Local Relaxation Effects in the Crystal Structure of Vanadium-Doped Zircon. An ab Initio Perturbed Ion Calculation (original) (raw)
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V4+ doping into SiO2, ZrO2 and ZrSiO4 structures. Anab initio perturbed ion study
International Journal of Quantum Chemistry, 1993
An ab initio perturbed ion study using X-ray diffraction data has been carried out for ZrSi04 (zircon), Zr02 (monoclinic zirconia, baddeleyite), and SiOz (a-cristobalite) crystal lattice structures. The different substitutions of V4+ for Zr4+ and Si4+ occumng in these host lattices have been analyzed. Geometry optimizations have been performed with the aim of determining the relative stability, cell parameters, and force constants of radial displacement associated with the local relaxation for pure and doped structures. Numerical results are confronted against experimental data and compared with previous results. The geometrical cell parameters of different structures obtained by computer simulation and the results of the X-ray diffraction studies agree with previous experimental data. For the zircon lattice, the substitution of V4+ for Zr4+ at an eightfold-coordinated site is energetically favorable while the substitution of V4+ for Si4+ at a fourfold-coordinated site is unstable. For Zr02, the substitution of V4+ for Zr4+ is energetically favorable while the substitution of V4+ for Si4+ in SiO2 is energetically unfavorable. There is less sensitive influence of the crystal lattice parameters for substitutions occurring at the eightfold-coordinated ion site in ZrSi04 and S O z structures. The doping process produces a decrease of force constant (k) values associated with the breathing fundamental vibrational mode for all structures. The k associated with the radial displacement in dodecahedra1 substitution in the ZrSi04 structure is especially high. The force constants for this movement in tetrahedral substitution in the ZrSiOd, ZrOz, and SiOz structures have a lower value. The differences between ionic radii reported by Shannon and Prewitt of the species concerned in the doping process are not capable of explaining the relaxation of crystal lattice parameters in the Zr02 and SiOz structures.
Spectroscopic Studies on the Localization of Vanadium(IV) in Vanadium-Doped Zircon Pigments
Journal of the American Ceramic Society, 2005
The origin of the blue color observed for vanadium-doped zircon powders that have been prepared either in the presence or the absence of fluxes (NaF) has been investigated by analyzing the localization and distribution of the vanadium species in the zircon matrix. In both cases, the unit-cell parameters determined from the X-ray diffraction patterns of the samples and the results obtained from X-ray absorption (X-ray absorption near-edge structure and extended X-ray absorption fine structure), infrared, and electron spin resonance spectroscopies indicated that V 4؉ cations form a solid solution with the zircon lattice, substituting for both Si 4؉ and Zr 4؉ cations, although to a greater extent for the former. These vanadium cations are heterogeneously distributed in the zircon matrix, being mainly located in the outer layers of the particles in two different situations: ''aggregated'' in nearby lattice positions with V 4؉ −V 4؉ distances (d V−V ) of <8 Å (0.8 nm), and isolated with respect to other V 4؉ cations (d V−V > 8 Å). The strongest blue color that is observed for samples prepared in the presence of NaF seems to be due to the higher amount of V 4؉ cations incorporated to the zircon lattice, because this flux agent does not have any structural role.
Hyperfine Characterization of Pure and Doped Zircons
Journal of Solid State Chemistry, 2000
The aim of this work has been to investigate the in6uence of two coloring dopant ions on the ZrSiO 4 host lattice. Pure, vanadium-doped and praseodymium-doped zircon powders have been synthesized by the ceramic method and analyzed using X-ray di4raction and the perturbed angular correlations (PAC) hyper5ne technique, which probes the nearest environments of zirconium ions. XRD results indicated that vanadium and praseodynamium incorporations in the host lattice result in crystalline solid solutions. In turn, PAC information allowed in both cases the determination of two contributions to the hyper5ne interaction. In the case of vanadium-doped zircon, V 4؉ substitutes mainly for Zr 4؉ and in a minor fraction for Si 4؉ or else it localizes at a tetrahedral interstitial site. In praseodymiumdoped zircon, a low fraction of Pr 4؉ cations locate at Zr 4؉ sites. A slight modi5cation of the zircon lattice is probably caused by the incorporation of the remaining cations at tetrahedral interstitial sites of the solid solution.
International Journal of Hydrogen Energy, 2016
Zirconium based alloys have potential applications in automobile, energy and spacecraft industries. The structural, electronic, elastic, vibrational and thermodynamic properties of orthorhombic ZrNi and ZrNiH 3 are studied in detail using the projector augmented wave method and the generalized gradient approximation based on density functional theory. The optimized lattice parameters of both ZrNi and ZrNiH 3 agree well within ±1% from the experimental values. The electronic structure analysis shows significant contribution of 4d and 3d orbitals of Zr and Ni to the fermi energy level, respectively and along with signatures of two types of hydrogen atoms occupying the lattice of ZrNiH 3. Elastic property calculation of these two compounds showed mechanical stability and anisotropy at ambient pressure. In addition, the phonon calculation of both the compounds showed that ZrNi is dynamically stable but ZrNiH 3 is dynamically unstable. The formation energies (D f H) of ZrNi and ZrNiH 3 at 0 K, after zero point energy correction, have been estimated to be À41.87 kJ/mol and À78.25 kJ/mol H 2 , respectively. The temperature dependent thermodynamic functions of ZrNi and ZrNiH 3 have also been calculated from the Debye-Grüneisen quasi-harmonic approximation.
Electrochemistry of vanadium-doped ZrSiO4
Electrochimica Acta, 2004
The electrochemistry of vanadium-doped zircon (V x ZrSiO 4 , 0 < x < 0.10) has been studied using abrasive-conditioned paraffinimpregnated graphite electrodes. It is compared with that of ZrSiO 4 , ZrO 2 , and vanadium-doped tetragonal and monoclinic zirconias. In contact with acetic/acetate and HCl + NaCl electrolytes, zirconium materials are reduced to Zr(III) at potentials near to À0.5 versus AgCl/Ag and to Zr metal at potentials more negative than À1.2 V, via proton-assisted reductive processes, influenced by the complexing action of chloride ions. Vanadium-centred oxidation processes appear at potentials from +0.2 to +0.7 V enabling for a distinction between different coordinative arrangements. ZrSiO 4 exerts a significant electrocatalytic effect on nitrite oxidation in acetic/acetate buffers, slightly enhanced in the presence of increasing vanadium loadings. Electrocatalytic data are indicative that only V centres substituting Zr are catalytically active, whereas V substituting Si are catalytically silent. ß
Bond Valence Analysis of Tetragonal Zirconias
Journal of Solid State Chemistry, 1999
In tetragonal zirconia, the cation is coordinated by two interpenetrating tetrahedra of oxygen ions, implying two di4erent cation+oxygen bond lengths. On substituting the di4erent tetravalent ions Ge, Ti, Sn, and Ce into tetragonal ZrO 2 +2 mol% Y 2 O 3 , the mean value of the shorter cation+anion bond length varies linearly with the concentration of the substituent ion where the bond length increases or decreases depending on whether the substituted ion is larger or smaller than the zirconium ion it replaces. It is argued in this paper that the length of the longer bond is determined by the requirement that the bond valence sum remains constant. In each case the length of the longer bond calculated on this basis is in good agreement with the measured bond length (from neutron di4raction), and following small adjustments of the bond valence constants, excellent agreement is obtained. The requirement for the bond valence sum evidently accounts for the physics of the situation, and at the same time the available bond length data allow very precise determination of the bond valence constants of the di4erent ions in the tetragonal zirconia environment. It is shown how these bond length considerations provide an explanation for the variation with composition of oxygen position and lattice parameters in all of the materials considered. Among the interesting features accounted for by this analysis are the increase in cell volume occurring when Zr is replaced by the smaller Sn ion, and slight departures from Vegard:s law observed in the substitution of Zr by Ti.
Ab initio simulation and investigation of a novel Ta-doped Zirconia material
2015
Ab-initio materials modeling based on the Density Functional Theory is used to investigate the structural and electronic properties of a novel oxide material obtained by doping zirconium oxide (ZrO2-zirconia) with Ta atoms. The material may have interesting applications in medical and electronic technologies. The work is motivated and driven by X-Ray Diffraction experiments (XRD), which measure the lattice parameters, atomic positions and space groups of these zirconia-based materials. The actual composition and structure of the powders are not known precisely. The Rietveld fit applied to the XRD data, taken at room temperature and pressure, shows that, by increasing the Ta doping from 0 to 12 %, the main structures found in the powder are Monoclinic (P21/c), Orthorhombic (Pca21), and Monoclinic(C2/c) at higher Ta concentration. The calculations are focused on the orthorhombic and monoclinic phases. It is experimentally demonstrated that at high doping percentage, Ta-doped orthorhombic zirconia is stable in a wide range of temperatures, after a synthesis that involve a thermal treatment in vacuum. Calculations provide insight into the thermodynamic and structural stability of the pure and Ta-doped oxides and predict the minimum-energy crystal structure of the orthorhombic phase. Moreover the results allow for the characterization of the electronic band structure of the material and on the effects of the point defects on the electronic properties. In particular, when Ta substitute for a Zr ion forming a point defect, there is the appearance of gap states, which may be of interest for technological electronic applications. The stabilization process might occur as the ion-ion substitution induces the generation of oxygen vacancies for charge compensation. The phase transition should be related to the amount of tantalum introduced and so to the amount of vacancies generated. The computed results will be useful for interpreting the existing measurements and for prompting new experiments that, on the basis of this new fundamental understanding, may better exploit the most interesting features of this new material.
Chemistry of …, 2011
Polarizable interaction potentials, parametrized using ab initio electronic structure calculations, have been used in molecular dynamics simulations to study the conduction mechanism in doped zirconias. The influence of vacancy-vacancy and vacancy-cation interactions on the conductivity of these materials has been characterized. Although the latter can be minimized by using dopant cations with radii which match those of Zr 4þ (as in the case of Sc 3þ ), the former appears as an intrinsic characteristic of the fluorite lattice that cannot be avoided and that is shown to be responsible for the occurrence of a maximum in the conductivity at dopant concentrations between 8 and 13%. The weakness of the Sc-vacancy interactions in Sc 2 O 3 -doped zirconia confirms that this material is likely to present the highest conductivity achievable in zirconias.
The dynamic properties of zircon studied by single-crystal X-ray diffraction and Raman spectroscopy
European Journal of Mineralogy, 2001
An investigation of the dynamic properties of synthetic non-metamict zircon, ZrSiO 4 , was undertaken using X-ray single-crystal diffraction and polarized single-crystal Raman spectroscopy. The X-ray results at room temperature show that the Zr cation located on the triangular dodecahedral site is tightly bonded. The atomic displacement parameters from different crystals and refinements on zircon are compared and shown to be different. This is probably a result of experimental problems associated with extinction and/or slight degrees of metamictization in natural samples. A calculation of the librational motions of the rigid SiO 4 tetrahedron give a mean-square libration of 3.3(3) degree 2 along the a axes and 6.9(6) degree 2 along the c axis. The Raman spectrum shows some unusual features, such as the low wave number SiO 4 bending motion, υ 2 , at 266 cm -1 , that can be explained by the structural properties of zircon. The structure is characterized by open 'channels' running parallel to [001] and they influence the energies of the SiO 4 bending modes depending upon their symmetries. The Raman modes differ strongly in intensity and this can be explained by vibrational interactions with the electronic state of Zr 4+ . In contrast to garnet, which shares a few structural similarities with zircon, the Zr cation residing in the large dodecahedral site shows little dynamic disorder. In addition, all the external modes in zircon are harmonic in comparison to a few modes in pyrope garnet, for example, that soften with decreasing temperature.
Hydrogen incorporation in crystalline zircon: Insight from ab initio calculations
American Mineralogist, 2013
The OH stretching vibration frequencies of crystalline zircon that contains water have been investigated by quantum mechanical calculation using CRYSTAL09 and several hybrid functionals. Incorporation mechanisms considered for H in zircon include: (1) hydrogarnet and partial hydrogarnet-type substitution;