高氧空位简并锐钛矿TiO2半导体电子寿命的第一性原理研究

为了研究锐钛矿TiO₂晶体中高氧空位浓度对电子寿命的影响,利用基于局域密度泛函理论框架下的广义梯度近似平面波超软赝势方法, 用第一性原理对含高氧空位浓度的锐钛矿TiO₂晶体进行了结构优化处理、能带分布和态密度分布计算, 表明在温度一定和高氧空位浓度的条件下, 锐钛矿TiO₂的电子寿命随氧空位浓度的增大而减小;电子浓度的大小对电子寿命无影响.同时,锐钛矿TiO₂晶体中高氧空位浓度时,发现有莫特相变的现象. 关键词： 高氧空位 2半导体')" href="#">锐钛矿TiO₂半导体 电子寿命 第一性原理     

钨酸铅晶体中与铅空位有关的电子结构和色心模型研究

根据密度泛函理论对钨酸铅晶体中铅空位周围的晶格进行了结构优化，计算得到铅空位周围的晶格结构、电子态密度分布和偏振吸收光谱；计算结果表明钨酸铅晶体中铅空位的两价负电性是通过铅空位周围的氧共同抓获两个空穴来维持局部电中性的，即铅空位周围形成的色心模型为［O₂^3--V_Pb^2--O₂^3-］，不同的氧离子抓获空穴方式不同形成不同的色心，对应不同的吸收带；根据吸收带的偏振性质，提出了各个 关键词： 钨酸铅晶体 色心模型 电子结构 偏振特性     

La0.7Sr0.3-x□xCoO3(0≤x≤0.2)钴氧化物中锶空位诱导的自旋态转变效应

研究了锶空位对La_0.7Sr_0.3-x_xCoO₃ (0≤x≤0.2)多晶钴氧化物结构、磁性和输运性质的影响.结果表明：随着锶空位浓度x的增大,A位阳离子无序度增大,导致铁磁双交换作用减弱及反铁磁超交换作用增强,两者相互竞争,出现团簇自旋玻璃态;空位浓度超过10%后,Co—O键长迅速减小,导致晶体场劈裂能加大,大部分三价钴离子以低自旋态出现,系统基态为类超顺磁态, 关键词： 空位掺杂 钴氧化物 自旋转变     

PbWO4晶体空位型缺陷电子结构的研究

采用基于密度泛函理论的相对论性离散变分和嵌入团簇方法计算了PbWO4晶体中与氧空位和铅空位相关缺陷的态密度分布,并运用过渡态方法计算了其激发能.结果表明:PbWO4晶体中WO3+VO缺陷的O2p→W5d跃迁可引起350nm和420nm附近的吸收,并且发现VPb的存在可以使WO42基团的禁带宽度明显变小 关键词： PbWO4晶体 密度泛函 氧空位和铅空位 态密度分布     

KMgF3晶体的色心和自陷态激子研究

应用从头计算方法对KMgF3₃离子晶体中的色心和自陷态激子(STE)进行了模拟研 究.对包含F心的量子团簇进行几何结构优化, 并计算了F心处于1s基态和2p激发态的Mulliken电荷分布. 模拟结果表明，F心周围的晶格弛豫较小，处于基态的F心电子主要局域在阴离子空位处，而处于激发态的F心电子波函数则比较扩展. 计算结果表明，VK_K心移向邻近的 间隙位置，但仍保持分子轴向与［110］晶向平行.自陷态激子的弛豫是分子轴向平移与转动的叠加.计算得到的F心、VK关键词： 色心 自陷态激子 从头计算 3离子晶体')" href="#">KMgF3₃离子晶体     

锂离子电池富锂锰基三元材料中氧空位簇的形成:第一原理计算

采用第一原理方法计算了两种不同镍含量的锂离子电池富锂锰基三元正极材料Li_1.2Ni_0.32Co_0.04Mn_0.44O₂(空间群为■)和Li_1.167Ni_0.167Co_0.167Mn_0.5O₂ (空间群为C2/m)中氧空位簇的形成能.结果表明,含镍量较少的Li_1.167Ni_0.167Co_0.167Mn_0.5O₂正极材料中氧空位簇的形成能总是高于含镍量较多的Li_1.2Ni_0.32Co_0.04Mn_0.44O₂材料中的氧空位簇形成能,这说明含镍量较高的正极材料中氧空位簇更容易形成.无论是含镍量较高的富锂锰基材料,还是含镍量较少的同类材料,过渡金属边上的氧空位簇的...     

GaN及其Ga空位的电子结构

用团簇埋入自洽计算法对宽禁带半导体GaN的电子结构进行了自旋极化的、全电子、全势场的从头计算,得到了与实验值符合的GaN晶体禁带宽度以及价带中N2p带、N2s带和Ga3d带之间的相对位置.在此基础上Ga空位计算(无晶格畸变)显示,Ga空位周围的费米面显著高于正常GaN晶格的费米面.因此Ga空位周围N原子的处于费米面上的2p电子很易被激发成正常晶格处的传导电子 关键词： GaN 电子结构 团簇埋入自洽计算     

掺镧PbWO4闪烁晶体的缺陷研究

利用正电子湮没寿命谱(PAT)和X射线电子能谱(XPS)研究了掺镧所引起的PbWO₄ 晶体缺陷的变化.结果表明：掺镧后,PbWO₄晶体中的正电子捕获中心铅空位(V< sub>Pb)浓度增加,并进一步诱导低价氧浓度的增加.讨论了掺La的作用机制,认为掺 La将抑制晶体中的氧空位,增加铅空位浓度. 关键词： 掺镧钨酸铅晶体 正电子湮没寿命谱 X射线电子能谱 缺陷     

钨酸铅晶体中与氧空位相关的色心研究

采用基于相对论性密度泛函理论的离散变分和嵌入团簇方法计算了PbWO4晶体中与氧空位VO及其相关色心的态密度分布，并运用过渡态方法计算了其激发能.结果表明：VO的存在可以使WO42-基团的禁带宽度明显变小；PbWO4晶体中VO可以产生F+，并且F+的存在可以使晶体引起420nm的吸收，另外PbWO4晶体中VO可能不会产生F心. 关键词： PbWO4晶体 密度泛函 氧空位 态密度分布     

高氧空位简并锐钛矿TiO2半导体电子寿命的第一性原理研究

为了研究锐钛矿TiO2晶体中高氧空位浓度对电子寿命的影响,利用基于局域密度泛函理论框架下的广义梯度近似平面波超软赝势方法,用第一性原理对含高氧空位浓度的锐钛矿TiO2晶体进行了结构优化处理、能带分布和态密度分布计算,表明在温度一定和高氧空位浓度的条件下,锐钛矿TiO2的电子寿命随氧空位浓度的增大而减小;电子浓度的大小对电子寿命无影响.同时,锐钛矿TiO2晶体中高氧空位浓度时,发现有莫特相变的现象.     

X α studies of laser crystals III. Electronic structures of vacancy and impurity centers in cubic yttria stabilized zirconia hosts

The electronic structures of oxygen-ion vacancies and substitutional divalent and trivalent cobalt ions in cubic yttria stabilized zirconia hosts were calculated by the discrete-variational (DV)-X method. Various clusters account for different configurations of the oxygen-ion vacancies and different coordinations of the cobalt ions. The one-electron eigenvalues and charge distributions of the clusters were reported. The energies of optical transitions were obtained by transition state calculations, and compared with available experimental data. The effects of composition and thermal processing on the absorption spectra of the crystal were discussed.     

A close correlation between induced ferromagnetism and oxygen deficiency in Fe doped In2O3

We report on the reversible manipulation of room temperature ferromagnetism in Fe (5%) doped In₂O₃ polycrystalline magnetic semiconductor. The X-ray diffraction and photoemission measurements confirm that the Fe ions are well incorporated into the lattice, substituting the In³⁺ ions. The magnetization measurements show that the host In₂O₃ has a diamagnetic ground state, while it shows weak ferromagnetism at 300 K upon Fe doping. The as-prepared sample was then sequentially annealed in hydrogen, air, vacuum and finally in air. The ferromagnetic signal shoots up by hydrogenation as well as vacuum annealing and bounces back upon re-annealing the samples in air. The sequence of ferromagnetism shows a close inter-relationship with the behavior of oxygen vacancies (V_o). The Fe ions tend to a transform from 3+ to 2+ state during the giant ferromagnetic induction, as revealed by photoemission spectroscopy. A careful characterization of the structure, purity, magnetic, and transport properties confirms that the ferromagnetism is due to neither impurities nor clusters but directly related to the oxygen vacancies. The ferromagnetism can be reversibly controlled by these vacancies while a parallel variation of carrier concentration, as revealed by resistance measurements, appears to be a side effect of the oxygen vacancy variation.     

Experimental evidence for fast cluster formation of chain oxygen vacancies in YBa2Cu3O7−δ as the origin of the fishtail anomaly

We report on three different and complementary measurements, namely magnetization measurements, positron annihilation and NMR spectroscopy, which give evidence that the formation of oxygen vacancy clusters is the origin of the fishtail anomaly in YBa₂Cu₃O_7−δ. In contrast to YBa₂Cu₃O_7.0, where the anomaly is intrinsically absent, it can be suppressed in the optimally doped state where vacancies are present. Thus, clusters of oxygen vacancies rather than single vacancies or point defects are responsible for this anomaly.     

Local structure analysis of YSZ by Y-89 MAS-NMR

The local coordination structure around Yttrium ions in yttria stabilized zirconia (YSZ) has been investigated by ⁸⁹Y MAS-NMR. The NMR spectrum showed multiple peaks corresponding to yttrium ions in different coordination numbers. The compositional dependence of spectra was observed. Yttrium ions of different oxygen coordination number were quantified. The oxygen vacancy concentration around the cations was determined. It was found that the vacancies were distributed around Zirconium ions in lower Y₂O₃ concentrations, and the vacancy concentration located to Yttrium began increasing at concentrations above 10 mol% Y₂O₃. The local structure change was able to be directly observed by ⁸⁹Y NMR measurements.     

Structure of ionic and electronic defects in cobalt bronze NaxCoO2

Thermogravimetric and electrical conductivity measurements of cobalt bronze Na_0.7CoO₂ have been carried out in the temperature range 990-670 K and over oxygen pressures from 100 to 10 kPa. It has been stated that in addition to defects in the sodium sublattice (sodium vacancies and electron holes) in this material there are also ionic defects in the oxygen-cobalt sublattice. The proposed method of analysis of defect type, based on the calculation of derivative of the function ln (def) = f(lnP_O2) shows the existence of cobalt ions Co³⁺ in the sodium sites.     

Influence of defects on magnetism of Co-doped ZnO

Two kinds of Zn_0.97Co_0.03O powders were prepared by precursor thermal decomposition under different conditions. One grown at low temperature has a positive Curie-Weiss temperature Θ, while the other grown at high temperature has a negative Θ. Both of them contain oxygen vacancies. There are more shallow donors in the former than those in the latter. It is proposed that coexistence of oxygen vacancies and shallow donors is necessary to induce ferromagnetic coupling between Co ions.     

Change of the stoichiometry and electrocoloration due to the injection of Li and O ions into lithium niobate crystals

Congruently grown LiNbO₃ single crystals show both high oxygen and lithium ion conductivity at temperatures above 500 °C. The high oxygen ion conductivity can be understood in terms of a certain amount of oxygen vacancies already present in congruently grown LiNbO₃ single crystals. Thermal treatment of LiNbO₃ produces additional oxygen vacancies. The absorption bands introduced by this procedure are investigated. It is found that the electrons which are generated during the reduction process are homogeneously distributed among all oxygen vacancies in the LiNbO₃ single crystals. The electrocoloration phenomenon in LiNbO₃ single crystals is due to the process of injection of lithium ions and electrons into LiNbO₃ by a double charge mechanism. Investigations of the optical and electrical properties of electrocolored LiNbO₃ crystals are reported. It is shown that the absorption spectra of thermally and electrochemically reduced samples are identical and that the origin of the absorption processes has to be therefore the same in both cases. That means, additional electrons produced by the double charge injection of lithium ions and electrons are also homogeneously distributed among the oxygen vacancies. This supports our hypothesis that a certain amount of oxygen vacancies has to be present already in as-grown LiNbO₃ single crystals.     

ZnO-doped LiNbO<Subscript>3</Subscript> single crystals studied by X-ray and density measurements

Energy Dispersive X-ray Fluorescence Spectroscopy, X-ray diffraction and density measurements were conducted on undoped and ZnO-doped congruent LiNbO₃ single crystals grown by the Czochralski method. Based on the experimental results, an intrinsic defect evolution model was proposed. When ZnO was doped into the congruent LN crystals, the Zn ions replaced first the Li ions and increased the density. Then, the Zn ions simultaneously replaced the Li ions and the antisite Nb_Lis until all Nb_Li ions were replaced, which increased the density further. After that, the Zn ions substituted Nb ions in the Nb-sublattice sites with the reduction of the Li vacancies as self-compensation and thus reduced the density. When the Li vacancies disappeared completely, the Zn ions substituted simultaneously both Li ions in the Li-sublattice sites and Nb ions in the Nb-sublattice sites. The simultaneous substitution might finally lead to the generation of oxygen vacancies and decreased the density further. PACS 61.72.-y; 06.30.Dr; 61.10.-i     

Investigation on dependence of BiFeO3 dielectric property on oxygen content

The influence of oxygen content on the dielectric property of BiFeO3 ceramics is studied by experiment and firstprinciples calculation.The experimental result demonstrates that the dielectric constant of BiFeO3 is strongly dependent on introduced oxygen and oxygen vacancies.By comparison with BiFeO3,the introduced oxygen and oxygen vacancies can lead to a reduction in dielectric constant of BiFeO δ at a lower frequency.The first-principles calculation also shows a similar result when photon energy is in a range of 2.0-4.1 eV.A likely explanation is that this oxygen content dependence may be ascribed to the distortion of Fe-O octahedron structure due to oxygen vacancies or excess oxygen ions in the crystal structure of BiFeO3.     

Mictomagnetic State in an EuBaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5.5–δ</Subscript> Single Crystal

The magnetic properties of an EuBaCo_1.9O_5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo₂O_5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 Å, b = 7.833 Å, and c = 7.551 Å. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below T_C = 242 K. At T < 300 K, all three spin states of the Co³⁺ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co³⁺ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co³⁺ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo_1.9O_5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo_1.9O_5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.