Rare-earth orthovanadates: Covalency,chemical bonding,and optical spectra

Investigations of electronic structure and optical spectra were made for yttrium orthovanadate, and for rare earth orthovanadates RVO₄, where R = Ce, Nd, Eu, Tb, Dy, Gd, and Yb. The Hartree-Fock-Slater model was used in conjunction with a numerical discrete variational method to calculate energy levels and wavefunctions for molecular clusters (VO₄)^3? and (RO₈)^13? found in the orthovanadate crystal lattice. Analysis of the MO charge and spin densities reveals a significant involvement of rare earth 4f orbitals in chemical bonding, through hybridization of of R-5p and mixing with O-2p atomic orbitals. The MO energy level diagrams provide a satisfactory semiquantitative interpretation of the experimental excitation, reflection, and luminescence spectra. Energy transfer from the vanadate ion to the rare-earth ion is understood in terms of covalent mixing between metal and shared O-2p orbitals for neighboring (VO₄)^3? and (RO₈)^13? clusters. The relative luminescent efficiency of some rare-earth elements is explained on the basis of the calculated energy level diagrams.     

Dimorphism in triple orthovanadates MIILaTh (VO4)3 (MII = Sr or Pb)

Triple orthovanadates, SrLaTh(VO₄)₃ and PbLaTh(VO₄)₃ are found to be dimorphic, the crystalline form depending upon the firing temperature. The low-temperature form has zircon structure and the corresponding high-temperature form exhibits its well-known monoclinic modification. The X-ray powder diffractograms and infrared spectra are recorded and discussed in comparison with those of structurally related compounds.     

Structure and conductivity of strontium-doped cerium orthovanadates Ce1−xSrxVO4 (0≤x≤0.175)

A-site substituted cerium orthovanadates, Ce_1−xSr_xVO₄, were synthesised by solid-state reactions. It was found that the solid solution limit in Ce_1−xSr_xVO₄ is at x=0.175. The crystal structure was analysed by X-ray diffraction and it exhibits a tetragonal zircon structure of space group I4₁/amd (1 4 1) with a=7.3670 (3) and c=6.4894 (1) Å for Ce_0.825Sr_0.175VO₄. The UV-vis absorption spectra indicated that the compounds have band gaps at room temperature in the range 4.5-4.6 eV. Conductivity measurements were performed for the first time up to the strontium solid solution limit in air and in dry 5% H₂/Ar with conductivity values at 600 °C ranging from 0.3 to 30 mS cm⁻¹ in air to 30-45 mS cm⁻¹ in reduced atmosphere. Sample Ce_0.825Sr_0.175VO₄ is redox stable at a temperature below 600 °C although the conductivity is not high enough to be used as an electrode for solid oxide fuel cells.     

Hydrogen bond: Second order effects on potentials calculated by CNDO/2 method

Second order perturbation theory (SOP) has been used to introduce electronic correlation effects on CNDP/2 calculated quantities like stabilization energy, proton potential curves, intermolecular dependence on distance and orientation. The HCHO…H₂O' model has been studied. The SOP energy as a function of the R_O…O' distance introduces changes in the potential minima which amount to 14% of the CNDO/2 value and in the asymmetry of the potential energy curve. For the proton curve, an extra stabilization energy of 4 kcal/mole at the minimum is found as well as changes in the shape of the potential curve. Effects on orientation dependence are also reported.     

Phase formation in the Ag2O-ZnO (CdO)-V2O5 systems

The phase composition of the Ag₂O-ZnO(CdO)-V₂O₅ systems has been studied. Two new orthovanadates have been synthesized: AgZnVO₄ (monoclinic space group P21/n, a = 5.68710, b = 12.54080, c = 5.65947 Å, β = 116.209°) and AgCd₄(VO₄)₃ (orthorhombic space group Pnma, a = 9.82438 b = 7.01250, c = 5.37393 Å). One double metavanadate Ag₂Cd(VO₃)₄ has been synthesized. A continuous solid solution formulated as Ag_{3 ? 2x} Cd_{3 + x} (VO₄)₃ has been found to exist between AgCd₄(VO₄)₃ and already described AgCdVO₄. The Ag₂O-ZnO (CdO)-V₂O₅ systems have been triangulated in the subsolidus region.     

Die Infrarotspektren einiger Orthovanadate mit Apatitstruktur

The infrared spectra (4000–300 cm^?1) of orthovanadates with apatite structure, of the formula M₅(VO₄)₃X (M = Ca, Sr, Ba and X = F, Cl, Br) are measured and interpreted. The influence of the cations and of the halogen anions about the internal vibrations of the VO ions is discussed.     

Equilibrium structure and vibrational frequencies of the O4 molecule

SCF closed shell calculations were performed to determine the equilibrium structure and vibrational frequencies of the O₄ molecule by means of Payne's method and with the help of the molecule's symmetry coordinates. The equilibrium geometry corresponds to symmetry group D_2d with R = 1.505 Å and h = 0.094 Å. The vibrational frequencies are: ν₅(E) = 885.5 cm^?1, ν₃(B₁) = 1051.9 cm^?1, ν₁(A₁) = 1018.3 cm^?1, ν₄(B₂) = 880.3 cm^?1. The second vibrational coordinate (A₁) corresponds to a double-well potential. The first vibrational levels were calculated by a variational method.     

The synthesis and crystal structure of 5-acetyl-2'-deoxyuridine

5- Acetyl - 2' - deoxyuridine (1) has been synthesised by treating 2' - deoxy - 5 - ethynyluridine with dilute sulphuric acid. Condensation of the trimethylsilyl derivative of 5-acetyluracil with 2-deox-3, 5-di-O-p-toluoyl-α-d-erythropen chloride gave a mixture of α- and β-anomeric blocked nucleosides from which the α-anomer was isolated and the p-toluoyl groups removed to give 5 - acetyl -1 - (α - d - 2- deoxyerythropentofuranosyl) uracil. Only a poor yield of the β-anomer (1) was obtained by this procedure. The UV spectra and m.p. obtained for 1 differed from the values quoted in the literature. The crystals of 1 are monoclinic, space group P2₁, with a = 9.525, b = 12.16, c = 5.22 Å, β = 92.03° and two molecules in the unit cell. The structure was refined by least-squares calculations to R 3.4% for 1426 observed counter amplitudes. The pyrimidine ring is essentially planar with the acetyl group inclined at 6° to it. The sugar ring has the highly unusual C(4')-exo conformation and the arrangement about C(4')-C(5') is such that O(5') is oriented gauche with respect to both O(1') and C(3'). The glycosidic torsion angle O(1')-C(1')-N(1)-C(6) is 56° (anti conformation).     

Enthalpies of formation of rare earth orthovanadates, REVO4

Rare earth orthovanadates, REVO₄, having the zircon structure, form a series of materials interesting for magnetic, optical, sensor, and electronic applications. Enthalpies of formation of REVO₄ compounds (RE=Sc, Y, Ce-Nd, Sm-Tm, Lu) were determined by oxide melt solution calorimetry in lead borate (2PbO·2B₂O₃) solvent at 1075 K. The enthalpies of formation from oxide components become more negative with increasing RE ionic radius. This trend is similar to that obtained for the rare earth phosphates.     

Spectrophotometric determination of copper with α, β, γ, δ-tetraphenylporphine trisulfonate

Water-soluble porphyrin, α, β, γ, δ-tetraphenylphorphine trisulfonate (TPPS, H₂R), was found to be a very useful agent for both the direct spectrophotometric determination and the photometric titration of copper(II). The molar absorptivity of H₄R²⁺ at 434 nm is 5.0·10⁵ and the spectrophotometric sensitivity is 0.00013 μg Cu cm^?2 for A=0.001. Beer's law is followed in the range 0.006 μg–0.06 μg Cu ml^?1. Among twenty-two elements examined, only zinc(II) seriously interfered. Acid dissociation constants and salt effects on the spectra of TPPS were evaluated.     

On the hydroxyl ions in apatites

The possible mechanism originating the stretching and librational bands of the OH groups in ir spectra of various apatites are discussed. On the basis of relating the ν_s frequency of the V₃ hindered potential barrier, it is deduced that hydrogen bonds between the OH groups and the nearest O's of PO^3?₄ ions exist, and that they are responsible for the observed frequencies of the stretching and librational modes. The additional ir OH bands of heated nonstoichiometric hydroxylapatites are also discussed.     

Ba2(VO2)(PO4)(HPO4)·H2O, a New Barium Vanadium(V) Phosphate Hydrate Containing Trigonal Bipyramidal VO5Groups

The hydrothermal synthesis, single crystal structure, and some physical properties of Ba₂(VO₂)(PO₄)(HPO₄)·H₂O, a new barium vanadium(V) phosphate hydrate, are reported. This phase is built up from one-dimensional chains of unusual VO₅trigonal bipyramids and (H)PO₄tetrahedra, fused together via V–O–P linkages. These anionic chains propagate along the polar [010] direction. 11-Coordinate barium cations and water molecules occupy the interchain regions and link the chains together. Structural data for this phase and other known barium vanadium phosphates are briefly compared. Crystal data: Ba₂(VO₂)(PO₄)(HPO₄)·H₂O,M_r=566.57, monoclinic, space groupP2₁(No. 4),a=5.0772(5) Å,b=8.724(2) Å,c=10.806(1) Å,β=90.795(8)°,V=478.6(1) ų,Z=2,R=2.65%,R_w=2.89% [147 parameters, 1893 observed reflections withI>3σ(I)].     

Synthesis,structure and magnetic properties of a quasi-two-dimensional compound [Cu(C5H4NCOO)2] · 2H2O

A new compound [Cu(C₅H₄NCOO)₂] · 2H₂O has been prepared. Its structure consists of a chain built from [Cu(C₅H₄NCOO)₂] units, bridged through weak interactions between the copper(II) atoms and the O atoms of the carboxylate groups of a adjacent [Cu(C₅H₄NCOO)₂] units. I.r. spectra, thermal analysis and elemental analysis have been recorded for the complex. Room temperature X-band e.s.r. spectra of powered samples and variable-temperature magnetic susceptibility studies indicate that the compound exhibits a weak ferromagnetic interaction through Cu—O—C—O—Cu pathways.     

Formation of gels and gel-glasses in the VO2(V2O5)-SiO2 systems

Blue-coloured gels have been prepared in the VO₂-SiO₂ system up to 80 mol% VO₂ by sol-gel technology using TEOS and aqueous solutions of VOSO₄·5H₂O. It is established by means of VIS and ESR spectra that at low temperatures VO²⁺ complexes are formed. An oxidation of V⁴⁺ has taken place with increasing temperature, and V₂O₅ and cristobalite have been separated. Silica gel glasses stable up to 800°C have been obtained from gels containing 1–3 mol% VO₂.     

Displacive disorder and dielectric relaxation in the stoichiometric bismuth-containing pyrochlores, Bi2MNbO7 (M=In and Sc)

The structural disorder and temperature-dependent dielectric properties of two Bi-based niobate pyrochlore systems which have both previously been reported to occur at the ideal Bi₂(M^IIINb^V)O₇ stoichiometry without any compositional disorder on the pyrochlore A site, namely the Bi₂InNbO₇ (BIN) and Bi₂ScNbO₇ (BSN) pyrochlore systems, have been carefully re-investigated. It is established that A site stoichiometric, Bi-based niobate pyrochlores can indeed exist. Electron diffraction is used to investigate the nature of the displacive disorder therein both at room temperature as well as at close to liquid nitrogen temperature. The characteristic structured diffuse scattering observed arises from β-cristobalite-like, 1-d correlated rotations and associated translations of chains of corner-connected O′Bi₄ tetrahedra. The temperature-dependent dielectric properties including the low temperature dielectric relaxation properties of these A site stoichiometric, Bi-based niobate pyrochlores are also reported as are the micro-Raman spectra thereof. The experimental results suggest that the dipoles as well as the glassy relaxation behaviour in these Bi-based pyrochlores are directly related to these β-cristobalite-like, correlated rotations of 〈110〉 chains of corner-connected O′Bi₄ tetrahedra.     

X-ray absorption spectroscopy of transition-metal trichalcogenides

The sulfur K and metal L_III absorption spectra of transition-metal trichalcogenides (TMTC's) were measured. The matrix element effect plays an important role in these spectra. It was considered that the structures up to 5 eV above the absorption edge reflect the chalcogen antibonding band, the metal nonbonding d_z² band, and the metal d bands, and that the higher energy structures are derived from the metal s and p bands. The chalcogen antibonding band arises from chalcogen pairing and the metal d, s, and p bands are the mixture bands with chalcogen p orbitals. Evidence that shows that the lowest conduction band of the group IV TMTC's is the chalcogen antibonding band is presented. The overlap of the metal d and metal s bands is promoted by increasing the atomic number of chalcogen atoms.     

Stability and oxygen ionic conductivity of zircon-type Ce1−xAxVO4+δ (A=Ca, Sr)

Zircon-type Ce_1−xA_xVO_4+δ (A=Ca, Sr; x=0-0.2) are stable in air up to approximately 1300 K, whilst further heating or reducing oxygen partial pressure leads to formation of A-site deficient zircon and CeO_2−δ phases. The stability boundaries of Ce_1−xA_xVO_4+δ are comparable to those of vanadium dioxide and calcium orthovanadate. At oxygen pressures lower than 10⁻¹⁵ atm, perovskite-type CeVO_3−δ is formed. The oxygen ion transference numbers of Ce_1−xA_xVO_4+δ, determined by faradaic efficiency measurements in air, vary in the range from 2×10⁻⁴ to 6×10⁻³ at 973-1223 K, increasing with temperature. The oxygen ionic conductivity has activation energy of 87-112 kJ/mol and is essentially independent of A-site dopant content. Contrary to the ionic transport, p-type electronic conductivity and Seebeck coefficient of Ce_1−xA_xVO_4+δ are influenced by the divalent cation concentration. The average thermal expansion coefficients of Ce_1−xA_xVO_4+δ, calculated from high-temperature XRD and dilatometric data in air, are (4.7-6.1)×10⁻⁶ K⁻¹.     

The singlet nπ* states of para-benzoquinone

The vibronic nπ* singlet spectra of p-benzoquinone-h₄ and p-benzoquinone-d₄ have been observed in a supersonic jet and some as yet unknown excited state fundamentals in the vapor phase have been assigned. The electric dipole forbidden, magnetic dipole allowed origin of the ¹B_1g ← ¹A_g transition is observed at 20045 cm^?1. The origin of the¹A_u ← ¹A_g, transition has been indirectly determined at 19991 cm^?1 from the vibronic excitation spectra. Neither shows a deuterium shift.     

Structural Study of an Unusual Cubic Pyrochlore Bi1.5Zn0.92Nb1.5O6.92

Single-phase pyrochlore-type specimens of Bi_1.5Zn_0.92Nb_1.5O_6.92 were studied using combined electron, X-ray and neutron powder diffraction techniques. Rietveld refinements using neutron powder diffraction data confirmed an average pyrochlore structure A₂B₂O₆O′ (Fd&3macr;m, a=10.5616(1) Å) with both Bi and Zn mixed on the A-sites. However, refinements revealed significant local deviations from the ideal pyrochlore arrangement which were caused by apparent displacive disorder on both the A and O′ sites. The best fit was obtained with a disordered model in which the A-cations were randomly displaced by ∼0.39 Å from the ideal eight-fold coordinated positions. The displacements occur along the six 〈112〉 directions perpendicular to the O′-A-O′ links. In addition, the O′ ions were randomly displaced by ∼0.46 Å along all 12 〈110〉 directions. Crystal-chemical considerations suggest the existence of short-range correlations between the O′ displacements and both the occupancy of the A-sites (i.e., Bi or Zn) and the directions of the A-cation displacements. The combined A-cation and O′ displacements change the coordination sphere of the A-cations from 8 to (5+3); the resulting coordination environment of the A-cations bears similarities to that of the (5+1)coordinated Zn in zirconolite-like Bi₂Zn_2/3Nb_4/3O₇. The observed displacive disorder in the A₂O′ network of the Bi_1.5Zn_0.92Nb_1.5O_6.92 structure involves atoms associated with the lowest-frequency vibrational bending mode, and is likely responsible for both the high dielectric constant and the dielectric relaxation reported for this compound.     

Synthesis,structural and spectral investigations of new types of dioxovanadium(V) complexes withS-methylthiosemicarbazones derived from salicyl and 2,4-dihydroxybenzaldehydes. X-ray structure of ammonium(salicylaldehydeS-methylthiosemicarbazonato) dioxovanadate(V) monohydrate

Summary The x-ray crystal structure of [VO₂(HL¹)] (where L¹ denotes the dianion of theS-methylthiosemicarbazone of salicylaldehyde) has been determined and refined to R=0.058 (R_w=0.063) for 3377 observed reflections.There are two symmetrically independent molecules in the asymmetric unit, showing no significant differences in their geometries. The vanadium atom is pentacoordinated in a distorted square-pyramidal arrangement.Absorption spectra of [VO₂(HL¹)] and [VO₂(HL²)], in the 10000–45000 cm^–1 range, were calculated from the measured reflectance spectra, applying Kubelka-Munk's theory. The extinction coefficients were determined from the absorption spectra of the solutions of these complexes in methanol. The observed maxima are interpreted on the basis of intraligand transition and charge-transfer spectra on the basis of the presence of approximate groupC _2v for both complexes.