Search

W-substituted rare-earth niobates (LnNb1 − xWxO4 + δ (Ln = Nd, Sm, Eu, Gd)) have been systematically synthesized by a solid-state reaction method. Powder X-ray diffraction at room temperature revealed that the monoclinic fergusonite phase structure of unsubstituted samples gradually changes to a tetragonal scheelite phase structure with increasing W substitution. The crystal structures of the samples with x ≥ 0.3 can be refined well (goodness-of-fit indicator S ≤ 1.5) as the tetragonal scheelite structure. The W substitution improves the ionic conductivities of the four rare-earth niobates, and all of their optimum W substitution amount (x) are 0.2. The activation energy of the conductivity for the substituted samples is almost constant regardless of the type of rare-earth ion, the temperature, and the W substitution amount. [ABSTRACT FROM AUTHOR]

• A novel Ca 1-3x-0.0200 Mn 0.0200 □ x Pr 2x (MoO 4) 1-3x (WO 4) 3x (□ denotes vacancies) solid solution. • A homogeneity range is 0 < x ≤ 0.25. • High-temperature solid state reaction as well as citrate-nitrate combustion route. • Nonlinearly changes of the insulators band gap with Pr3+ concentration increase. A novel tetragonal, scheelite-type Ca 1-3x-0.0200 Mn 0.0200 □ x Pr 2x (MoO 4) 1-3x (WO 4) 3x (0 < x ≤ 0.2500, and □ denotes vacancies) solid solution was successfully synthesized via high-temperature solid state reaction as well as combustion method. A change in lattice parameters (both a and c as well as parameter ratio c / a) with increasing Pr3+ concentration are observed. Pr3+-doped and Mn2+-co-doped materials are insulators and their direct band gap nonlinearly changes with increasing Pr3+ concentration. [ABSTRACT FROM AUTHOR]

Polycrystalline samples of new scheelite-type tungstates, Pb1−3x xPr2xWO4 with 0.0098 ⩽ x ⩽ 0.20, where denotes cationic vacancies have been successfully prepared by a high-temperature solid-state reaction method using Pr2(WO4)3 and PbWO4 as the starting reactants. The influence of the Pr3+ substitution in the scheelite framework on the structure and optical properties of prepared new ceramic materials has been examined using powder X-ray diffraction method (XRD) and UV-Vis-NIR spectroscopy. The results of dielectric studies of Pb1−3x xPr2xWO4 samples showed both low values of dielectric constant (below 14) and loss tangent (below 0.2). The electrical conductivity and thermoelectric power measurements revealed a low conductivity (∼2 × 10−9 S/m) and the sign change of thermoelectric power around the temperature of 366 K suggesting the p-n transition. These results are discussed in the context of vacancy, acceptor and donor levels as well as the Maxwell-Wagner model. [ABSTRACT FROM AUTHOR]

Polycrystalline samples of rare-earths molybdates and tungstates, i.e., CdREMoO (RE = Eu, Gd, Y, Ho) and Pb□EuWO (0 < x ≤ 0.1296 and □ denotes cationic vacancies) have been successfully prepared by high-temperature annealing of adequate CdMoO/REMoO and PbWO/Eu(WO) mixtures, respectively. According to the X-ray diffraction analysis, the CdREMoO compounds crystallize in a cubic, fluorite-related-type structure with space group $$Pn\bar{3}n$$ . In turn, new Pb□EuWO phases crystallize in the scheelite-type, tetragonal symmetry, space group I4/ a. Cadmium and rare-earth molybdates decompose in the solid state and the solid products of their decomposition are two RE molybdates, i.e., REMoO and RE(MoO). Thermal stability of CdREMoO decreases with decreasing of RE radius. The melting point of each sample of Pb□EuWO solid solution is lower than melting point of pure matrix, i.e., PbWO (1116 °C), and it decreases with increasing in Eu content. Both CdREMoO as well as Pb□EuWO samples are insulators, and their optical band gap ( E ) is bigger than 3 eV. [ABSTRACT FROM AUTHOR]

A novel double molybdate LiSm(MoO 4) 2 composition was prepared through a conventional solid-state ceramic route for its potential application as a solid-state Li+-ion conductor for low-loss dielectric ceramics. Ceramic disks were prepared by uniaxial pressing and sintered at the low temperature of 620 °C. Phase formation was confirmed through powder X-ray diffraction (XRD), which was indexed to the tetragonal crystal system with a Scheelite -type structure. Laser Raman spectroscopy showed that the MoO 4 2− tetrahedra in the LiSm(MoO 4) 2 material are more regular, suggesting high symmetry for the compound. Scanning Electron Microscopy (SEM) revealed the formation of polygonal grains with an average grain size of less than 1 μm. The electrochemical properties of the ceramic system were investigated by Electrochemical Impedance Spectroscopy (EIS) in O 2 , air, and Ar (250 °C – 500 °C). Data were systematically analyzed using an Equivalent Circuit Model (EQM) combined with a Distribution Function of Relaxation Times (DFRT) analysis. The results revealed the coexistence of both a delocalized (long-range conductivity) and a localized (dipole relaxation) phenomenon, mainly distinguishable at lower temperatures (i.e. , for T ≤ 350 °C) and in a pure O 2 atmosphere. This localized phenomenon was tentatively attributed to the presence of trapped hole-vacancy pairs with reduced mobility. Conversely, at higher temperatures in O 2 (i.e. , for T ≥ 350 °C) and in the whole temperature range in less oxidizing conditions (air and Ar), such a localized phenomenon was no longer observed, as a likely result of a decrease in hole concentration with decreasing oxygen partial pressure (p O2). The current work, thus, underscores a different perspective for the development and characterization of ceramic-based solid-state Li+-ion conductors for their application as low-loss dielectric ceramics. [ABSTRACT FROM AUTHOR]

Monophasic polycrystalline samples of Pb□Pr(MoO)(WO) solid solution with limited homogeneity (0 < x ≤ 0.2222) and cationic vacancies (□) have been prepared by high-temperature annealing of PbMoO/Pr(WO) mixtures composed of 40.00 mol% and less of praseodymium tungstate. Initial reactants and obtained ceramic materials were characterized by XRD, simultaneous DTA-TG, IR and UV-Vis-NIR techniques. The X-ray diffraction analysis showed that the monophasic samples crystallize in a tetragonal symmetry, with space group I4 /a (a scheelite-type structure), and PbMoO is a matrix of Pb□Pr(MoO)(WO) solid solution. Thermal stability of samples under study strongly depends on concentration of Pr ions. The Pb□Pr(MoO)(WO) solid solution ( x = 0.0238) shows the highest melting point (1055 °C), and this value is slightly higher than the melting point of PbMoO (1040 °C). Lead molybdate and samples of Pb□Pr(MoO)(WO) solid solution are insulators having indirect band gap E > 3 eV. The observed band gap of monophasic samples shows a nonlinear variation with a change of Pr ions concentration in the scheelite framework. [ABSTRACT FROM AUTHOR]

Abstract: The scheelite-type polymorph of GdCrO4 has been obtained from the corresponding zircon-type compound under high pressure and temperature conditions, namely 4GPa and 803K. The crystal structure has been determined by X-ray powder diffraction. This GdCrO4 scheelite crystallizes in a tetragonal symmetry with space group I41/a (No. 88, Z=4), a=5.0501(1)Å, c=11.4533(2)Å and V=292.099(7)Å3. The thermal decomposition leads to the formation of the zircon–polymorph as intermediate phase at 773K to end in the corresponding GdCrO3 distorted perovskite-structure at higher temperatures. Magnetic susceptibility and magnetization measurements suggest the existence of long-range antiferromagnetic interactions which have been also confirmed from specific heat measurements. Neutron powder diffraction data reveal the simultaneous antiferromagnetic Gd3+ and Cr5+ ordering in the scheelite-type GdCrO4 with a T N ∼20K. The magnetic propagation vector was found to be k=(000). Combined with group theory analysis, the best neutron powder diffraction fit was obtained with a collinear antiferromagnetic coupling in which the and magnetic moments are confined in the tetragonal basal plane according to the mixed representation Γ 6 ⊕ Γ 8. [Copyright &y& Elsevier]

new group of cadmium and rare-earth metal molybdato–tungstates with the formula Cd RE□(MoO)(WO) ( RE = Pr, Nd, Sm–Dy, □—vacancies in cation sublattice) were synthesized by a high-temperature solid-state reaction between REMoO and CdWO mixed at the molar ratio of 1:3. Powder X-ray diffraction measurements showed that obtained phases adopt the scheelite-type structure. The phases melt congruently in the temperature range of 1382–1458 K. [ABSTRACT FROM AUTHOR]

Stoichiometric CaWO4 and SrWO4 thin films were synthesized using a chemical solution processing, the so-called polymeric precursor method. In this soft chemical method, soluble precursors such as strontium carbonate, calcium carbonate and tungstic acid, as starting materials, were mixed in an aqueous solution. The thin films were deposited on glass substrates by means of the spinning technique. The surface morphology and crystal structure of the thin films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Nucleation stages and surface morphology evolution of the thin films on glass substrates were studied by atomic force microscopy. The films nucleate at 300 °C, after the coalescence of small nuclei into larger grains yielding a homogeneous dense surface. XRD characterization of these films showed that the CaWO4 and SrWO4 phases crystallize at 400 °C from an inorganic amorphous phase. No intermediate crystalline phase was identified. The optical properties were also studied. It was found that CaWO4 and SrWO4 thin films have an optical band gap, Egap=5.27 and 5.78 eV, respectively, of a direct transition nature. The excellent microstructural quality and chemical homogeneity confirmed that this soft solution processing provides an inexpensive and environmentally friendly route for the preparation of CaWO4 and SrWO4 thin films. [Copyright &y& Elsevier]

Heat capacity measurements were carried out on Pb1-xLaxWO4+x/2 ( x=0.2) and Pb1-xLa2x/3WO4 ( x=0.2, 0.5) solid solutions prepared by sintering and mechanical alloying (MA) methods. For all the solid solutions, sintered samples showed slightly larger heat capacity around 100 K in comparison with MA samples, which was presumably caused by the excitation of mobile oxide ion motion. For sintered scheelite-type structured PbWO4s, high-temperature synthesis introduced oxide ion interstitials even for the Pb1-xLa2x/3WO4 system, which resulted in the excess heat capacity at low temperature for excitation. On the other hand, for the samples prepared by room-temperature MA technique, oxide ion seemed to occupy the regular sites rather than interstitial ones and excess heat capacities were not observed. [ABSTRACT FROM AUTHOR]

Structure refinements have been carried out on pure and lanthanide-substituted PbWO4 using powder neutron diffraction, and the defect structures of La- and Pr-substituted PbWO4 are discussed. The Rietveld refinement structure of pure PbWO4 agreed well with previous results. In the Pb1−xLaxWO4+x/2 system, the oxide ion interstitials seemed to be rather localized around z=0.3 of the 8e position (0 0 z) or the adjacent general position of the scheelite structure-type with I41/a symmetry. The deduced interstitial position is considered in terms of the oxide ion conduction path. A similar defect structure has been determined for the praseodymium-substituted compound, Pb1−xPrxWO4+x/2. In contrast, Pb1−xLa2x/3WO4-type substitution resulted in the formation of a cation-deficient structure. [Copyright &y& Elsevier]

Abstract: RMoO4 (RMO, R=Ca, Sr, Ba) films were fabricated on LaAlO3 (LAO) substrates using chemical solution deposition method. The derived RMO films are highly (00l)-orientated with good crystallization. The photoluminescence (PL) emission spectra show that the CaMoO4 (CMO) and SrMoO4 (SMO) films have an intrinsic broad green emission band centered at 500nm whereas BaMoO4 (BMO) film has a broad orange emission band resulting from the PL of molybdate group with an oxygen ion deficiency. The highly (00l)-oriented CMO film on LAO has better PL property than that of CMO film on Si substrate, which is ascribed to higher crystalline integrity and lower light scattering. [Copyright &y& Elsevier]

Synthesis of a novel solid solution system GdNb 1- x W x O 4+ δ through substituting W for Nb has been investigated by a solid-state reaction method. It is found that the structure of the system changed from a monoclinic fergusonite phase to a tetragonal scheelite phase with increasing W substitution and that a single tetragonal scheelite phase could be obtained at room temperature with the composition of 0.3 ≤ x ≤ 0.7. The structural phase transition temperature decreases with increasing W substitution and the tetragonal scheelite phase can be stabilized by the W substitution, as reveals by Synchrotron powder X-ray diffraction. The conductivity of GdNbO 4 is significantly improved by the substitution of W for Nb and that of GdNb 1- x W x O 4+ δ (0.4 ≤ x ≤ 0.6) samples reach 10−5 S cm−1 at an intermediate temperature of 600 °C. The activation energy of the conductivity for the substituted samples is almost constant regardless of the temperature and the W substitution amount. • A novel solid solution system of GdNb 1- x W x O 4+ δ was successfully synthesized by substituting Nb for W. • The structural phase transition temperature from fergusonite phase to scheelite phase decreased with W substitution. • The conductivity of GdNb 1- x W x O 4+ δ (0.4 ≤ x ≤ 0.6) samples reached 10-5 S cm-1 at an intermediate temperature of 600 °C. [ABSTRACT FROM AUTHOR]

Scheelite-type Ca1−xMnxMoO4 (x = 0.0, 0.01, 0.05, 0.10 and 0.15) nanomaterials were successfully synthesized via a combustion route. Dielectric studies showed a weak n-type electrical conductivity characteristic for insulators and low relative permittivity (εr < 15) decreasing with increasing Mn2+ content. CaMoO4 and Mn2+-doped nanomaterials are chemically compatible with Al and Ag electrodes and promising for low-temperature co-fired ceramic applications. Magnetic studies showed, at room-temperature diamagnetism for pure CaMoO4, the balance between diamagnetism and paramagnetism for Ca1−xMnxMoO4 (x = 0.01) and paramagnetic behaviour when 0.05 ≤ x ≤ 0.15 as well as the short-range antiferromagnetic interactions growing in strength as Mn2+ content increases. The Landé factor fitting procedure showed a spin-only contribution to the magnetic moment. CaMoO4 matrix unexpectedly revealed the residual paramagnetism at low temperatures derived probably from the molybdenum ions having unpaired 4d electrons as well as a paramagnetic-diamagnetic transition at 70 K. [ABSTRACT FROM AUTHOR]