Rare Earth Vanadates Research Articles

Binary rare earth-vanadium systems

Available data concerning the phase equilibria in the rare earth vanadium binary systems were accumulated and evaluated. The atypical Sc-V, Y-V, Ce-V, Eu-V and Y-V systems were deleted. The experimental data for each of the remaining systems was then used to evaluate an initial set of interaction parameters. Refined parameters were developed by plotting the initial parameters against the atomic numbers of the rare earths and smoothing the trends. These refined parameters were then used to calculate phase equilibria for the various systems. The refined parameters show quite good agreement with estimated values generated by the Mi edema approximations.

A high-pressure Raman study of yttrium vanadate (YVO 4) and the pressure-induced transition from the zircon-type to the scheelite-type structure

The pressure dependences of the Raman active modes in yttrium vanadate (YVO 4) crystallizing in the zircon-type structure D 4 h 19 ( I4 1 amd ), have been studied using a diamond anvil cell up to 15 GPa. At room temperature the zircon-type structure transforms to the scheelite-type structure C 4 h 6( I4 1 a ) with a = 5.04 Å and c = 11.24 Å near 7.5 GPa. The density changes from 4.24 to 4.74 g cm 3 in the transition, and the corresponding volume decrease is Δ V = 5.07 cm 3 mole . The high-pressure phase is retained on release of pressure. The optical absorption edge shifts from 3.87 to 2.77 eV at the transition, presumably due to charge transfer, or due to V-O bond length change of the VO 4 tetrahedra. The internal mode frequencies of the VO 4 tetrahedra also decrease abruptly at the transition. The V-O bond stretching frequency decreases from 891 to 828 cm −1 in the scheelite-type phase, which suggests a fundamental change in the VO 4 unit. The general question of the stability of the zircon-type structure under high pressure is discussed in the light of the high-pressure behavior of a large number of rare earth vanadates and arsenates.

Xes characterization of rare earth vanadates

Measurements have been made on the Kβ1,3 and Kβ5 X-ray emission lines of vanadium and on the Lβ2,15, Lγ1, Lγ2 and Lγ4 lines of rare earths. These lines were analysed in PrVO4, EuVO4, TbVO4, DyVO4, HoVO4 TmVO4 and LuVO4 single crystals, as well as in standards: V2O5, Pr2O3, Eu2O3, metallic V and other rare earths. The measurements have been performed using a single-crystal spectrometer of the Johann type with LiF analysing crystal in EPMA. The investigated crystals have been grown from PbO·V2O5 flux. The presence of V4+ ions in TbVO4 and TmVO4 crystals was detected by the analysis of the energy separation of the 3p and 3d levels. It was revealed by the observation of the multiplet splittings of the 4d52 and 4d32 levels and by the analysis of the 4f → 5d electronic transfer that the real valency of Eu and Tb ions in vanadates differs from 3 +. This was confirmed by the observation of the 5d → 2s electronic transitions.

Electrical transport in heavy rare-earth vanadates

This paper reports measurements of electrical conductivity (σ) and Seebeck coefficient (S) between 300 and 1250 K and differential thermal analysis (DTA) and thermogravimetric analysis (TGA) between 300 and 1200 K, together with X-ray diffraction studies of heavy rare-earth vanadates (RVO4 with R=Tb, Dy, Ho, Er and Yb). All these vanadates have been found to have a tetragonal unit cell. The DTA study shows a flat dip in the temperature interval 1075 to 1300 K, indicating a possible structural phase transition of these compounds. Practically no weight loss has been observed in TGA from 300 to 1200 K in any of the vanadates. All RVO4 are semiconducting materials with the room-temperatureσ value lying in the range 10−12 to 10−3 Ω−1 m−1, becoming of the order of 10−2 Ω−1 m−1 around 1000 K. The electrical conductivity of all vanadates exhibits an exponential increase in the temperature intervals 420 K toT 1 andT 1 toT 2, with different values of the activation energy. A logσ againstT −1 plot shows a peak aroundT 3 and drops to a minimum value aroundT 4, before increasing again with temperature.T 4 >T 3 >T 2 >T 1 are different for different vanadates and these are termed “break temperatures”.T 4 lies well within the temperature range of the DTA peak and can be termed the phase transition temperature. In the lower temperature interval the electrical conduction is essentially extrinsic. The localized charge carriers on defect centres conduct by a hopping mechanism. The defect centres are V4+ ions in all vanadates with R4+ centres in some of them. It is concluded that in the temperature intervalT 1 <T <T 2 the conduction mechanism is of the intrinsic band type, with oxygen 2p and vanadium 3d as the valence and conduction bands, respectively. Related parameters like the energy band gap and the mobilities of the charge carriers have also been evaluated. The low values of mobility suggest that large polarons with intermediate coupling are the charge carriers rather than bare electrons in the intrinsic region. All these vanadates tend to become metallic, but before this is achieved the phase change makes the conductivity smaller.

Electrical transport in light rare-earth vanadates

Electrical transport in light rare-earth vanadates

Single crystal growth conditions and characterization of some rare-earth vanadates

The crystal growth conditions, solubility curves, supercooling and relative supersaturation data measured by the thermogravimetric method for PrVO 4 and EuVO 4 grown from PbO · V 2O 5 flux are presented. The influence of Yb and Gd as well as Mo, Fe, Cr, and Ti dopants on crystal growth conditions and crystal structure have been investigated. It was found that when the amount of dopant does not exceed 1% of the basic RE ion, the crystallisation temperature changes are very small, but the changes in crystal size are significant. The structure of the valence bands of PrVO 4 and EuVO 4 has been investigated by X-ray emission spectroscopy. The vanadium Kβ and Lγ lines of RE (rare earth) ions were analysed.

Unusual magnetic behaviour of heavy rare earth vanadates at higher temperature

Unusual magnetic behaviour of heavy rare earth vanadates at higher temperature

Unusual magnetic behaviour of light rare earth vanadates at higher temperature

Mesure de la susceptibilite magnetique et de l'aimantation de RVO 4 (avec R=Gd, Tb, Dy, Ho, Er et Yb) entre 295 et 925 K

The secondary effect of the cooperative Jahn-Teller distortion in rare earth vanadate

A short range interaction between displacement of the rare-earth ion and the local deformation of its environment in rare earth vanadate is noticed. Anomalies in the dielectric constant as well as the specific heat and others are discussed based on this secondary effect accompanying the cooperative Jahn-Teller phase transition.

ChemInform Abstract: STUDIES ON THE HIGH‐PRESSURE REACTION OF RARE EARTH SESQUIOXIDES WITH VANADIUM DIOXIDE

AbstractSeltenerdmetallsesquioxide Ln2O3 (Ln: z.B. La, Nd, Sm, Y, Ho, Er) reagieren bei 1400°C und 50 oder 30 kbar mit V02 unter Reduktion des 4wertigen V zu den Seltenerdmetallvanadaten(III), LnVO3.

Studies on the High-pressure Reaction of Rare-earth Sesquioxides with Vanadium Dioxide

Abstract The reaction of rare-earth sesquioxides (Ln2O3 with vanadium dioxide (VO2) at 1400 °C and 50 kbar and 30 kbar was studied. Quadrivalent vanadium ions were reduced to the trivalent state, rare-earth vanadates(III) (LnVO3) being obtained. For LnVO3 a typical transformation of the ABO3-type compounds, one from the aragonite-type to the perovskite-type via the vaterite- or calcite-type structure was observed with a decrease in the ionic radius of the rare-earth ions. The magnetic properties of the vaterite-type LnVO3 were studied.

Thermal conductivity of rare earth vanadates and arsenates undergoing a cooperative Jahn-Teller effect

Presents results of thermal conductivity experiments performed on TmVO4, TmAsO4, TbVO4 and DyVO4 single crystals in the temperature range 0.1-40K. Preliminary calculations within the Callaway model are developed and discussed. The authors also report measurements on TmVO4 and TmAsO4 with an applied magnetic field and discuss the very strong effect observed.

Thermal properties of rare earth vanadates and arsenates with a view to magnetic refrigeration applications

RXO 4 compounds (R = rare earth and X = V, As or P) exhibit a crystallographic phase transition sensitive to the magnetic field. They furthermore have a large thermal conductivity which is increased by application of the field. The possibility of using these compounds for magnetic cooling is examined. TmVO 4 and DyVO 4 are studied with particular interest.

RVO4-new oxide ferroelectrics (R = Nd, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Sc)

Dielectric measurements of the rare earth vanadates (R = Nd, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu) and Sc have been carried out. On all of the samples at 0 to 10°C, piezoelectric effects were discovered. In the region — 35° to 10°C at electric fields of E = 0.5 + 1.5 k V/cm, dielectric hysteresis loops were observed. At 20 to 25°C anomalous changes of e were established. Thus, like the formerly discovered family of the rare earth arsenates, RVO4 crystals are also ferroelectric.

A study of cooperative Jahn-Teller phase transitions in rare-earth vanadates by linear birefringence. II. DyVO4

For pt.I see ibid., vol.10, p.2937 (1977). The authors have measured changes in linear birefringence ( Delta n) associated with the cooperative Jahn-Teller phase transition of DyVO4 near 14K as a function of temperature and magnetic fields, B, between 0.024 and 0.095 T. Theoretical arguments show that Delta n is directly proportional to the order parameter of the transition and that B2 is equivalent to the conjugate ordering field. By extrapolation to zero field they obtain the temperature dependence of the order parameter and the susceptibility. The data are compared with calculations based on a mean-field 'compressible' Ising model. For a reasonable choice of adjustable parameters this classical description gives a good fit to the data close to TD consistent with general theoretical arguments and more detailed calculations, but it deviates progressively away from TD presumably because of the known importance of short-range interactions in the system.

V51NMR studies of transferred hyperfine effects in the rare-earth vanadates

$^{51}\mathrm{V}$ ($I=\frac{7}{2}$) NMR investigation in the whole set of rare-earth vanadates ($R\mathrm{V}{\mathrm{O}}_{4}$) in the polycrystalline form at 300 K has revealed transferred hyperfine effects. It has been observed that the $^{51}\mathrm{V}$ NMR line shape strongly depends on the relative strength of the magnetic and quadrupolar parts of the transferred hyperfine interaction. On the basis of the powder pattern the $R\mathrm{V}{\mathrm{O}}_{4}$ system may be divided into two halves. The first-half members always exhibit a quadrupolar split central transition in the frequency range 2-12 MHz, whereas the members of the second half exhibit at higher frequencies an asymmetric line which, however, splits as the frequency is lowered. The hyperfine-interaction parameters, viz., the isotropic shift ${K}_{\mathrm{iso}}$, the anisotropic shift ${K}_{\mathrm{an}}$, and the quadrupolar coupling constant $\frac{{e}^{2}\mathrm{qQ}}{h}$ have been estimated from the central transition. The linear plot of ${K}_{\mathrm{iso}}$ against calculated $〈{S}_{z}〉$ values of rare-earth ions is indicative of the contact nature of ${K}_{\mathrm{iso}}$. Though ${K}_{\mathrm{an}}$ is dominated by the dipolar part, a small amount of anisotropic hyperfine part ${K}_{\mathrm{an}}^{\mathrm{hf}}$ could be separated from the experimental shift. Thus the results of both ${K}_{\mathrm{iso}}$ and ${K}_{\mathrm{an}}^{\mathrm{hf}}$ seem to uphold the presence of a small amount of covalent contribution (${f}_{s}\ensuremath{\sim}0.01%$ and ${f}_{s}\ensuremath{\sim}0.0001%$) in these basically ionic compounds. A comparison of the ligand NMR shifts in $3d$ and $4f$ systems (oxysalts) suggests that the mechanism of spin transfer within the oxyanion is not the same in these two series. Finally, the $\frac{{e}^{2}\mathrm{qQ}}{h}$ values (0.400-0.300 MHz) exhibit a gradual decrease along the isostructural series of $R\mathrm{V}{\mathrm{O}}_{4}$. The value of the coupling constant and its variation across the rare-earth series in different compounds have been qualitatively interpreted by taking into account the bonding effects.

Preparation and crystal chemistry of some rare earth vanadium (IV) pyrochlores (RE) 2V 2O 7; RE = Lu, Yb, Tm, Lu 1−xY x, Lu 1−xSc x

The preparation of the phases (RE) 2V 2O 7 where RE = Lu, Yb, Tm, and the solid solutions Sc xLu 1−x (x = .10 → .50) and Y xLu 1−x (x = .20 and .40), using a CO CO 2 buffer system are described. Precision lattice constants were determined and compared with previous work, finding good agreement with (1) but not with (2). The solid solution (Sc xLu 1−x) 2V 2O 7 exists as a pyrochlore to x = .30, compared with x = .55 for the similar (Sc xLu 1−x) 2 Ti 2O 7 system. Chemical analysis on the members x = .40 and x = .50 indicate significant amounts of V(III) and X-ray data show a diminishing intensity for the “ordering” reflections (h, k, l all odd). These results suggest that Lu(III), Sc(III), V(III) and V(IV) are partially disordered over both cation sites tending toward a C-type (RE) 2O 3 structure or a defect fluorite structure with increasing x.

Solubility curves in high-temperature melts for the growth of single crystals of rare earth vanadates and phosphates

With thermoanalytical methods the solubility of rare earth salts in fluxes used for the growing of single crystals are investigated and the systems YVO4NaVO3 and REPO4Pb2P2O7 (RE = Tb to Lu, Y) are measured. The various REPO4 show different solubilities. Borax and lead oxide as additions to the fluxes increase the solubilities of the vanadates and phosphates respectively. The results allow the exact determination of starting temperatures and compositions for the flux growth of better crystals of these compounds. Mit Hilfe thermoanalytischer Methoden werden die Loslichkeiten von RE-Salzen in Flusmitteln untersucht, die zur Kristallzuchtung dieser Verbindungen dienen. Gemessen werden die Systeme YVO4NaVO3 und REPO4Pb2P2O7 (RE = Tb bis Lu, Y). Die verschiedenen REPO4 zeigen unterschiedliche Loslichkeiten. Borax und PbO-Zusatze zu den Flusmitteln fur Vanadate und Phosphate erhohen deren Loslichkeiten in den betreffenden Flusmitteln. Aus den Mesergebnissen lassen sich die Versuchsparameter, wie Anfangstemperaturen und Zusammensetzung, zur Zuchtung besserer Kristalle dieser Verbindungen entnehmen.

Precipitation of rare earth vanadates from aqueous solution

The chemistry of vanadium in solution has been investigated over the entire pH range, with special emphasis on rare earth precipitates. Whereas prior investigators were confronted with an instability range between pH 5.5–7.5 due to precipitation of hydrous V 2O 5, the present work did not encounter such a barrier. Thus, the complete pH range could be surveyed by starting with VO 2+ ion plus a rare earth in strong acid solution and titrating with strong base plus an oxidizing agent. The VO 2+ ion appears to be oxidized early in the titration to form VO 2 + ion which then condenses in solution to form vanadate anionic precipitates.

Rotational and finite strain contributions to spin-lattice interactions in paramagnets

A theory is developed to treat rotational and finite strain contributions to the spin-lattice coupling in paramagnets. The spin system is described by pseudospin operators whose spatial rotational properties may be unknown. Measurements of the magnetoelastic shifts in the elastic constants of two rare earth vanadates are in excellent agreement with the theory.