Rare earth niobium oxynitrides, LnNbON2−δ (Ln = Y, La, Pr, Nd, Gd, Dy): Synthesis, structure and properties

Ammonolysis of rare earth niobates of the type LnNbO₄ (Ln = Y, La, Pr, Nd, Gd, Dy) yields oxynitrides of different structures. When Ln = La, Nd and Pr, the structure is that of an orthorhombic perovskite of the general formula LnNbON₂. As the size of the rare earth decreases, the oxynitride has a nitrogen-deficient defect fluorite (Ln = Pr, Nd, Gd), or pyrochlore (Ln = Y) structure. The IR spectra of the oxynitrides and the corresponding oxides are significantly different. Thermogravimetric analysis suggests the formation of an intermediate phase wherein the N₂ molecule is attached to the oxide lattice above 400 °C and decomposes to give the oxide on heating in an oxygen atmosphere. Raman spectra of the intermediate phases show evidence for the NN stretching vibration. Gadolinium niobium oxynitride is found to be paramagnetic.     

Study of the R-(Zr,W)-(O,N) (R = Y, Nd, Sm, Gd, Yb) oxynitride system

The replacement of tantalum by the couple Zr/W within the RTa-O-N systems (R = Y, Nd, Sm, Gd, Yb), enables the preparation of novel oxide and oxynitride phases in the R-Zr-W-O-N system. R₂Zr_2−xW_xO_7+x oxides exhibit the fluorite-type (x < 0.9) and scheelite (x ∼ 1) structures. Corresponding oxynitride compositions are of the fluorite-type and show different colors, for example in the case of ytterbium: pale yellow (x = 0.2 or 0.25), green (x = 0.5-0.8) and brown for the tungsten-rich samples (x = 0.9, 1). Photocatalytic activity measurements have been performed to investigate the overall water splitting behavior of these colored phases.     

An investigation of structural, magnetic and dielectric properties of R2NiMnO6 (R = rare earth, Y)

We have investigated the structure, magnetic and dielectric properties of the double perovskite oxides, R₂NiMnO₆ (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho and Y). We could refine powder X-ray diffraction patterns of all the phases on the basis of monoclinic (P2₁/n) double perovskite structure where Ni and Mn atoms are ordered at 2c and 2d sites, respectively. All the phases are ferromagnetic insulators exhibiting relatively low dielectric loss and dielectric constants in the range 15-25. The ferromagnetic ordering temperature of the R₂NiMnO₆ series seems to correlate better with the radius of R³⁺ atoms than with the average Ni-O-Mn angle (φ) in the double perovskite structure. These results are consistent with all samples having Mn⁴⁺ and Ni²⁺ with minimal antisite disorder.     

Optical, magnetic and electronic properties of Ln2O2Te (Ln = La, Sm and Gd)

The synthesis, the optical and magnetic properties and the electronic structure of the rare-earth oxytellurides of formula Ln₂O₂Te (Ln = La, Sm and Gd) are reported. The magnetic measurements show that La₂O₂Te exhibit a diamagnetic behavior. The value of the magnetization M of Gd₂TeO₂ at our highest available field (50 kOe) is about 5.5μ_B, well below the expected value for two independent Gd ions (2gJ = 14μ_B) confirming the antiferromagnetic character of this compound. The magnetic properties of Sm₂TeO₂ reveal that Sm is in its 3+ oxidation state with a large Van Vleck contribution. The electronic structure calculations, studied by means of a first principles DFT approach, are consistent with the optical measurements and suggest an indirect band gap semiconductor behavior.     

Synthesis and structural characterization of new defect pyrochlore type AxLnySb2O6+z antimoniates (0<x, y, z<1; A = Rb, Cs, Tl; Ln = Eu, Gd, and Y) prepared by coprecipitation

New cubic defect pyrochlore type A_xLn_ySb₂O_6+z antimoniates (0<x,y,z<1; A=Rb, Cs, Tl; Ln=Eu, Gd, and Y) (space group Fd3m) were prepared by calcination in air at 650 °C of compositions obtained by a coprecipitation method. Their crystal structures were refined by the Rietveld procedure and further confirmed by X-ray absorption spectroscopy (EXAFS) experiments. Refinements of the X-ray powder diffraction data show a partial and statistical occupation of the 8b site by alkaline or thallous ions and oxygen and lead to values close to 0.33 for the only refinable positional x parameter of the 48f oxygenated site.     

Preparation and characterization of rare earth orthoborates, LnBO3 (Ln = Tb, La, Pr, Nd, Sm, Eu, Gd, Dy, Y) and LaBO3:Gd, Tb, Eu by metathesis reaction: ESR of LaBO3:Gd and luminescence of LaBO3:Tb, Eu

Lanthanide orthoborates of composition LnBO₃ (Ln = Tb, La, Pr, Nd, Sm, Eu, Gd, Dy, Y) and LaBO₃:Gd, Tb, Eu have been prepared by metathesis reaction. This method provides a convenient route for the synthesis of orthoborates and its solid solutions at low temperatures. Powder X-ray diffraction and FT-IR spectroscopy were used to characterize these borates. Rare earth borates, (LnBO₃) are isomorphous with different forms of CaCO₃ depending on the radius of rare earth ion. LaBO₃, LaBO₃:Gd, Tb, Eu, PrBO₃, NdBO₃ crystallized in aragonite structure, SmBO₃ crystallized in H-form and TbBO₃, EuBO₃, GdBO₃, DyBO₃, YBO₃ crystallized in vaterite structure. The structural analysis of TbBO₃ was carried out. The morphology of these borates was obtained from Scanning electron microscopy. Spin-Hamiltonian parameters for Gd³⁺ are deduced from room temperature electron spin resonance spectrum of LaBO₃:Gd. The luminescence of LaBO₃:Tb, Eu gave characteristics peaks corresponding to Tb³⁺, Eu³⁺ respectively.     

Luminescence and microstructures of Eu-doped in triple phosphate Ca8MgR(PO4)7 (R = La, Gd, Y) with whitlockite structure

Eu³⁺-doped triple phosphate Ca₈MgR(PO₄)₇ (R = La, Gd, Y) was synthesized by the general high temperature solid-state reaction. This phosphor was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and emission spectra. XRD and FT-IR analysis indicated that Ca₈MgR(PO₄)₇ (R = La, Gd, Y) crystallized in single-phase component with whitlockite-like structure (space group R3c) of β-Ca₃(PO₄)₂. Under the excitation of UV light, the phosphors show bright red emission assigned to the transition (⁵D₀ → ⁷F₂) at 612 nm. The crystallographic sites of Eu³⁺ ions in Ca₈MgR(PO₄)₇ (R = La, Gd, Y) host were discussed on the base of site-selective excitation and emission spectra, luminescence decay and its host crystal structure.     

Diffuse scattering in the cesium pyrochlore CsTi0.5W1.5O6

The structure of the defect pyrochlore CsTi_0.5W_1.5O₆ has been investigated using electron, synchrotron X-ray and neutron diffraction methods. The material is cubic a = 10.2773 Å with displacive disorder of the Cs cations along the 〈1 1 1〉 direction. The local structure, revealed by the diffuse structure in the electron diffraction patterns shows there is correlated displacement of the heavy Cs atoms along the 〈1 1 0〉 directions. The thermal expansion of the material is also described.     

Synthesis and characterization of K2NiF4-type CaLnCoO4 (Ln = Sm and Gd)

K₂NiF₄-type CaLnCoO₄ (Ln = Sm and Gd) has been synthesized at 1173 or 1223 K in air using citric acid (CA) and ethylene glycol (EG). CaLnCoO₄ (Ln = Sm and Gd) has an orthorhombic structure with the space group Bmab. The average particle sizes are approximately 300 nm for CaSmCoO₄ and approximately 170 nm for CaGdCoO₄, respectively. The global instability index (GII) indicates that the crystal structure of CaGdCoO₄ is more stable than that of CaSmCoO₄. CaLnCoO₄ (Ln = Sm and Gd) is a p-type semiconductor and shows paramagnetic behavior above 5 K. The 1/χ-T curve of CaSmCoO₄ deviates from the Curie-Weiss law, whereas the 1/χ-T curve of CaGdCoO₄ follows the Curie-Weiss law in the temperature range of 5 ≤ T ≤ 300 K. From the values of the observed effective magnetic moment (μ_eff) of CaLnCoO₄ (Ln = Sm and Gd), it is considered that the spin state of the Co³⁺ ion is low.     

Synthesis, structure and dielectric characterization of Ln2Ti2−2xM2xO7(Ln=Gd, Er; M=Zr, Sn, Si)

The progress in wireless communications and information access has demanded the use of electronic ceramics exhibiting desired properties. To further our understanding of these properties, compounds in the Ln₂Ti_2-2xM_2xO₇ (Ln=Gd, Er; M=Zr, Sn, Si) systems were synthesized by ceramic methods and characterized by powder X-ray diffraction. The ZrO₂-doped Gd₂Ti_2−2xZr_2xO₇ compounds adopt the pyrochlore structure type and form a complete solid solution. Er₂Ti_2−2xZr_2xO₇ forms a pyrochlore solid solution for x<0.1. However, stoichiometric Er₂Zr₂O₇ does not form; instead Er₄Zr₃O₁₂ forms a with defect fluorite structure. The Sn-doped Ln₂Ti_2−2xSn_2xO₇ (Ln=Gd, Er) compounds form complete solid solutions, and the Si compounds adopt the pyrochlore structure up to x=0.05. At ambient temperature, dielectric constants range from 10 to 61 for Er₂Ti_2−2xZr_2xO₇ and 16-31 for Gd₂Ti_2−2xZr_2xO₇ with low dielectric loss (1×10⁻³) at 1 GHz.     

6H hexagonal structure of low loss Ba3MTiWO9 (M = Mg, Zn) dielectrics

The crystal structure of Ba₃MTiWO₉ (M = Mg, Zn) oxides has been found to be 6H hexagonal, space group P6₃/mmc, parameters of unit cell: a = 5.7943(1) Å, c = 14.1642(1) Å and a = 5.7993(1) Å, c = 14.1626(1) Å for M = Mg and Zn, respectively. The tungsten and titanium atoms are randomly distributed in pairs of face-sharing octahedra separated by octahedral layers containing magnesium or zinc. It was revealed that the structures are well ordered which is believed to result in a low dielectric loss at a microwave frequency reported for these materials.     

High-pressure synthesis, electrical and magnetic properties of new filled skutterudites LnOs4P12 (Ln = Eu, Gd, Tb, Dy, Ho, Y)

New filled skutteudites LnOs₄P₁₂ (Ln: Eu, Gd, Tb, Dy, Ho and Y) have been prepared at high temperatures and at high pressures. X-ray diffraction of these compounds is studied at room temperature. The relationship between lattice constants and atomic numbers of lanthanide (including Y) is obtained for LnOs₄P₁₂ (Ln: lanthanide). Electrical and magnetic properties of the new filled skutterudites with heavier lanthanide have been investigated at low temperatures. EuOs₄P₁₂ and GdOs₄P₁₂ show ferromagnetic transitions at around 15 and 22 K, respectively. The valence states of both compounds are +2 for the Eu compound and +3 for the Gd compound. DyOs₄P₁₂ does not show the magnetic transition down to 2 K. However, a small electrical anomaly is found at around 10 K. YOs₄P₁₂ exhibits a superconducting transition at around 3 K. This compound is a new superconductor. Electrical and magnetic anomalies of new filled skutterudites with heavier lanthanide LnOs₄P₁₂ (Ln: Eu, Gd, Dy and Y) are discussed. We have also found the electrical anomaly based on the magnetic transition at around 22 K for GdFe₄P₁₂.     

Sintering, microstructure and microwave dielectric properties of rare earth orthophosphates, RePO4 (Re = La, Ce, Nd, Sm, Tb, Dy, Y, Yb)

Various rare earth orthophosphates, such as monazite and xenotime RePO₄ (Re = La, Ce, Nd, Sm, Tb, Dy, Y, Yb) were prepared by a conventional solid-state reaction method. The crystal structure and microstructure of the sintered ceramics were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), respectively. The dielectric properties were measured in the microwave region using a network analyzer. It was found that the monazite RePO₄ could be sintered near 1400 °C, although the xenotime RePO₄, which had a smaller Re³⁺ ionic radius, could be sintered above 1600 °C. The permittivity (?_r) of the monazite structures was higher than that of the xenotime structures. This difference was explained by the differences in ionic polarizability and bond strength. Both monazite and xenotime rare earth orthophosphates, however, exhibited a high quality factor (Q × f), where Q = 1/tan δ and f is the measuring frequency, of greater than 60,000 GHz. The temperature coefficient of resonant frequency (τ_f) was a negative value, ranging between −17 and −56 ppm/°C.     

Influence of Temperature on the Mutual Solubility of Components in the Binary Systems Tetradecane—Rare-Earth Metal (Nd,Sm, Gd,Y, Lu) Nitrate Solvate with TBP

The phase diagrams of binary liquid systems tetradecane—rare-earth metal (Nd, Sm, Gd, Y, Lu) nitrate solvate with TBP were studied in the temperature range 273.15–373.15 K. These diagrams contain the fields of homogeneous solutions and the field of separation into two liquid phases (I, II). Phases I and II are enriched, respectively, in tetradecane and Ln(NO₃)₃(TBP)₃ (Ln = Nd, Sm, Gd, Y, and Lu). With increasing temperature, the fields of separation into two liquid phases contract. The upper critical solution temperatures are dependent on the kind of rare-earth metal(III).     

Synthesis and characterization of Sr(10−x)Cdx(PO4)6Y2 (Y = OH and F): A comparison of apatites containing two divalent cations

Cadmium substituted strontium hydroxy- and fluoro-apatites, Sr_(10−x)Cd_x(PO₄)₆Y₂ (Y = OH and F), have been prepared as single phases in the 0 ≤ x ≤ 4 interval for the former and 0 ≤ x ≤ 6 for the latter compound, respectively. The refinements of the X-ray full powder patterns allowed the structure determination of nine samples, and showed a preference of cadmium atoms for the M(1) site in fluoroapatite samples. IR investigation gave information about the nature of the metal oxygen interactions. The obtained results are used for a comparative discussion about the factors which drive the mutual substitution of Ca, Sr, Cd and Pb in apatites. The important role of electronegativity and polarisability, as well as of ionic radii, is evidenced.     

R3Mn1.5CuV0.5O9 (R = Y, Ho, Er, Tm, Yb and Lu) and Lu3Mn3−3xCu2xVxO9: New noncentrosymmetric oxides related to YMnO3

We report formation of new noncentrosymmetric oxides of the formula, R₃Mn_1.5CuV_0.5O₉ for R = Y, Ho, Er, Tm, Yb and Lu, possessing the hexagonal RMnO₃ (space group P6₃cm) structure. These oxides could be regarded as the x = 0.5 members of a general series R₃Mn_3−3xCu_2xV_xO₉. Investigation of the Lu-Mn-Cu-V-O system reveals the existence of isostructural solid solution series, Lu₃Mn_3−3xCu_2xV_xO₉ for 0 < x ≤ 0.75. Magnetic and dielectric properties of the oxides are consistent with a random distribution of Mn³⁺, Cu²⁺ and V⁵⁺ atoms that preserve the noncentrosymmetric RMnO₃ structure.     

Synthesis of the new layered oxides NaRbLnMO5 (Ln = La, Nd, Sm, Eu, Gd; M = Nb, Ta)

The new layered transition metal oxides NaRbLnMO₅ (Ln = Nd, Sm, Eu, Gd; M = Nb, Ta) were synthesized by direct solid-state reaction. NaRbLaNbO₅ crystallizes with a tetragonal unit cell [a=5.839(6) Å, c=8.313(1) Å] analogous to that of the related compound NaKLaNbO₅, while NaRbLaTaO₅ indexes to a larger monoclinic unit cell [a=9.577(2) Å, b=5.834(1) Å, c=8.323(2) Å, β=93.00(2)]. NaRbLnNbO₅ can be prepared for Ln = Nd, Sm, Eu, Gd, and NaRbLnTaO₅ can be prepared for Ln = Nd, Sm. Both series of compounds show the expected decrease in unit cell volume as the size of the lanthanide decreases. NaRbLaNbO₅ is also amenable to ion exchange, forming Li_2−xRb_xLaNbO₅ upon reaction with molten lithium nitrate.     

Compositionally controlled rhombohedral to cubic phase transition in CsTe2−xWxO6 related to pyrochlore structure

CsTe₂O₆ adopts a rhombohedrally distorted pyrochlore related structure due to the 1:3 ordering of Te⁴⁺ and Te⁶⁺ in the octahedral sites respectively. Phases of the type CsTe_2−xW_xO₆ were found to have the cubic pyrochlore structure from x = 0.2 to 0.5. These phases all contain Te⁴⁺ and Te⁶⁺ (mixed with W⁶⁺) and are disordered in octahedral sites of the pyrochlore structure. This mixed valence situation results in strong optical absorption in the visible region of the spectrum but does not produce a measurable electrical conductivity.     

Luminescent and scintillation properties of orthotantalates with common formulae RETaO4 (RE = Y,Sc, La,Lu and Gd)

Samples of undoped and Ce-, Eu- and Pr-doped orthotantalates with common formulae RETaO₄ (RE = Y, Sc, La, Lu and Gd) have been obtained by solid-state reaction. Optical and luminescent characteristics in visible and UV bands have been explored. The most intense light output is determined for GdTaO₄, YTaO₄, and LuTaO₄ (for GdTaO₄ it is 111% compared to BGO). Among lanthanide-doped samples, the highest light output is achieved with Eu-doped YTaO₄, GdTaO₄ and LuTaO₄ (up to 136% compared to BGO). Undoped rare-earth tantalates may be considered as promising materials for high energy physics due to extremely high density, substantial light output, and fast decay. Eu³⁺-doped orthotantalates are the brightest among the studied samples for X-ray radiography, or other application where slow decay is acceptable.     

Oxygen treatment effects on properties of MgO thin films grown on single-crystalline diamond (1 0 0)

Oxygen post-treatment effects on the electronic structure and electrical properties of MgO films grown on homoepitaxial single-crystalline (1 0 0) diamond have been studied. MgO films examined were deposited at room temperature (RT) using an electron beam evaporator and were subsequently either annealed at 573-773 K for 12 h in oxygen ambient or treated by O₂ plasma for 10-40 min. RT resistivities remarkably increased after the O₂ annealing and plasma treatment, indicating that the post treatments play an essential part on the formation and positioning of bandgap states. Cathodoluminescence (CL) spectra had a broad band feature in a wavelength region from 360 to 530 nm, which were decomposed to several peaks originating mainly from the oxygen-vacancy-related F and F⁺ centers and the interstitial vacancies of MgO film. A prominent rectifying behavior of I-V property was observed for a Au/MgO/p-diamond layered structure. Based on temperature dependences of the electrical properties in a temperature region from RT to 600 K, the electrical conduction mechanism in the MgO films is discussed in relation to polaron-related conduction as well as the ionic conduction.