Rare Earth Molybdates Research Articles

Electrical and Magnetic Properties of LaMo8-xO14 Containing Isolated Mo8 Clusters

Single crystal specimens of LaMo8-xO14 (x = 0 and 0.3) suitable for transport property measurements have been prepared by electrolyzing fused mixtures of sodium and rare earth molybdates. Electrical resistivity measurements with the current parallel to the ab plane of LaMo8-xO14 containing the quasi-discrete units of Mo8 clusters showed semiconducting behavior down to 20 K for both compounds. The electrical resistivity in the ab plane (ρ300Kab = 5 × 10-3ohm · cm) of LaMo7.70O4 is an order of magnitude smaller than the corresponding value for the fully stoichiometric compound (ρ300Kab = 4.4 × 10-2 ohm · cm). Furthermore, the ρ300Kab of La Mo7.70O14 is a factor of 4 lower than that measured along the c-axis (ρ300Kc = 2 × 10-2 ohm · cm) implying quasi-low-dimensional behavior. The thermal activation energy for electrical conductivity estimated from the lnρ vs 1000/T plots showed a significant decrease near ∼70 K and 130 K for LaMo7.70O14 and LaMo8O14, respectively. The magnetic susceptibility data collected on a batch of randomly oriented crystals revealed the presence of local moments in LaMo7.7O14, while the stoichiometric analogue was diamagnetic. The semiconducting nature of the LaMo8-xO14 phases may be attributed to the unfavorable intercluster distance (3.08 Å) as well as positional and configurational disorder of the Mo8 clusters.

Magnetic phase transitions in CsEr(MoO4)2 and KDy(MoO4)2-chain-layered Ising compounds (abstract)

The crystals investigated belong to the family of the orthorhombic layered rare-earth double molybdates. These crystals have aroused interest because of the diversity of structural and magnetic phase transitions, which are due to the crystalline structure and the electron energy spectrum of rare-earth ions. These ions are distinguished for their Ising behavior. The magnetization and susceptibility measurements were performed at temperatures between 0.5 and 300 K in magnetic fields up to 0.4 T along three principal magnetic axes. The 3D magnetic ordering is found to occur at Tc=0.84 K for CsEr(MoO4)2 and Tc=1.1 K for KDy(MoO4)2. The magnetic properties are strongly anisotropic both above and below Tc. In the magnetic ordered state spin-orientation phase transitions were found in both of these compounds. For CsEr(MoO4)2 a typical metamagnetic transition was observed along one magnetic field direction (b). There is a more intricate situation in the case of KDy(MoO4)2. The spin-orientation phase transition has a rather complex character: it is anitferromagnetic along the a axis, and ferromagnetic along the b and c axes. The magnetic structures of these compounds were determined. In the case of CsEr(MoO4)2 a collinear two-sublattice structure of antiferromagnetically ordered ferromagnetic chains is realized. For KDy(MoO4)2, the magnetic structure is defined as a complicated noncollinear four sublattice structure with magnetic moments rotated 35° in the ac plane and 10° in the bc plane.

Elemental analysis of gel-grown rare earth molybdate crystals using energy-dispersive X-ray fluorescence spectrometry

Rare earth molybdate crystals such as those based on lanthanum, neodymium and some mixed molybdates, grown by the gel method, were analysed for major and trace impurity elemental concentrations using energy-dispersive x-ray fluorescence spectrometry in order to understand the characteristics of the material in terms of structural perfection and composition. The method used permits the simultaneous determination of rare earth elements such as La and Nd and also Mo and impurities such as Fe and Zn. Based on the observed concentrations of major elements, it is shown that the formulae so far assumed for some of these composites may be incorrect and may need a major refinement. Monoenergetic x-rays of 22.1 and 59.6 keV were employed for fluorescence excitation and optimum detection efficiency was obtained for a large number of elements. Pulverized samples mixed with cellulose powder and specimens prepared in the form of pellets were used. The fundamental parameter method was used for the quantitative estimation of elemental concentration.

Magneto-electro-elastic effects in some rare earth molybdates and related properties

A giant magneto-electric effect was observed in single-crystal samples of metastable orthorhombic ferro-electric ferro-elastic paramagnetic beta ' phases of rare earth molybdates Tb2(MoO4)3 and TbGd(MoO4)3. The electric polarization in these compounds showed large and sharp changes in pulsed magnetic fields of 100 kOe at 78 K. Magnetic fields applied in a certain direction produced irreversible alterations in ferro-electric domain structure of these compounds that were observed visually. This effect was explained qualitatively in terms of large spin-orbit coupling in rare earth triply positive-charged ions and of interaction between the crystal field and the anisotropic charge density cloud of 4f electrons of rare earth triply charged ions of non-zero angular momentum. Related anomalies in the magnetostriction and in magnetization were also observed. The photoluminescence related to Tb3+ ions was observed at temperatures in the range 4.2-300 K. The most intense photoluminescence takes place in the green part of the spectrum. Here two groups of narrow sharp lines were observed, each group consisting of three lines. The photoluminescence exhibited a strong dependence of the polarization upon the ferro-electric domain orientation. Some applications of the effect are proposed.

Irreversible alterations of ferroelectric domain structure in paramagnetic rare earth molybdates induced by a magnetic field

The effect of a magnetic field on the ferroelectric domain structure of single crystal samples of the metastable orthorhombic ferroelectric ferroelastic β′-phase of Sm2(MoO4)3, Gd2(MoO4)3, Tb2(MoO4)3, and TbGd(MoO4)3 was investigated at 78 K in pulsed magnetic fields up to 100 kOe. It was shown that a magnetic field of certain orientation may change a ferroelectric domain structure in Tb2(MoO4)3 and TbGd(MoO4)3 that was accompanied by large changes of the electric polarization of the sample under investigation. The effect is explained by large magnetostrictive deformations. The prediction is made that there must be a positive longitudinal magnetostriction in a magnetic field applied along the [100] crystallographic axis and a negative transverse magnetostriction in the (001) plane in a magnetic field applied along the [100] crystallographic axis in Tb2(MoO4)3 and in TbGd(MoO4)3 β′-phases at 78 K and at a magnetic field strength of about 100 kOe.

Electrical transport in light rare-earth molybdates

This paper reports electrical transport studies of rare-earth vanadates (RVO4 with R = Ce, Pr, Nd, Sm, Eu and Gd), prepared by solid state reaction and characterized by X-ray diffraction studies. TGA study (300 to 1200 K) shows no weight loss; possible phase transitions in the range 1075 to 1300 K have been indicated by DTA studies. All these vanadates are typical semiconducting compounds with room temperature electrical conductivity (σ) lying between 10−4 and 10−2Ω−1m−1. Measurements of σ and the Seebeck coefficient (S) are reported in the temperature interval 400 to 1200 K. Two linear regions 400 to T1K and T1 to T2K have been obtained from the log σ against T−1 as well as the S against T−1 plots followed by a peak around T3 and minima around T4K. T4>T3>T2>T1, are different for different vanadates. It has been concluded that in the interval 400 to T1K, conduction is of the extrinsic hopping type with Ce4+ in CeVO4, Pr4+ in PrVO4 and V4+ in Nd to Gd vanadates as dominant defect centres. In the temperature interval T1, to T2K, the conduction has been shown to be of the intrinsic band type in all vanadates with polarons of intermediate coupling strength as the dominant charge carriers. Above T2 all vanadates tend to become metallic, but before this is achieved the phase change makes the conductivity smaller. T4 is close enough to the temperature corresponding to the DTA peak to be termed the phase transition temperature.

SmP1O5: Growth of rare-earth molybdate crystals

Abstract Growth conditions of high quality rare-earth molybdate crystals by modified Czochralski and Stepanov methods in automated regime are discussed.

Synthesis of charge with the growth of rare-earth molybdate crystals

Synthesis of charge with the growth of rare-earth molybdate crystals

Effect of Layered Structure on the Lattice Heat Capacity of the Rare Earth Molybdates

Temperature dependence of the heat capacity of 1ayered rare earth molybdates was measured in the temperature range from 0.4 K to 23 K. It is shown that the temperature dependence of the 1attice heat capacity can be described by the T, where n Ο 3. It is shown that differently from KDy(ΜoO4)2 and CsDyEu(MoO4)2 for CsDy(ΜoO4)2 and CsGd(MoO4)2 we obtained the temperature term with n < 3, which can be connected with the 2D behaviour, manifested also in the so-called membrane effect.

Preparation and characterization of ferroelectric rare-earth molybdate ceramics

Abstract Rare Earth Molybdates, R 2(MoO4)3 (R = rare-earth ions), in their β’ (ferroelectric) forms, have many interesting properties which can be utilized for industrial applications. As not much have been reported on some members of the family, we have prepared ferroelectric samples of pure Sm2(MoO4)3, Gd2(MoO4)3, Dy2(MoO4)3 and mixed rare earth molybdates by quenching them to room temperature from their high temperature phase. X-ray, thermal and dielectric properties of the compounds have been studied extensively to understand the existence of some interesting properties in them.

Differential scanning calorimetric studies of ferroelectric rare-earth molybdates

Gadolinium and samarium molybdates in their pure forms and substituted with other homovalent rare-earth ions (in small concentrations) have been studied using the DSC technique. The ferroelectric phase transitions in the said compounds have been detected using this technique. It has been observed that there is no change in the ferroelectric phase transition temperature when the homovalent cation is substituted to a small extent.

Microhardness measurements on single crystals of gel-grown rare-earth (Nd) molybdate and paramolybdate

Microhardness measurements on single crystals of gel-grown rare-earth (Nd) molybdate and paramolybdate

Preparation of tetragonal defect scheelite-type RE 2(MoO 4) 3 (RE=La TO Ho) by precipitation method

Ten isostructural rare earth molybdates RE 2(MoO 4) 3 (RE=La TO Ho) of tetragonal defect scheelite-type structure (a new phase for molybdates of Pr to Ho) have been prepared by reacting rare earth nitrates with ammonium paramolybdate in solution, and calcining the precipitates at 520°C. All of these, except La and Ce, have phase transition temperatures ranging from 880°C to 1020°C. Molybdates of Pr to Gd transform to the Eu 2(WO 4) 3 structure when heated at 850°C, and that of Pr through Dy to the ferroelectric phase when heated at 1040°C and cooled in air. The reactivity of lattice oxygen in redox reaction has been correlated with the 4f structure of the RE 3+ ions.

New rare earth molybdates with the β-Gd2(MoO4)3 structure

Abstract A crystal chemical study carried out on the binary systems Ln2(MoO4)3-MIII 2(MoO4)3 (Ln = rare earth, MIII = Bi, La, Er), has allowed us to identify new rare earth molybdates with the β-Gd2(MoO4)3 structure. The conditions of their thermal stability have been examined.

Electrical conduction in non-metallic rare-earth solids

Schematic energy band diagrams for the genesis of charge carriers in non-metallic rare-earth solids have been presented. It has been shown that positions of 4f bands have significant effect on the genesis and nature of charge carriers, their conduction mechanism and magnitude of electrical conductivity (σ) and Seebeck coefficient (S) of the solid. Relevant relations have been given for bothσ and S in different situations. Experimental data on rare-earth sesquioxides (R2O3), rare-earth tungstates [R2(WO4)3] and rare-earth molybdates [R2(MoO4)3] in the intrinsic range have been explained as examples for the validity of energy band diagrams.

Electrical transport in heavy rare-earth molybdates

Rare-earth molybdates of the type R 2(MoO 4) 3 with R = Gd, Tb, Dy, Ho, Er, Tm and Yb have been prepared, characterized and measurements of electrical conductivity (σ) and Seebeck coefficient (S) in the temperature intervals 420–1200K and 500–1200K, respectively, have been reported. It has been concluded that these molybdates are essentially insulating solids with a band gap E g ⋍3.7 eV for Gd molybdate which increases as we go down in the series and becomes 4.0 eV for Yb 2(MoO 4) 3. The log σ vs T −1 as well as S vs T −1 plots show in general four linear regions with three (namely T 3, T 2 and T 1) break temperatures that essentially occur due to changes in the conduction mechanism. For T > T 3 the conduction is intrinsic with polarons of intermediate coupling as the main entity of charge carrier. For T < T 2 it is extrinsic and is essentially due to hopping of electrons localized on defect centres in the temperature interval of T 1 < T < T 2; it is due to donor-like centres for T < T 1. The mobility of charge carriers for T > T 3 has also been determined.

Comment on "Irrelevant variables, Landau expansions, and cubic anisotropy."

The relationship of the $\mathrm{XY}$ model, which arises from a ${C}_{4v}$ image, to the phase transition in the rare-earth molybdate ${\mathrm{Tb}}_{2}$${(\mathrm{M}\mathrm{o}{\mathrm{O}}_{4})}_{3}$ (TMO) is discussed. We point out that this model does not describe the TMO transition, but that a Landau expansion arising from a ${C}_{4}$ image is the appropriate free-energy model.

Irrelevant variables, Landau expansions, and cubic anisotropy.

A two-component spin system (${\ensuremath{\varphi}}_{1}$,${\ensuremath{\varphi}}_{2}$) with cubic anisotropy is studied using a Landau expansion truncated at order eight (${\ensuremath{\varphi}}^{8}$ theory). The symmetry of the ordered phases is primarily fixed by the coefficient of ${\ensuremath{\varphi}}_{1}^{2}$${\ensuremath{\varphi}}_{2}^{2}$ in the expansion. While constant within a ${\ensuremath{\varphi}}^{4}$ theory this coefficient becomes temperature dependent within a ${\ensuremath{\varphi}}^{8}$ theory. A systematic investigation of the minima of the free energy is carried out as a function of the various Landau coefficients. Comparison of ${\ensuremath{\varphi}}^{4}$ and ${\ensuremath{\varphi}}^{8}$ theories shows that inclusion of sixth- and eighth-degree terms does not only generate a new symmetry breaking but changes the relative energy of the previous minima. Experimental data on the rare-earth molybdate ${\mathrm{Tb}}_{2}$(${\mathrm{MoO}}_{4}$${)}_{3}$ are shown to fit exactly a ${\ensuremath{\varphi}}^{8}$ theory.

Laser Raman and far-infrared spectra of Gd 2(MoO 4) 3, Tb 2(MoO 4) 3, GdTb(MoO 4) 3, GdDY(MoO 4) 3, Tb 1.8Eu 0.2(MoO 4) 3 and Gd 0.2Dy 0.9Tb 0.9(MoO 4) 3 single crystals in the external mode region

Laser Raman and Fourier transform far-infrared (FT-FIR) spectra of rare earth molybdate single crystals of Gd 2(MoO 4) 3, Tb 2(MoO 4) 3, GdTb(MoO 4) 3, GdDy(MoO 4) 3, Tb 1.8Eu 0.2(MoO 4) 3 and Gd 0.2Dy 0.9Tb 0.9(MoO 4) 3 were recorded. Based on C 2ν symmetry, group theoretical analysis was carried out and a vibrational assignment is proposed for the lattice modes of the rare earth and molybdate ions. For the first time bands are observed in the region 10–40 cm −1 in FT-FIR spectra and they are assigned as difference bands. LO-TO splittings in these noncentrosymmetric systems are also discussed.

Light scattering in rare-earth molybdates: Temperature dependence of the “true” soft modes in the ferroelectric phase

Abstract A Phenomenological theory of the soft modes Raman, tensor which allows, in particular, to explane the strong anisotropy of this tensor for one of the soft modes is considered. The contradiction between strong acoustic anomalies and weak temperature dependence of the soft mode side components is removed by taking into account the observed central peak. From experiment it is found that the “true” soft mode frequency depends really linearly on temperature near TO.