Crystal chemistry,magnetic, and electrical properties of hexagonal plutonium oxide chalcogenides: Pu2O2X, X = O,S, Se

A study was made of the influence of the chalcogen X on the magnetic and electrical properties of Pu₂O₂X (X = O, S, Se) compounds and on bonding in these hexagonal compounds, isostructural with the corresponding rare-earth compounds. The comparison of the cell volumes of Nd₂O₂X and Pu₂O₂X compounds showed that the Pu³⁺ crystal radius decreased from β-Pu₂O₃ to Pu₂O₂Se as “5f delocalization” and 5f-p overlap increased. These plutonium compounds were all found to be antiferromagnetic. This was caused by superexchange coupling interactions via p orbitals of the oxygen and chalcogen ions. The Néel temperature increased from β-Pu₂O₃ (26 K) to Pu₂O₂S (28 K) then to Pu₂O₂Se (34 K) showing that 5f-p covalency was enhanced as the chalcogen electronegativity decreased and the p radial extent increased. The hexagonal β-Pu₂O₃ was found to be an insulator while both other compounds were semiconductors with gaps around 0.5 eV. These gaps were interpreted as the energy separation between the 6d-7s conduction band and the np band of the chalcogen X with some overlap of the occupied 5f states and the np band. A simple electron band scheme of these compounds is proposed on such assumptions.     

The Family of Ln2Ti2S2O5 Compounds (Ln=Nd, Sm, Gd, Tb, Dy, Ho, Er, and Y): Optical Properties

The optical properties of Ln₂Ti₂S₂O₅ compounds (Ln=Nd, Sm, Gd, Tb, Dy, Ho, Er, and Y) have been measured. Diffuse reflectance spectra revealed a strong absorption band above 2 eV, which explains the color of the compounds. Moreover, other bands, with a lower intensity, have been attributed to 4f-4f rare earth transitions. In the case of neodymium the derived energy level scheme is rich enough to determinate a set of phenomenological crystal field parameters that correctly reproduce the spectrum. These parameters were also calculated from the crystallographic structure, in a good agreement with the experiment. Finally, the paramagnetic susceptibility, well reproduced by the calculation, confirms that the rare earth is in a trivalent state.     

Mild hydrothermal synthesis and ferrimagnetism of Pr3Fe5O12 and Nd3Fe5O12 garnets

Two pure light rare earth iron garnets Pr₃Fe₅O₁₂ and Nd₃Fe₅O₁₂ single crystals were synthesized under mild hydrothermal conditions and structurally characterized by single crystal and powder X-ray diffraction methods. Both compounds crystallize in cubic space group Ia3?d with lattice parameters a=12.670(2) Å for Pr₃Fe₅O₁₂ and a=12.633(2) Å for Nd₃Fe₅O₁₂, respectively. The synthesis of compounds was studied with regard to phase evolution and morphology development with hydrothermal conditions. We proposed the formation mechanisms and formulated a reasonable explanation for their growth habits. Ferrimagnetic Curie temperatures which have been inferred from thermo-magnetization curves were 580 K for Pr₃Fe₅O₁₂ and 565 K for Nd₃Fe₅O₁₂, and the transitions of long range order were also evidenced by differential scanning calorimetry method. The result of magnetic properties has shown that moments of the large radius Pr³⁺ and Nd³⁺ ions are parallelly coupled with net moments of iron ions.     

UV-VUV-excited photoluminescence of RE-activated CaLaP3O10 (RE=Eu,Tb)

Monazite-type polyphosphate CaLaP₃O₁₀ was synthesized by solid-state reaction at 1000 °C and their photoluminescence of Eu³⁺ and Tb³⁺ in CaLaP₃O₁₀ under ultraviolet (UV) and vacuum-ultraviolet (VUV) excitation were evaluated for the first time. The emission spectra of CaLaP₃O₁₀:Eu³⁺showed that Eu³⁺ are in a site with inversion symmetry because the magnetic dipole transition ⁵D₀-⁷F₁ was the strongest both upon 254 and 147 nm excitation. Monitored at 621 nm the excitation spectra consisted of host absorption bands, charge transfer band of Eu-O and the intraconfiguration 4f⁶ transition of Eu³⁺. Green phosphor CaLaP₃O₁₀:Tb³⁺exhibited better color purity when excited by 147 nm than that excited by 254 nm. With monitored at 542 nm the host absorption bands of CaLaP₃O₁₀:Tb³⁺ were also observed. Besides the host absorption bands there were strong f-d and weak f-f transitions of Tb³⁺.     

Systematic study of photoluminescence upon band gap excitation in perovskite-type titanates R1/2Na1/2TiO3:Pr (R=La, Gd, Lu, and Y)

Pr³⁺-doped perovskites R_1/2Na_1/2TiO₃:Pr (R=La, Gd, Lu, and Y) were synthesized, and their structures, optical absorption and luminescent properties were investigated, and the relationship between structures and optical properties are discussed. Optical band gap of R_1/2Na_1/2TiO₃ increases in the order R=La, Gd, Y, and Lu, which is primarily due to a decrease in band width accompanied by a decrease in Ti-O-Ti bond angle. Intense red emission assigned to f-f transition of Pr³⁺ from the excited ¹D₂ level to the ground ³H₄ state upon the band gap photo-excitation (UV) was observed for all compounds. The wavelength of emission peaks was red-shifted in the order R=La, Gd, Y, and Lu, which originates from the increase in crystal field splitting of Pr³⁺. This is attributed to the decrease in inter-atomic distances of Pr-O together with the inter-atomic distances (R, Na)-O, i.e., increase in covalency between Pr and O. The results indicate that the luminescent properties in R_1/2Na_1/2TiO₃:Pr are governed by the relative energy level between the ground and excited state of 4f² for Pr³⁺, and the conduction and valence band, which is primarily dependent on the structure, e.g., the tilt of TiO₆ octahedra and the Pr-Ti inter-atomic distance and the site symmetry of Pr ion.     

Growth and spectroscopic characterization of Nd:Sr6GdSc(BO3)6 crystal

A crystal of Nd³⁺:Sr₆GdSc(BO₃)₆ with the dimension of φ20×30 mm³ was grown by Czochralski method. The grown crystal was characterized by X-ray diffraction and DSC analysis. The DSC analysis showed that the crystal congruently melt at 1306.7°C. The absorption and emission spectra of Nd³⁺:Sr₆GdSc(BO₃)₆ were investigated. The absorption band at 806 nm has a FWHM of 13 nm. The absorption and emission cross-sections are 2.33×10⁻²⁰ cm² at 806 nm and 1.58×10⁻¹⁹ cm² at 1062 nm, respectively. The luminescence lifetime τ_f is 75 μs at room temperature.     

An investigation of the Nd2O3-MoO3 phase system: Thermal decomposition of Nd2Mo4O15 and formation of Nd6Mo10O39

A new neodymium molybdate, Nd₆Mo₁₀O₃₉, has been identified in the Nd₂O₃-MoO₃ phase system. Nd₆Mo₁₀O₃₉ appears to be a metastable phase, which does not form directly from a stoichiometric mixture of Nd₂O₃ and MoO₃ oxides. Instead, it can be obtained by thermal decomposition of Nd₂Mo₄O₁₅. Nd₂Mo₄O₁₅ usually decomposes into Nd₂(MoO₄)₃, and the formation of Nd₆Mo₁₀O₃₉ critically depends on the heating regime used.The structure of Nd₆Mo₁₀O₃₉ has been determined by single crystal X-ray diffraction. It crystallizes in the monoclinic space group C2/c, with unit cell parameters of , , , β=100.767(2)°, at 120 K. Nd atoms are seven and eight coordinate, and pairs of coordination polyhedra share edges and faces, respectively, to form Nd₂O₁₂ and Nd₂O₁₃ groups. All Mo atoms are in tetrahedral coordination environments, with some of the tetrahedra sharing corners to form pyromolybdate groups.     

Solid solubilities of (La Nd,)2(Zr,Ti)2O7 phases deduced by neutron diffraction

Time-of-flight powder neutron diffraction has been performed on oxides with composition (La_1−xNd_x)₂Zr₂O₇ and Nd₂(Zr_1−xTi_x)₂O₇, where x=0, 0.2, 0.4,…1.0, in order to determine the solid solution behaviour across each series. Between La₂Zr₂O₇ and Nd₂Zr₂O₇, a cubic pyrochlore phase is observed (, Z=8). A linear decrease in the lattice parameter from 10.8047 to 10.6758 Å indicates complete miscibility of the two end-members. For the same series, the 48f oxygen x-parameter increases from 0.3313 to 0.3348, suggesting increased distortion of the 6 coordinate B sites and reduced distortion of the 8 coordinate A sites. There is limited solubility of Nd₂Ti₂O₇ in Nd₂Zr₂O₇. Exsolution of a monoclinic phase (P2₁, Z=8) rich in Nd₂Ti₂O₇ is observed at approximately x=0.56. The compositional range over which a solid solution exists is more extensive than that which has been previously reported. The solubility of Nd₂Zr₂O₇ in Nd₂Ti₂O₇ is very low.     

(NH4)4P4O12 · 2Te(OH)6 · 2H2O,the first example of a tetrametaphosphate-tellurate

Ammonium tetrametaphosphate-tellurate dihydrate, (NH₄)₄P₄O₁₂ · 2Te(OH)₆ · 2H₂O, is triclinic with the following unit cell dimensions: a = 11.845(6), b = 8.554(5), c = 7.433(5) Å, α = 66.28(5), β = 95.91(5), γ = 76.00(5)° space group: $\text{P}\text{1}$ and Z = 1. The crystal structure has been determined with a final R value of 0.021. As in the previously described phosphate-tellurates, monophosphate-tellurate and trimetaphosphate-tellurates, the phosphoric anion (here the P₄O₁₂ ring) is independent of the octahedral Te(OH)₆ group. A complete pattern of the hydrogen bonds is given.     

The crystal and magnetic structures of Ca2YRuO6 and the electronic properties of the series M2+2X3+Ru5+O6 (M = Ca,Sr, Ba; X = La,Y)

Profile analysis of constant-wavelength powder neutron diffraction data has been used to refine the crystal structure of the ordered perovskite Ca₂YRuO₆. The material is monoclinic (space group $\text{P2}{}_1\text{n}$) with a disordered arrangement of calcium and yttrium on the A site and one of the B sites, such that the formula is best written as Ca_1.43Y_0.57[(Ca_0.57Y_0.43)Ru]O₆. Low-temperature neutron diffraction experiments showed that the material is a Type I antiferromagnet at 2.5 K with an ordered magnetic moment of 1.2(1)μ_B per Ru⁵⁺. It is suggested that the dominant factor in determining the electronic properties of the series M²⁺₂X³⁺Ru⁵⁺O₆ (M = Ca, Sr, Ba; X = La, Y) is the Ru-Ru separation distance.     

Crystal growth of a series of lithium garnets Ln3Li5Ta2O12 (Ln=La, Pr, Nd): Structural properties, Alexandrite effect and unusual ionic conductivity

We report the single crystal structures of a series of lanthanide containing tantalates, Ln₃Li₅Ta₂O₁₂ (Ln=La, Pr, Nd) that were obtained out of a reactive lithium hydroxide flux. The structures of Ln₃Li₅Ta₂O₁₂ were determined by single crystal X-ray diffraction, where the Li⁺ positions and Li⁺ site occupancies were fixed based on previously reported neutron diffraction data for isostructural compounds. All three oxides crystallize in the cubic space group (No. 230) with lattice parameters a=12.7735(1), 12.6527(1), and 12.5967(1) Å for La₃Li₅Ta₂O₁₂, Pr₃Li₅Ta₂O₁₂, and Nd₃Li₅Ta₂O₁₂, respectively. A UV-Vis diffuse reflectance spectrum of Nd₃Li₅Ta₂O₁₂ was collected to explain its unusual Alexandrite-like optical behavior. To evaluate the transport properties of Nd₃Li₅Ta₂O₁₂, the impedance data were collected in air in the temperature range 300?T(°C)?500.     

Growth and spectroscopic properties of Nd-doped Sr3Y2 (BO3)4 crystal

A new crystal of Nd³⁺:Sr₃Y₂ (BO₃)₄ with a dimension of Φ 15×30 mm³ was grown by the Czochralski method. The grown crystal was characterized using X-ray diffraction. The absorption and emission spectra of Nd³⁺:Sr₃Y₂ (BO₃)₄ were investigated. The absorption transition at 807 nm has an FWHM of 16 nm. The absorption and emission cross sections are 6.32×10⁻²⁰ cm² at 807 nm and 1.07×10⁻¹⁹ cm² at 1065 nm, respectively. The luminescence lifetime τ_f is 51.7 μs at room temperature.     

Hydrothermal Synthesis and Spectroscopic and Magnetic Behavior of the Mn7(HOXO3)4(XO4)2(X=As, P) Compounds. Crystal Structure of Mn7(HOAsO3)4(AsO4)2

The Mn₇(HOXO₃)₄(XO₄)₂ (X=As, P) compounds have been synthesized by using hydrothermal conditions. The arsenate phase was obtained under autogeneous pressure at 170°C. However, more drastic conditions at both pressure and temperature were necessary in the attainment of the phosphate compound. The crystal structure of Mn₇(HOAsO₃)₄(AsO₄)₂ was solved using single-crystal data. The unit-cell parameters are a=6.810(3) Å, b=8.239(2) Å, c=10.011(4) Å, α=104.31(2)°, β=108.94(3)°, γ=101.25(2)°. Triclinic, P-1 with Z=1. The isostructural Mn₇(HOPO₃)₄(PO₄)₂ phase was characterized from X-ray powder diffraction techniques. The crystal structure of both compounds consists of zig-zag chains constructed by dimeric edge-sharing Mn₂O₁₀ octahedra linked through the MnO₅ trigonal bipyramids. The three-dimensional framework is completed by the connection between isolated MnO₆ entities to the dimers octahedra and trigonal bipyramids. The existence of hydrogenarsenate and hydrogenphosphate anions has been confirmed by IR and Raman spectroscopies. Magnetic measurements indicate the existence of antiferromagnetic interactions in both compounds, which are slightly stronger in the arsenate phase.     

Order-disorder transition in the Nd2−yYyZr2O7 system: Probed by X-ray diffraction and Raman spectroscopy

A series of compositions having the general formula Nd_2−yY_yZr₂O₇ have been synthesized by heating of mixtures of oxides of the components cation and characterized by X-ray diffraction and Raman spectroscopy. Rietveld analysis on the XRD data of all the compositions has been performed which revealed a decrease in lattice parameter as a function of y in the series Nd_2−yY_yZr₂O₇ (y=0.0-0.8). Subsequently, a biphasic region starts which continues for y=1.2 and 1.6. The other end member, i.e. Y₂Zr₂O₇ is found to be defect fluorite. On the other hand, Nd³⁺ has been used as surrogate material for Am³⁺, which is a minor actinide found in spent nuclear fuel. In the pyrochlore range, the increasing trend of the x-parameter of 48f oxygen indicates the enhancement of disorder in the system. Raman spectroscopy has been employed to validate the data obtained from XRD. The involvement of 48f oxygen in disorder has also been verified by Raman spectroscopic investigation.     

Synthesis, Crystal Structure, Magnetism, and Optical Properties of Gd3[SiON3]O—An Oxonitridosilicate Oxide with Noncondensed SiON3 Tetrahedra

The novel oxonitridosilicate oxide (sion oxide) Gd₃[SiON₃]O was obtained by the reaction of gadolinium metal with its carbonate oxide and silicon diimide in a radiofrequency (r.f.) furnace at a temperature of 1400°C. The crystal structure of Gd₃[SiON₃]O (I4/mcm, a=649.1(2) pm, c=1078.8(6) pm, Z=4, R₁=0.0411, wR₂=0.0769, 405 F² values, 19 parameters, 123 K) is isotypic with that of Ba₃[SiO₄]O and Cs₃[CoCl₄]Cl. It can be derived from the perovskite structure type by a hierarchical substitution: Ti⁴⁺→O^2-, O^2-→Gd³⁺, Ca²⁺→[SiON₃]^7- resulting in the formation of large [OGd₆]¹⁶⁺ octahedra, which are twisted by ξ=16.47(1)° around [001]. The low-temperature single-crystal data investigation led to a crystallographic splitting of the central O atom which could not be resolved at room temperature. The UV-Vis absorption spectra in reflection geometry of the yellow title compound revealed two overlaying broad bands, one peaking at almost the same wavelength as observed in gadolinium oxide (340 nm) and a second red-shifted band at approximately 400 nm indicating a strong influence of nitrogen on the ligand field splitting of the 5d states of Gd³⁺. Temperature-dependent magnetic susceptibility measurements of Gd₃[SiON₃]O show Curie-Weiss behavior from 2 to 300 K with an experimental magnetic moment of 7.68(5) μ_B/Gd, indicating trivalent gadolinium. There is no evidence for magnetic ordering down to 2 K. According to the paramagnetic Curie temperature of −7(1) K, the exchange between the gadolinium magnetic moments is supposed to be only weak. The vibrational spectroscopic data (IR and Raman) are reported.     

室温下制备以{211}晶面为主的Ti1-xVxO2薄膜及其可见光催化性能

以金属Ti和V作为靶材,采用直流反应共溅射技术在室温下制备了以{211}晶面为主的锐钛矿相Ti_1-xV_xO₂薄膜,研究了不同V靶功率对Ti_1-xV_xO₂薄膜的薄膜成分、晶相结构和可见光催化性能的影响。研究表明,Ti_1-xV_xO₂薄膜的晶相结构为锐钛矿相,择优取向为(211),而结晶度受V靶功率的影响。随着V靶功率的增加,薄膜中V元素含量逐渐增加,同时,晶粒和沉积速率也逐渐增加。另外,当V靶功率为150W时,薄膜的表面粗糙度值有一个最大值。V的掺杂导致薄膜的能带间隙变窄,对光的吸收向可见光区偏移,从而有效地改善了薄膜的可见光催化能力。当V靶功率为150W时,Ti_1-xV_xO₂薄膜的能带间隙值为2.82eV,其在2h的可见光照射下分解了80%的RhB染料。这被归结于能带间隙窄,高能晶面{211}和结晶度高的共同作用。     

室温下制备以{211}晶面为主的Ti1-xVxO2薄膜及其可见光催化性能

以金属Ti和V作为靶材,采用直流反应共溅射技术在室温下制备了以{211}晶面为主的锐钛矿相Ti_1-xV_xO₂薄膜,研究了不同V靶功率对Ti_1-xV_xO₂薄膜的薄膜成分、晶相结构和可见光催化性能的影响。研究表明,Ti_1-xV_xO₂薄膜的晶相结构为锐钛矿相,择优取向为(211),而结晶度受V靶功率的影响。随着V靶功率的增加,薄膜中V元素含量逐渐增加,同时,晶粒和沉积速率也逐渐增加。另外,当V靶功率为150 W时,薄膜的表面粗糙度值有一个最大值。V的掺杂导致薄膜的能带间隙变窄,对光的吸收向可见光区偏移,从而有效地改善了薄膜的可见光催化能力。当V靶功率为150 W时,Ti_1-xV_xO₂薄膜的能带间隙值为 2.82 eV,其在2 h的可见光照射下分解了80%的RhB染料。这被归结于能带间隙窄,高能晶面{211}和结晶度高的共同作用。     

An unusual technique for the study of nonstoichiometry: The thermal emission of electrons. Results for Y2O3 and TiO2

The thermal emission of electrons is presented as a useful technique for the study of nonstoichiometric oxides at high temperature. Results are reported for yttria and titanium dioxide, very different in their respective properties. For these compounds the density of emitted current follows a simple law, J ∝ P^x_O2, where P_O2 is the oxygen partial pressure and x is a constant that is not dependent on temperature. The electrical conductivity, when measured under the same conditions, follows a similar law. Therefore there is some evidence that at high temperature the chemisorption is not an important process, and the emission characteristics are then discussed in terms of a bulk nonstoichiometry. Data are obtained for yttrium oxide, as the width of the band gap E_g = 5.5 eV, the electron affinity χ = 2 eV. A reasonable defect for this oxide consists of oxygen vacancies V^2·_O and oxygen interstitials O^2′_i. The situation in the case of rutile is much more complicated as this oxide has a wide nonstoichiometric field with several suboxides and a nonisotropic structure. When the deviation to the stoichiometry is low the oxygen sublattice is stable and the main defects are titanium interstitials Ti^4·_i. When the compound is more reduced a surface reorganization then occurs which seems related to a crystallographic transformation leading to the Ti_nO_2n?1 suboxides. This technique give a lot of data on the properties of nonstoichiometric compounds in the vicinity of the surface at high temperature.     

Synthesis, crystal structure and characterization of new 12H hexagonal perovskite-related oxides Ba6M2Na2X2O17 (M=Ru, Nb, Ta, Sb; X=V, Cr, Mn, P, As)

The new Ba₆Ru₂Na₂X₂O₁₇ (X=V, Mn) compounds have been prepared by electrosynthesis in molten NaOH and their crystal structures have been refined from single crystals X-ray diffraction, space group P6₃/mmc, Z=2, for X=V: , , R₁=4.76%, for X=Mn : , , R₁=3.48%. The crystal structure is a 12H-type perovskite with a (c′cchcc)₂ stacking sequence of [BaO₃]_c, [BaO₃]_h and [BaO₂]_c′ layers. The tridimensional edifice is formed by blocks of Ru₂O₉ dimers that share corners with NaO₆ octahedra. These blocks sandwich double sheets of X⁵⁺O₄ tetrahedra. Several isotypic Ba₆M⁵⁺₂Na₂X⁵⁺₂O₁₇ materials (X=V, Cr, Mn, P, As) and (M=Ru, Nb, Ta, Sb) have been prepared by solid state reaction and characterized by Rietveld analysis. The magnetic and electric properties have been investigated and show besides the Ru⁵⁺₂O₉ typical intradimer antiferromagnetic couplings, discrepancies of both χ and ρ versus T at 50 and 100 K for Ba₆Ru₂Na₂X₂O₁₇ (X=V, As). In this work, a review of the identified Ru-hexagonal perovskite materials is also reported in order to overview the wide variety of possibilities in the field of new compounds synthesis.     

The luminescence of Pr3+ in BaY4Si5O17

The cell constants of four new monoclinic compounds BaR₄X₅O₁₇ (R = Y, Gd; X = Si, Ge) are given. The luminescence of various RE activators in the silicates is reported. Pr³⁺-activated BaY₄Si₅O₁₇ shows efficient ultraviolet 5d → 4f emission and weak 4f → 4f emission (mainly red luminescence from the ¹D₂ level). The 5d → 4f emission is ascribed to Pr³⁺ on Y sites, the 4f → 4f emission to Pr³⁺ on Ba sites. Energy transfer from Pr³⁺ to Gd³⁺ has been observed. Gd³⁺ plays an intermediate role in the energy transfer from Pr³⁺ to Sm³⁺ and to Dy³⁺ in BaGd₄Si₅O₁₇. Upon activation with Tb³⁺ the silicates show characteristic green Tb³⁺ luminescence with a quantum efficiency of 75% for ultraviolet excitation.