Structure and properties of (1−x)(0.6Pb(Zn1/3Nb2/3)O3-0.4Pb (Mg1/3Nb2/3)O3)-xPbTiO3 thin films with perovskite phase promoted by polyethylene glycol

The preferential formation of a pyrochlore structure is a knotty problem in the preparation of Pb(Zn_1/3Nb_2/3)O₃ (PZN)-based thin film materials and its presence is significantly detrimental to the dielectric and piezoelectric properties. In this study, 40 mol% of PZN was replaced with Pb(Mg_1/3Nb_2/3)O₃ (PMN) for obtaining a perovskite composition around a morphotropic phase boundary (MPB), (1−x)(0.6PZN-0.4PMN)-xPT ((1−x)PZMN-xPT, PT: PbTiO₃) where x = 0.23. The thin films with this composition were prepared with a polyethylene glycol (PEG) modi-fied sol-gel method on LaAlO₃ substrates. The microstructural evolution of the films on heat treatment was examined with X-ray diffraction. With the aid of PEG, the formation of the pyrochlore phase was suppressed and the perovskite phase formed directly from the amorphous gel film. The multilayer films with a thickness around 0.25 μm showed a single perovskite phase without any detectable pyrochlore structure. Microscopic images showed uniform grain size of a few tens of nanometers. The role of the polymer dramatically promoting the perovskite phase was investigated with the aid of X-ray photoelectron spectroscopy and thermal analysis. The dielectric constant of the obtained film was 4160 at 1 kHz. The film demonstrated typical ferroelectric hysteresis loops and exhibited excellent piezoelectric performance.     

Reaction mechanism in the formation of perovskite Pb(Fe1/2Nb1/2)O3 by calcining of mixed oxides (CMO)

A calcining at 300°C and sintering process were proposed to obtain a pure perovskite phase Pb(Fe_1/2 Nb_1/2)O₃ from a 4PbO/Nb₂O₅/Fe₂O₃ mixture, which is calcined at 300°C for several days and sintered at various temperatures for 2h; the resultant powder was air quenched. The X-ray powder diffraction pattern of the sintered sample is carefully analyzed to identify intermediate phases. The effects of calcining at 300°C and sintering on obtaining PFN are based on the deformation of Pb₅Fe₄Nb₄O₂₁. A reaction mechanism for the calcining cycle of Pb(Fe_1/2 Nb_1/2)O₃ is proposed.     

(1-x)Pb(Ni1/3Nb2/3)O3-xPbTiO3陶瓷的制备

铌镍酸铅;钙钛矿相;驰豫铁电陶瓷;(1-x)Pb(Ni1/3Nb2/3)O3-xPbTiO3陶瓷的制备     

PbMg1/3Nb2/3O3 (PMN)和PbMg1/3Nb2/3-PbTiO3 (PMNT)中几种局域结构与晶格形变的从头算研究

建立了钙钛矿结构铌镁酸铅PbMg1/3Nb2/3O3(PMN)的2×2×3复晶胞模型; 采用ab initio方法讨论了PMN晶体各种可能构型的稳定性; 选取了PMN三种高、中、低稳定性的代表构型, 并对Ti替换B位离子后的结构进行了结构优化. 计算结果表明复晶胞刚性模型的最低和最高能量差约0.74 a.u. (1940 kJ); Pb2＋离子结构框架的形变是PMN晶格发生形变的主要因素; 在不考虑被替换离子电荷差异的情况下, MgO6含量越少越有利于Ti离子替换Nb与晶胞的形变. PMNT材料中构型的分布和局域形变取决于生长PMNT材料的工艺过程.     

La(Ni2/3Nb1/3)O3 by neutron powder diffraction

Lanthanum nickel niobium trioxide has been synthesized and its structure refined for the first time. It was found to be a member of the family of technologically important `double perovskites', crystallizing in the monoclinic space group P2₁/n. The structure is characterized by a strong orthorhombic pseudosymmetry and a concurrent exhibition of both 1:1 B‐cation ordering and a⁻a⁻c⁺‐type tilting of the (Ni/Nb)O₆ structural units. Trivalent lanthanum resides on the perovskite A site, which is strongly distorted owing to the tilting of the (Ni/Nb)O₆ sublattice. Ordering of divalent nickel and pentavalent niobium on the B sublattice is described in terms of two twofold special positions (2c and 2d), with nickel taking almost complete occupancy of the 2d site and the 2c position being occupied by a statistical distribution of nickel and niobium.     

First-principles calculations of 17O nuclear magnetic resonance chemical shielding in Pb(Zr(1/2)Ti(1/2))O3 and Pb(Mg(1/3)Nb(2/3))O3: linear dependence on transition-metal/oxygen bond lengths

First-principles density functional theory oxygen chemical shift tensors were calculated for A(B,B')O(3) perovskite alloys Pb(Zr(1/2)Ti(1/2))O(3) (PZT) and Pb(Mg(1/3)Nb(2/3))O(3) (PMN). Quantum chemistry methods for embedded clusters and the gauge including projector augmented waves (GIPAW) method [C. J. Pickard and F. Mauri, Phys. Rev. B 63, 245101 (2001)] for periodic boundary conditions were used. Results from both methods are in good agreement for PZT and prototypical perovskites. PMN results were obtained using only GIPAW. Both isotropic δ(iso) and axial δ(ax) chemical shifts were found to vary approximately linearly as a function of the nearest-distance transition-metal/oxygen bond length, r(s). Using these results, we argue against Ti clustering in PZT, as conjectured from recent (17)O NMR magic-angle-spinning measurements. Our findings indicate that (17)O NMR measurements, coupled with first-principles calculations, can be an important probe of local structure in complex perovskite solid solutions.     

PbMg1/3Nb2/3O3(PMN)和PbMg1/3Nb2/3-PbTiO3(PMNT)中几咱种局域结构与晶格形变的从头算研究

建立了钙钛矿结构铌镁酸铅PbMg1/3Nb2/3O3(PMN)的2×2×3复晶胞模型;采用ab initio方法讨论了PMN晶体各种可能构型的稳定性:选取了PMN 三种高、中、低稳定性的代表构型,并对Ti替换B位离子后的结构进行了结构优化.计算结果表明复晶胞刚性模犁的最低和最高能量差约0.74 a.u.(1940 kJ);Pb2+离子结构框架的形变是PMN晶格发生形变的丰要因素;在不考虑被替换离子电荷差异的情况下,MgO6含量越少越有利于Ti离子替换Nb与晶胞的形变.PMNT材料中构型的分布和局域形变取决于生长PMNT材料的工艺过程.     

Kristallchemische Untersuchungen an den Phasen: (I) Zn(V0,12Nb0,88)2O6, (II) Zn(V0,28Ta0,72)2O6 und (III) Zn(Nb0,58Ta0,42)2O6

Crystal Chemistry Examinations of the Phases: (I) Zn(V_0.12Nb_0.88)₂O₆, (II) Zn(V_0.28Ta_0.72)₂O₆, and (III) Zn(Nb_0.58Ta_0.42)₂O₆ Single crystals of the quaternary phases ( I ), ( II ), and ( III ) were prepared by high temperature technique and investigated by X-ray single crystal methods. Phase ( I ) crystallizes in the columbite Structure. Phase ( II ) deviates from the ternary compounds (ZnV₂O₆, ZnTa₂O₆) and appears in trirutile type. In phase ( III ) the octahedra arrangement of the tri-α-PbO₂ Structure is realized.     

Neue Phasen in den Systemen Nb2O5-WO3 und Ta2O5-WO3

Zusammenfassung Bei Untersuchungen im WO₃-ärmeren Bereich der Systeme Nb₂O₅-WO₃ und Ta₂O₅-WO₃ bis zum Molverhältnis Me₂O₅:WO₃=1:2 wurden folgende Phasen neu gefunden: 40 Nb₂O₅·WO₃–20 Nb₂O₅·WO₃ (Phasenbreite), 13 Nb₂O₅· ·4 WO₃, 9 Nb₂O₅·8 WO₃ (Tieftemperaturphase), 9 Ta₂O₅· ·8 WO₃; ferner eine Mischphase des T-Ta₂O₅, die bis zur Zusammensetzung 13 Ta₂O₅·4 WO₃ (bei 1300° C) reicht. Weitere Phasen wurden im System Nb₂O₅-WO₃ bei den Molverhältnissen 8:1–6: 1, 7:3, 8:5 und 9:8 (Hochtemperaturphase) beobachtet.49. Mitt.:H. Schäfer, R. Gruehn undF. Schulte, Angew. Chemie, im Druck.     

Dielectric properties of chemically co-precipitated tetragonal Pb(Mg1/3Nb2/3)0.65Ti0.35O3

Pyrochlore phase free Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃ ceramics have been synthesized successfully by chemical co-precipitation method. It has been noted that formation of perovskite phase Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃ without pyrochlore phase is tricky. The synthesized samples at optimized parameters were characterized using X-ray diffraction technique. Careful analysis of the XRD data predicts the tetragonal lattice structure. The morphological studies depict the presence of uniform grain size. Dielectric constant (ε′) and loss tangent (tan δ), at and well above room temperature, were studied. The variation of dielectric constant with temperature shows sharp peak at ferroelectric–paraelectric transition temperature. Further, the temperature dependent dielectric constant shows good fit with modified Curie–Weiss law, which suggests normal ferroelectric behavior of Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃.     

Quenching of I(2P1/2) by O3 and O(3P)

Oxygen-iodine lasers that utilize electrical or microwave discharges to produce singlet oxygen are currently being developed. The discharge generators differ from conventional chemical singlet oxygen generators in that they produce significant amounts of atomic oxygen. Post-discharge chemistry includes channels that lead to the formation of ozone. Consequently, removal of I(2P1/2) by O atoms and O3 may impact the efficiency of discharge driven iodine lasers. In the present study, we have measured the rate constants for quenching of I(2P1/2) by O(3P) atoms and O3 using pulsed laser photolysis techniques. The rate constant for quenching by O3, (1.8 +/- 0.4) x 10(-12) cm3 s-1, was found to be a factor of 5 smaller than the literature value. The rate constant for quenching by O(3P) was (1.2 +/- 0.2) x 10(-11) cm3 s-1.     

Solvothermal and surfactant-free synthesis of crystalline Nb(2)O(5), Ta(2)O(5), HfO(2), and Co-doped HfO(2) nanoparticles

A simple route to niobium, hafnium and tantalum oxide nanocrystals using a nonaqueous sol-gel route based on the solvothermal reaction of the corresponding metal chlorides with benzyl alcohol is presented. This approach can easily be extended to the preparation of high quality Co-doped HfO(2) nanoparticles of uniform size and shape and with a homogenous distribution of the magnetic ions. The structural characterization of all these nanomaterials as well as the magnetic properties of pure and doped hafnia, with special attention to the doping efficiency, are discussed. The obtained Co-doped hafnia exhibits paramagnetic properties with very weak antiferromagnetic interactions between Co ions moments.     

Dynamics of nano-cluster in a new relaxor system: (PbMg1/3Nb2/3O3)/(BiFeO3)

We report the dynamical role of nano-clusters in controlling the physical properties and parameters of a new relaxor complex, (PbMg_1/3Nb_2/3)O₃/(BiFeO₃). This complex shows a gradual structural change from cubic to hexagonal crystal system with increasing BiFeO₃ content. Transmission electron microscopy (TEM) reveals the presence of scattered nano-clusters for (PbMg_1/3Nb_2/3)O₃ relaxor system which starts assembling for increasing value of BiFeO₃. As a consequence of this assembling, a crossover from relaxor to ferroelectric and dielectric peak broadening was observed. Corresponding change in physical parameters such as; Curie–Weiss and Uchino–Nomura parameters i.e., ΔT_cm and diffusivity ‘γ’ was also observed. Vogel–Fulcher (V–F) fitting was employed to calculate the activation energy required to activate the dipolar glasses into a dynamical state or to re-establish a polarization fluctuation around an isolated nano-cluster. V–F fitting shows increased activation energy for increasing cluster size. The suppression in polarization value shows the inability of the total dipole moment to reorient on application of electric field with growth/assembling of nano-clusters.     

Neutron powder diffraction study and B-site ordering in microwave dielectric ceramics Ba(Ca1/3Nb2/3)O3

Microwave dielectric powder Ba(Ca_1/3Nb_2/3)O₃ with high B-site cation ordering was synthesized by the molten salt method. Neutron powder diffraction (NPD) and Raman scattering spectra were introduced to investigate the variable ordering degree during the sintering process. It was revealed that the as-synthesized Ba(Ca_1/3Nb_2/3)O₃ powder had a nearly completely ordered structure, and the sintered Ba(Ca_1/3Nb_2/3)O₃ presented a bit higher ordering degree based on the detailed quantitative NPD Rietveld full profile fitting. The complete Raman mode assignment for the Ba(Ca_1/3Nb_2/3)O₃ was presented. The phonon bands could also confirm the order–disorder structural model and increasing ordering degree with the increasing sintering temperature. Final microwave dielectric measurements on sintered samples showed the present compound to tailor other dielectric materials for microwave applications with the dielectric properties of ?_r = 47.11, and Qf = 1389 GHz, and τ_f was about 113 ppm/°C.     

过氧多酸盐[NBu4]3[M(O2)W5O18]的合成及表征

过渡金属过氧化合物是一类性能良好的催化剂,在有机合成中得到广泛的应用[1,2].过氧多酸催化过氧化氢氧化有机物的反应,近年来倍受重视[3～5].     

Synthesis of new visible light active photocatalysts of Ba(In(1/3)Pb(1/3)M'(1/3))O3 (M' = Nb, Ta): a band gap engineering strategy based on electronegativity of a metal component

We have synthesized new, efficient, visible light active photocatalysts through the incorporation of highly electronegative non-transition metal Pb or Sn ions into the perovskite lattice of Ba(In(1/3)Pb(1/3)M'(1/3))O3 (M = Sn, Pb; M' = Nb, Ta). X-ray diffraction, X-ray absorption spectroscopic, and energy dispersive spectroscopic microprobe analyses reveal that tetravalent Pb or Sn ions exist in the B-site of the perovskite lattice, along with In and Nb/Ta ions. According to diffuse UV-vis spectroscopic analysis, the Pb-containing quaternary metal oxides Ba(In(1/3)Pb(1/3)M'(1/3))O3 possess a much narrower band gap (E(g) approximately 1.48-1.50 eV) when compared to the ternary oxides Ba(In(1/2)M'(1/2))O3 (E(g) approximately 2.97-3.30 eV) and the Sn-containing Ba(In(1/3)Sn(1/3)M'(1/3))O3 derivatives (E(g) approximately 2.85-3.00 eV). Such a variation of band gap energy upon the substitution is attributable to the broadening of the conduction band caused by the dissimilar electronegativities of the B-site cations. In contrast to the ternary or the Sn-substituted quaternary compounds showing photocatalytic activity under UV-vis irradiation, the Ba(In(1/3)Pb(1/3)M'(1/3))O3 compounds induce an efficient photodegradation of 4-chlorophenol under visible light irradiation (lambda > 420 nm). The present results highlight that the substitution of electronegative non-transition metal cations can provide a very powerful way of developing efficient visible light harvesting photocatalysts through tuning of the band structure of a semiconductive metal oxide.     

Fe x Ti1−2x M x O2 (M=Nb,Ta) rutile solid solutions from gels

In this study, Fe _x Ti_1–2x M _x O₂ (M=Nb, Ta) rutile solid solutions have been synthesized from gels made from Fe(III) acetylacetonate, NbCl₅, TaCl₅, Ta(V) ethoxide, TiCl₄ and Ti(IV) isopropoxide. The results obtained are compared with those obtained by the ceramic method. The solid solutions synthesized from gels were obtained at lower temperatures than these synthesized by the ceramic method.