compositional glass forming ranges support that the equilibrium compounds Nb₄Si is stable only at high temperature. Thermal decomposition of the amorphous NbSi films proceeds via several intermediate stages. In particular, the AuCu₃-type configuration of Nb₃Si was found to be formed. In addition, amorphous NbSi films around 20 at.% si show the formation of an undentified metastable compounds upon post-irradiation annealing. Amorphous VSi films proved to be stable up to at least 500°C. Above this temperature they decompose directly into their respective equilibrium phases.     

Effects of substrate orientation on aluminum grown on MgAl2O4 spinel using molecular beam epitaxy

Al thin films have been grown on single-crystal MgAl₂O₄ spinel substrates using solid source molecular beam epitaxy. The structural properties of Al layers were systematically investigated as a function of substrate orientation. X-ray diffraction reveals that Al layers are coherently grown on both (0 0 1)- and (1 1 1)-oriented spinel substrates. However, scanning electron microscopy and atomic force microscopy show that Al layers on (0 0 1) spinel substrates display smoother surface morphology than those grown on (1 1 1) spinel substrates. Additionally, electron backscatter diffraction and transmission electron microscopy demonstrate the presence of a high density of twin domain structures in Al thin films grown on (1 1 1) spinel substrates.     

Structural characterization and magnetic properties of the spinel compound CoIn0.5Cr1.5S4

Single crystals of the magnetic semiconductor CoIn_0.5Cr_1.5S₄, belong to the system CoIn_(2‐2X)Cr_(2X)S₄ with x = 0.75, was grown by the chemical transport method. X‐ray powder diffraction characterization by the Rietveld method indicated that CoIn_0.5Cr_1.5S₄ crystallizes in the space group Fd‐3m, Z = 8, with a = 10.0700(6) Å and V = 1021.2(1) ų, in a normal spinel structure. The temperature dependence of the DC magnetization suggests that the studied compound presents a ferromagnetic behavior with a Curie temperature Tc = 220 K. Sharp spin‐glass like behavior was found also. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The role of vanadium valence states and coordination on electrical conduction in lithium iodide borate glasses mixed with small concentration of silver iodide

LiI–AgI–B₂O₃ glasses mixed with different concentrations of V₂O₅ (ranging from 0 to 1.0 mol%) were prepared. Electrical and dielectric properties over wide ranges of frequency (10^?2–10⁷ Hz) and temperature (173–523 K) have been studied. Additionally spectroscopic properties viz., optical absorption and ESR spectra have been investigated. The optical absorption and ESR studies have revealed that vanadium ions do exist in both V⁴⁺ and V⁵⁺ states and the redox ratio is the highest in the glasses containing 0.8 mol% of V₂O₅. The results of conductivity measurements have indicated that there is a mixed conduction (both ionic and electronic). The ionic conduction seems to be dominant over polaron hopping only in the glasses containing V₂O₅ more than 0.8 mol% of V₂O₅. The impedance spectra have also indicated that the conduction is predominantly polaronic in nature. The frequency and temperature dependence of the electrical moduli as well as dielectric loss parameters have exhibited relaxation character attributed to the vanadyl complexes. The relaxation effects have been analyzed by the graphical method and from this analysis it has been established that there is a spreading of relaxation times. The results have been further discussed quantitatively in the light of different valance states of vanadium ions with the aid of the data on spectroscopic properties.     

Specific features of cation distribution in the crystal structure of mariinskite BeCr2O4 (Derivative of olivine-type structure)

Two samples of mineral mariinskite have been investigated by single-crystal X-ray diffraction, and their formulas were established to be Cr_1.43Al_0.64Be_0.9O₄ (sample 1) and Cr_1.32Al_0.74Be_0.9O₄ (sample 2). The parameters of their orthorhombic cells are, respectively, a = 4.487(1) Å, b = 5.629(1) Å, and c = 9.732(2) Å and a = 4.478(1) Å, b = 5.620(1) Å, and c = 9.746(2) Å; sp. gr. P2₁2₁2₁. The structures of samples 1 and 2 were solved by direct methods and refined in the anisotropic approximation of thermal atomic vibrations to R = 8.9 and 6.06%, respectively. Two components coexist in the mineral crystal structure, which belong to olivine-type structure and are interrelated through reflection in the mirror symmetry plane passing through close-packed layers. The presence of additional octahedral sites (which are statistically occupied by Cr³⁺ ions in the same way as vacancies in principal sites (occupancy ～80%)) in the mariinskite structure lowers the mineral symmetry from the centrosymmetric sp. gr. Pcmn, characteristic of olivine-group minerals, to the acentric sp. gr. P2₁2₁2₁.     

Dielectric relaxation investigations of the isotropic and nematic phases of 4-n-Butyloxyphenyl-4-n-hexylbenzoate (BOHB)

Experimental values of the complex dielectric permittivity in a wide frequency range are analized for the isotropic and nematic phase of BOHB. The results are discussed from the point of view of different of molecular reorientation mechanisms which are related to the structure of the molecule. It can be concluded that the molecules in the nematic phase exhibit a stronger tendency to reorient as “rigid bodies”.