Electronic Structure of Oxygen Vacancies in the Orthorhombic Noncentrosymmetric Phase Hf<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>O<Subscript>2</Subscript>

The electronic structure of stoichiometric and oxygen-depleted Hf_0.5Zr_0.5O₂ in the orthorhombic noncentrosymmetric phase has been studied by X-ray photoelectron spectroscopy and quantum-chemical simulation based on the density functional theory. It has been established that the ion-etching-induced peak in the photoelectron emission spectrum with the energy above the top of the o-Hf_0.5Zr_0.5O₂ valence band is due to oxygen vacancies. A method of estimating the density of oxygen vacancies from the comparison of the experimental and theoretical photoelectron spectra of the valence band has been proposed. It has been established that oxygen polyvacancies in o-Hf_0.5Zr_0.5O₂ are not formed: the energetically favorable spatial arrangement of oxygen vacancies in a crystal corresponds to noninteracting oxygen vacancies distant from each other.     

Effect of carbon coating process on the structure and electrochemical performance of LiNi<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>O<Subscript>2</Subscript> used as cathode in Li-ion batteries

LiNi_0.5Mn_0.5O₂ powder was synthesized by a coprecipitation method. LiOH.H₂O and coprecipitated [(Ni_0.5Mn_0.5)C₂O₄] precursors were mixed carefully together and then calcined at 900°C. Surface modified cathode materials were obtained by coating LiNi_0.5Mn_0.5O₂ with a thin layer of amorphous carbon using table sugar and starch as carbon source. Both parent and carbon-coated samples have the characteristic layered structure of LiNi_0.5Mn_0.5O₂ as estimated from X-ray diffractometry measurements. Transmission electron microscope showed the presence of C layer around the prepared particles. TGA analysis emphasized and confirmed the presence of C coating around LiNi_0.5Mn_0.5O₂. It is obvious that the carbon coating appears to be beneficial for the electrochemical performance of the LiNi_0.5Mn_0.5O₂. A capacity of about 150 mAh/g is delivered in the voltage range 2.5–4.5 V at current density C/15 for carbon coated LiNi_0.5Mn_0.5O₂ in comparison with about 165 mAh/g obtained for carbon free LiNi_0.5Mn_0.5O₂ at the same current density and voltage window. About 92% and 82% capacity retention was obtained at 50th cycle for coated LiNi_0.5Mn_0.5O₂ using sucrose and starch, respectively; whereas, 75% was retained after only 30th cycle for carbon free LiNi_0.5Mn_0.5O₂. This improvement is mainly attributed to the presence of thin layer of carbon layer that encapsulate the nanoparticles and improve the conductivity and the electrochemical performance of LiNi_0.5Mn_0.5O₂.     

Investigation of ion transport in Li<Subscript>8</Subscript>ZrO<Subscript>6</Subscript> and Li<Subscript>6</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> solid electrolytes

Lithium ionic conductivity and spin-lattice relaxation rates were measured in Li₈ZrO₆ and Li₆Zr₂O₇ solid electrolytes. It was found that the Li₈ZrO₆ solid electrolyte undergoes a transition to the superionic state in the temperature range 673–703 K. It was shown that Li⁺ ions are mobile in particular lattice positions of the Li₆Zr₂O₇ phase, and that ionic conductivity is monotonic at an activation energy of 79.4 kJ/mol.     

Charge transport mechanism in thin films of amorphous and ferroelectric Hf0.5Zr0.5O2

The charge transport mechanism in thin amorphous and ferroelectric Hf_0.5Zr_0.5O₂ films has been studied. It has been shown that the transport mechanism in studied materials does not depend on the crystal phase and is phonon-assisted tunneling between traps. The comparison of the experimental current–voltage characteristics of TiN/Hf_0.5Zr_0.5O₂/Pt structures with the calculated ones provides the trap parameters: thermal energy of 1.25 eV and the optical energy of 2.5 eV. The trap concentration has been estimated as ~10¹⁹–10²⁰ cm^–3.     

Structure,the lattice dynamic,and the dielectric characteristics of Sr<Subscript>0.5</Subscript>Ba<Subscript>0.5</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> films

Solid solution Sr_0.5Ba_0.5Nb₂O₆ films have been synthesized on a (111)Pt/(001)Si substrate by rf deposition in an oxygen atmosphere. The depolarized Raman spectra, the structure, and the dielectric characteristics of the films have been studied over a wide temperature range. It is found that the films were singlephase, had the tetragonal tungsten bronze structure, and had a pronounced axial texture with axis 001 directed perpendicular to the substrate surface. It is shown that the film material undergoes a diffuse phase transition to the state of a relaxor ferroelectric in the temperature range 300–425 K. Possible reasons of the regularities observed are discussed.     

RETRACTED ARTICLE: Refractive-index tailoring and morphological evolutions in Gd<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> composite thin films

Refractive-index tailoring and morphological evolutions in two different thin film composite systems of gadolinia–silica (Gd₂O₃:SiO₂) and zirconia–silica (ZrO₂:SiO₂) deposited through reactive electron-beam codeposition processes are discussed in this research paper. For Gd₂O₃:SiO₂ the refractive-index tuning has been achieved from 1.45 to 2.18, whereas in the case of ZrO₂:SiO₂ the achieved tunable range is from 1.45 to 2.45 in the ultraviolet region. Under certain compositional mixings with lower silica fractions both the systems demonstrated relative microstructural and morphological densifications. Such evolutions were very successfully derived through phase-modulated ellipsometry and atomic force microscopy. The composition-dependent refractive-index tailoring and microstructural densifications have been investigated by adopting Tauc–Lorentz and single-effective-oscillator models. The morphological correlation functions have also very aptly supported such evolutions in these composite films. These experimental results indicate their favourable properties and applicability down to the extreme ultraviolet wavelength region of the electromagnetic spectrum. PACS 42.79.Wc; 78.66.-w; 78.20.Ci; 61.16.Ch; 42.70.-a;68.55.-a; 68.35.Bs; 81.15.Ef     

Comparative study of solid-state reaction and sol-gel process for synthesis of Zr-doped Li<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> solid electrolytes

We investigated the synergistic influences of synthesis methods (solid-state reaction vs. sol-gel process) and Zr⁴⁺ doping on the structure and ionic conductivity of perovskite-structured Li_0.5La_0.5TiO₃ (LLTO) solid electrolytes. The lithium-ion conductivity of Li_0.5La_0.5Ti_1???x Zr _x O₃ ceramic specimens was evaluated as a function of x value and compared carefully between those two synthesis methods. Regarding the conductivity, sol-gel process is better for the synthesis of LLTO than solid-state reaction. As a result, the highest grain conductivity is obtained in the sol-gel-derived pure LLTO sample with x?=?0, reaching 1.10?×?10^?3 S?·?cm^?1. Partial substitution of Zr⁴⁺ enlarges the LLTO’s grain aggregate size and increases the total superficial area of aggregates. Consequently, Zr⁴⁺ substitution not only affects the grain (bulk) conductivity, but more importantly, also improves the grain boundary conductivity and the total conductivity. The highest total conductivity is 5.84?×?10^?5 S?·?cm^?1 with x?=?0.04 by sol-gel process.     

Study on the spin-states of cobalt-based double-layer perovskite Sr<Subscript>2</Subscript>Y<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>Co<Subscript>2</Subscript>O<Subscript>7</Subscript>

The spin-states of cobalt based perovskite compounds depend sensitively on the valence state and local crystal environment of Co ions and the rich physical properties arise from strong coupling among charge, spin, and orbital degrees of freedom. While extensive studies have been carried out in the past, most of them concentrated on the isotropic compound LaCoO₃. In this paper, using the unrestricted Hartree-Fock approximation and the real-space recursion method, we have investigated the competition of various magnetically ordered spin-states of anisotropic double-layered perovskite Sr₂Y_0.5Ca_0.5Co₂O₇. The energy comparison among these states shows that the nearest-neighbor high-spin-intermediate-spin ferromagnetically ordered state is the relevant magnetic ground state of the compound. The magnetic structure and sizes of magnetic moments are consistent with the recent experimental observation.     

Fine structure and mechanical properties of the shape-memory Ni<Subscript>50</Subscript>Ti<Subscript>32</Subscript>Hf<Subscript>18</Subscript> alloy rapidly quenched by spinning

We have reported the results of investigations of the structure and chemical and phase compositions of the amorphous Ni₅₀Ti₃₂Hf₁₈ alloy prepared by rapid quenching from melt by spinning and subjected to heat treatments. The specific features of the fine polycrystalline alloy structure formation depending on the heat-treatment mode have been studied by transmission and scanning electron microscopy, chemical microanalysis, electron diffraction, and X-ray diffraction analysis. According to the data on the temperature behavior of electrical resistivity, critical temperatures of devitrification and subsequent thermoelastic martensitic transformation B2 → B19′ have been determined. The mechanical properties in different heat-treatment modes have been investigated.     

Microstructure,dielectric and piezoelectric properties of (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript>-Ba(Ti<Subscript>0.95</Subscript>Zr<Subscript>0.05</Subscript>)O<Subscript>3</Subscript> lead-free ceramics with CuO sintering aid

Using an ordinary ceramic fabrication technique, we fabricated lead-free (1-x)(K_0.5Na_0.5)NbO₃-xBa(Ti_0.95Zr_0.05)O₃ ceramics with CuO sintering aid . Ba(Ti_0.95Zr_0.05)O₃ diffuses into (K_0.5Na_0.5)NbO₃ to form a new solid solution. The ceramics with perovskite structure possess orthorhombic phase at x≤0.04 and become tetragonal phase at x≥0.06. Both the paraelectric cubic–ferroelectric tetragonal and the ferroelectric tetragonal–ferroelectric orthorhombic phase transition temperatures decrease with increasing the concentration of Ba(Ti_0.95Zr_0.05)O₃. The doping of CuO effectively promotes the densification of the ceramics. The coexistence of the orthorhombic and tetragonal phases at 0.04<x<0.06 and the improvement in sintering performances of the ceramics significantly enhance the piezoelectric and dielectric properties at room temperature. The ceramics with x=0.04–0.06 and y=0.75–1.50 possess excellent properties: d₃₃=119–185 pC/N, k_P=37–44%, k_t=35–49%, ε=341–1129, cosδ=0.7–4.4% and T_c=312–346 °C. PACS 77.65.-j; 77.84.Dy; 77.84.-s     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

Features of the formation of an oxide layer on the surface of amorphous Zr<Subscript>41.2</Subscript>Ti<Subscript>13.8</Subscript>Cu<Subscript>12.5</Subscript>Ni<Subscript>10.0</Subscript>Be<Subscript>22.5</Subscript> alloy

Field investigations were performed into the nature of oxidation of Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 alloy (Vitreloy-1), a new alloy highly promising for in -vessel mirrors of the ITER (International Thermonuclear Experimental Reactor). The main methods of investigation were X-ray photoelectron spectroscopy and multi-angle ellipsometry. The resistance of the optical properties of Vitreloy-1 against radiation impact was explained by the oxidation of the surface layer, based on the features of the diffusion process in amorphous alloys and of interaction between amorphous metal alloys with hydrogen.     

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> and Ni<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript>: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co₃V₂O₈ and Co₃V₂O₈ single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co₃V₂O₈ is shifted toward low energies by 0.25–0.3 eV as compared to Co₃V₂O₈. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.     

Structure and dielectric properties of ultra-thin ZrO<Subscript>2</Subscript> films for high-<Emphasis Type="Italic">k</Emphasis> gate dielectric application prepared by pulsed laser deposition

ZrO₂ thin films have been prepared on Pt-coated silicon substrates and directly on n-Si(100) substrates by the pulsed laser deposition (PLD) technique using a ZrO₂ ceramic target under different deposition conditions. X-ray diffraction showed that the films prepared at 400 °C in 20 Pa oxygen ambient remained amorphous. Differential thermal analysis was carried out to study the crystallization behavior of ZrO₂. The dielectric constant of ZrO₂ was determined to be around 24 by measuring a Pt/ZrO₂/Pt capacitor structure. Sputtering depth profile X-ray photoelectron spectroscopy was used to investigate the interfacial characteristics of ZrO₂/n-Si stacks. A Zr silicate interfacial layer was formed between the ZrO₂ layer and the silicon substrate. The equivalent oxide thickness (EOT) and leakage current densities of the films with 6.6 nm physical thickness post-annealed in O₂ and N₂ ambient were investigated. An EOT of 1.65 nm with a leakage current of 36.2 mA/cm² at 1 V gate voltage for the film post-annealed in N₂ has been obtained. ZrO₂ thin films prepared by PLD have acceptable structure and dielectric properties required by a candidate material of high-k gate dielectrics. PACS 77.55.+f; 81.15.Fg; 73.40.Qv     

Synthesis of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> nano/microspheres with adjustable hollow structures for lithium-ion battery

High-voltage spinel LiNi_0.5Mn_1.5O₄ nano/microspheres with adjustable hollow structures have been fabricated based on the Kirkendall effect. The main characteristic is that the wall thickness of the hollow structure as well as the cavity size of the hollow structure can be adjusted by the different ratio of mixed precipitation agents. Especially, the diagrammatic sketch for the formation process of various LiNi_0.5Mn_1.5O₄ materials with adjustable hollow structures is discussed. Besides, the results of electrochemical performance test show that LiNi_0.5Mn_1.5O₄ obtained from 10:1 Na₂CO₃/NaOH (in mole) ratio is worth looking forward to, owing to its special hierarchical nano/microsphere and moderate hollow structures.     

Spectral properties of Nd<Superscript>3+</Superscript> ions in samples of transparent Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramics

The spectral and luminescent characteristics of samples of Y₂O₃:Nd³⁺ ceramics obtained from different precursors under different preparation conditions (the concentration of an HfO₂ compacting additive, the temperature and time of synthesis) are studied at 300 and 77 K. It is shown that the spectral positions of absorption and luminescence lines of ceramics correspond to those of a Y₂O₃:Nd³⁺ single crystal. At the same time, the absorption and luminescence spectra show an inhomogeneous broadening, characteristic of disordered crystals and glass. The energies of the ⁴ I _9/2 and ⁴ F _3/2 Stark states of the Nd³⁺ ion are calculated. The calculation results nearly coincide with the data from the literature for the Y₂O₃:Nd³⁺ single crystal and transparent ceramics. Samples containing the compacting additive show additional lines, whose intensities correlate with its concentration and the method of preparation of Y₂O₃:Nd³⁺ ultradispersed powders. It is assumed that these lines are related to the fact that either Nd³⁺ ions enter the composition of the HfO₂ compacting additive or Hf⁴⁺ ions are present in the nearest environment of Nd³⁺ ions at the boundaries of granules enriched with HfO₂.     

X-ray spectroscopic examination of thin HfO<Subscript>2</Subscript> films ALD- and MOCVD-grown on the Si(100) surface

HfO₂ films 5 nm thick grown on Si(100) substrates by the methods of MOCVD hydride epitaxy and atomic layer deposition (ALD) are studied using X-ray photoelectron spectroscopy combined with Ar⁺ ion etching and X-ray reflectometry. It is found that (i) the ALD-grown HfO₂ films are amorphous, while the MOCVD-grown films show signs of a crystal structure; (ii) the surface of the ALD-grown films is more prone to contamination and/or is more reactive; and (iii) the amount of interfacial silicon dioxide in the case of the MOCVD-grown film is greater than in the case of the films synthesized by ALD. It is also shown that the argon ion etching of the HfO₂ film results in the formation of a metallic hafnium layer at the interface. This indicates that HfO₂ can be used not only as a gate dielectric but also as a material suitable for fabricating nanodimensional conductors by direct decomposition.     

Formation of ZrO<Subscript>2</Subscript> cubic phase microcrystals during crystallization of amorphous films deposited by laser ablation of Zr in an oxygen atmosphere

The structure and phase transformations during annealing of zirconium dioxide films grown by pulsed laser sputtering of a Zr target in an oxygen atmosphere have been studied by transmission electron microscopy and electron diffraction methods. The conditions of the formation of both amorphous and cubic ZrO₂ phases have been determined. The electron beam impact on the amorphous film in vacuum is accompanied by the formation of zirconium dioxide microcrystals with fcc lattice. The average grain size in the crystallized film is ～0.5 μm. The phase transformation is accompanied by film material densification. The relative change in the density during ZrO₂ crystallization is 10.27 ± 2.14%.     

Fast ionic conduction in cubic hafnium garnet Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Hf<Subscript>2</Subscript>O<Subscript>12</Subscript>

A new member of the family of garnets with fast lithium ion conduction has been found with the composition Li₇La₃Hf₂O₁₂. The anion arrangement corresponds to the oxygen framework in garnets, e.g., in Ca₃Fe₂Si₃O₁₂. Hafnium is coordinated octahedrally while the lanthanum environment can be described as a distorted cube. Lithium occupies a large number of positions with tetrahedral, trigonal planar, and metaprismatic coordination. Li₇La₃Hf₂O₁₂ shows a lithium bulk ion conductivity of 2.4 × 10⁻⁴ Ω⁻¹ cm⁻¹ at room temperature with an activation energy of 0.29 eV.     

Interfacial properties of Ce<Subscript>0.75</Subscript>Zr<Subscript>0.25</Subscript>O<Subscript>2</Subscript> supported noble metals (Pd,Pt) from first principles

First-principles electronic structure calculations of noble metals (NM=Pd, Pt)/Ce_0.75Zr_0.25O₂ systems are presented. It is found that: the NM adatoms do not prefer to stay at the atop or the bridge sites of the cations (Ce and Zr), but prefer to be adsorbed at or around the anion sites. The most preferable adsorption sites for both the Pd and Pt adatoms are the O-bridge sites neighboring the Zr dopant. The Pt adatom show much stronger interaction with the Ce_0.75Zr_0.25O₂(111) surface than does the Pd adatom. The interactions of the NM/Ce_0.75Zr_0.25O₂(111) interfaces are stronger than those of the corresponding NM/ceria(111) interfaces. There are some metal induced gap states (MIGS) appeared in the gaps of the NM/Ce_0.75Zr_0.25O₂(111) interfaces, which are important to catalytic properties of the NM/Ce_0.75Zr_0.25O₂(111) catalysts.