第一原理研究LuPO4中氧空位的性质

磷酸结构的晶体在掺杂二价阳离子后容易形成产生焦磷酸结构(P₂O₇) ^4-, 这种含有焦磷酸结构的氧化物材料十分适合做质子导体、燃料电池、气体传感器以及陶瓷膜等。本文利用第一性原理研究了LuPO₄晶体中氧空位的结构性质, 结果显示当氧空位带二价正电时, 会引发氧空位周围原子奇特的畸变, 形成焦磷酸结构。为了解释这种结构畸变的机理, 本文利用过渡态搜索计算了结构变化过程中势能面的变化情况, 正一价氧空位形成焦磷酸 结构需要越过2.4 eV的势垒, 而正二价氧空位形成焦磷酸结构则不需要越过任何势垒, 因此很容易形成焦磷酸 结构。最后给出氧空位不同带电态的晶格结构、电子态密度以及电荷密度分布等基本物理性质, 氧空位处于正二价态结构下, 氧空位附近的P原子与O原子成键, 又由于O原子有较强的电负性, P的s轨道电子向O的p轨道转 移。P的s、p轨道在禁带中出现了与总态密度对应的缺陷能级, 结果表明带正二价氧空位的晶体性质发生了明显变化。     

四面体掺杂尖晶石结构Co1-xRexCr2O4(Re=Li、Na、K、Rb)的电子结构和光学性质

采用基于密度泛函理论(DFT)的平面波超软赝势方法, 计算了CoCr₂O₄及Li、Na、K和Rb四面体掺杂CoCr₂O₄的基态结构、电子结构和光学性质。计算结果表明: 一价离子四面体掺杂都导致晶格有微小的畸变, 使体系的稳定性降低, Rb掺杂的体系最稳定; 电子态密度的计算结果表明: 掺杂体系的导带主要有Co-3d和Cr-3d轨道电子构成, 掺杂离子改变了CoCr₂O₄导带的电子结构, 主要引起了导带Co-3d态密度峰的下移, 随着掺杂浓度的增大, 费米能级进入价带更深; 光学性质计算表明: 掺杂体系的吸收光谱发生红移, 并在低能区有很强的吸收, 表明掺杂能极大地提高CoCr₂O₄对可见光的吸收和光催化效率。     

AlF3掺杂氧化铝陶瓷的高灵敏辐照诱导相分离行为

放电等离子体烧结的AlF₃掺杂氧化铝陶瓷在透射电镜(TEM)常规观察条件下发现了一种电子辐照诱导快速相分离行为。在透射电镜的电子辐照下, 球形纳米晶Al颗粒在几秒钟内从原始氧化铝晶粒表面析出。高分辨TEM观察结合衍射花样分析发现原始的F掺杂氧化铝晶粒表面为高度缺陷态, 电子辐照后, 随着Al纳米颗粒析出, 氧化铝晶粒表面的缺陷消失。通过对掺杂过程缺陷反应及氧化铝阳离子亚晶格的深入分析, 提出了一种缺陷辅助间隙原子偏析机理来解释这一现象。即掺杂F离子首先占据氧空位的同时Al离子占据间隙位, 当氧空位被全部占据时, F和Al离子同时占据基体八面体间隙位, 并形成了亚稳定的掺杂态。在氧化铝基体1/3 [11ˉ00]不全位错的作用下, 畸变的阳离子亚晶格产生双聚八面体间隙位。当这些双聚八面体空位被外来Al离子占据时, 正如高分辨图像所观察的, 形成了包含有三个原子层左右的堆垛层错。同时, 沿着层错偏聚在双聚八面体位的掺杂Al离子扮演了析出物早期的角色, 在电子辐照下随着F离子的烧蚀, 不稳定的偏聚Al离子析出成为纳米颗粒并伴随着基体氧化铝的晶格重构。     

Pt和Au修饰锐钛矿型TiO2(101)面的第一性原理研究

采用平面波超软赝势方法研究了Pt和Au修饰锐钛矿型TiO₂(101)面的结构稳定性及电子结构。结果显示贵金属原子在TiO₂(101)符合化学计量比的条件下, 在其表面的吸附作用不强, 对电子结构的影响也较小。但是发现在富O条件下, Pt和Au原子容易吸附在表面Ti空位的位置, 与Au原子不同, Pt原子有从TiO₂表面扩散进入体相晶格中的趋势。而在富Ti条件下, Pt和Au原子容易吸附在O1空位的位置。对可能存在的几种空位缺陷吸附模型进行了电子结构的计算。结果表明: 空位缺陷的产生不仅有利于Pt和Au原子“湿化”TiO₂(101)表面, 也有利于带隙中产生贵金属原子的5d杂质能级。     

S和Al掺杂单层g-C3N4电子结构与光学性质的第一性原理研究

g-C₃N₄是一种典型的聚合物半导体材料，在可见光下就能完成对半导体要求较高的光催化反应。采用基于密度泛函理论的第一性原理平面波超软赝势方法研究了单层g-C₃N₄、S单掺g-C₃N₄、Al单掺g-C₃N₄和S-Al共掺g-C₃N₄的形成能、电子结构及光学性质。结果表明：S掺杂空隙I位置、Al掺杂N2位置时，杂质原子最易掺入g-C₃N₄体系。与单层g-C₃N₄相比，掺杂后的体系均发生了晶格畸变以及红移现象，拓展了体系的光吸收范围，可推测出S、Al掺杂能够提高g-C₃N₄体系的光催化性。其中，S-Al共掺杂体系的光催化性是最优的，原因是共掺杂体系的分子轨道有较强的离域性，有利于提高载流子的迁移率，并且共掺杂能使单掺杂引入的深能级变浅，减少杂质能...     

CsCdF3:Cr3+晶格畸变及其Cd2+-空位研究

按照零场分裂（ＺＦＳ）的三阶微扰理论和叠加晶场模型,建立了ＺＦＳ参量Ｄ与ＣｓＣｄＦ_３：Ｃｒ^３＋晶格结构之间的定量关系;同时考虑了晶格畸变和Ｃｄ^２＋空位对零场分裂参量Ｄ的贡献,计算了ＣｓＣｄＦ_３：Ｃｒ^３＋晶体的零场分裂参量Ｄ,计算结果与实验符合甚好．证明了晶格畸变和Ｃｄ^２＋空位的存在;同时得到包围 Ｃｒ^３＋离子的 Ｆ^－离子向中心 Ｃｒ^３＋离子分别移动Ｘ_１＝０．００２９１ｎｍ,Ｘ_２＝０．００１ｎｍ,Ｘ_３＝０．００２６ｎｍ．ＣｓＣｄＦ_３：Ｃｒ^３＋晶体基态的ＺＦＳ主要来自Ｃｄ^２＋空位,但晶格畸变的贡献是不能忽略的．     

Pd插层NbSe2化合物的制备、晶体结构和电学性质研究

通过固相反应法合成一系列插层化合物Pd_xNbSe₂ (x=0~0.17)。它们与2H-NbSe₂相同, 属于六方晶格, 空间群为P6₃/mmc。Pd占据NbSe₂层间的八面体空位。随着Pd含量的增加, 晶格常数c线性增大, 而a几乎不变。X射线单晶衍射结果表明, Pd_0.17NbSe₂的晶格常数为a=b=0.34611(2) nm, c=1.27004(11) nm。每个Pd原子与六个Se原子键合形成[PdSe₆]八面体来连接相邻的Nb-Se层, 使晶体结构变得更加稳定, 从而提高化合物的热稳定性。电学测试表明, 随着Pd含量的增加, Pd_xNbSe₂的剩余电阻比减小。此外, 超导转变温度也随着Pd含量的增加而下降, 说明Pd的引入不利于NbSe₂的超导态。     

Bi掺杂BaTiO3陶瓷缺陷性质的第一性原理研究

采用第一性原理计算了Bi掺杂BaTiO3陶瓷3种不同的晶格缺陷结构,分别为单独的BiBa掺杂缺陷模式(BTB模型),1个BiBa掺杂缺陷与1个VBa钡空位同时存在(BTB1模型),符合化学计量比的BiBa掺杂缺陷与VBa钡空位缺陷模式(BTB2模型)。BTB模型显示缺陷结构是由施主掺杂机制控制的,Bi与周围的O原子形成典型的离子键,Ti 4+被还原成Ti 3+。在BTB1模型中钡空位的存在则影响了Bi在晶格中与O的相互作用,Bi偏离初始的中心位置,与邻近的3个氧原子形成了弱的共价键,而正是由于这些弱的共价键导致缺陷附近的[TiO6]八面体产生较大畸变,削弱了Ti 4+的极化能力,使缺陷附近的[TiO6]八面体极化能力减弱,此时缺陷结构是由Ba2+离子空位补偿机制控制的。而BTB2模型可以看成是BTB模型与BTB1模型的叠加,因此缺陷结构是由施主掺杂机制与Ba2+离子空位补偿机制共同控制的。     

氧控In2O3薄膜的光电性能

对比在控氧条件下制备态和退火态In₂O₃薄膜的微观结构和光电性能,分析两种状态中不同的氧作用机制。两种控氧行为都能够有效提高In₂O₃薄膜的晶格有序度和降低氧空位浓度,使其载流子浓度下降、迁移率提高和光学带隙变窄;等离子体制备过程中氧以高活性非平衡方式注入晶格,而退火时氧以低活性平衡态扩散的方式进入晶格;不同的氧作用机制使得退火态薄膜比制备态薄膜具有更少的结构缺陷、更高的氧空位浓度和更佳的透光导电性。     

AlF_3掺杂氧化铝陶瓷的高灵敏辐照诱导相分离行为（英文）

放电等离子体烧结的AlF_3掺杂氧化铝陶瓷在透射电镜(TEM)常规观察条件下发现了一种电子辐照诱导快速相分离行为。在透射电镜的电子辐照下,球形纳米晶Al颗粒在几秒钟内从原始氧化铝晶粒表面析出。高分辨TEM观察结合衍射花样分析发现原始的F掺杂氧化铝晶粒表面为高度缺陷态,电子辐照后,随着Al纳米颗粒析出,氧化铝晶粒表面的缺陷消失。通过对掺杂过程缺陷反应及氧化铝阳离子亚晶格的深入分析,提出了一种缺陷辅助间隙原子偏析机理来解释这一现象。即掺杂F离子首先占据氧空位的同时Al离子占据间隙位,当氧空位被全部占据时,F和Al离子同时占据基体八面体间隙位,并形成了亚稳定的掺杂态。在氧化铝基体1/3[1100]不全位错的作用下,畸变的阳离子亚晶格产生双聚八面体间隙位。当这些双聚八面体空位被外来Al离子占据时,正如高分辨图像所观察的,形成了包含有三个原子层左右的堆垛层错。同时,沿着层错偏聚在双聚八面体位的掺杂Al离子扮演了析出物早期的角色,在电子辐照下随着F离子的烧蚀,不稳定的偏聚Al离子析出成为纳米颗粒并伴随着基体氧化铝的晶格重构。     

Effect of Mn on defect states in ZnIn2Se4

ZnIn₂Se₄ polycrystalline samples were synthesized with small amounts of manganese and their optical absorption spectra examined. It was found that pure material shows the presence of defect states very close to the conduction and valence bands; however, the presence of manganese atoms reduces the density of defects and, at 0.3 at% of Mn, the defect states located very close to the conduction and valence bands are eliminated. The observed results are explained by considering that the Mn atoms tend to occupy the Zn vacancy sites and reduce antisite-induced disorder.     

Local density calculation of structural and electronic properties for Ca10(PO4)6F2

The local-density-functional pseudopotential approach is applied to the calculation of the lattice properties and electronic structure of the fluoroapatite Ca₁₀(PO₄)₆F₂ which is one of the simplest apatites. The calculated lattice parameters agree well with the experiment. Valence charge density and Mulliken population are analysed to understand the nature of the bond between the different atoms.     

A comparative study of the atomic and electronic structure of F centers in ferroelectric KNbO3: Ab initio and semi-empirical calculations

The linear muffin-tin-orbital method combined with density functional theory (in a local density approximation) and the semi-empirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the supercell study of the F centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO₃ crystals. The two electrons are found to be considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus, the F center in KNbO₃ resembles much more electron defects in the partly covalent SiO₂ crystal (the so-called E₁′ center) rather than usual F centers in ionic crystals like MgO and alkali halides. This covalency is confirmed by the analysis of the electronic density distribution. The absorption energies were calculated by means of the INDO method using the ΔSCF scheme after a relaxation of atoms surrounding the F center. For the orthorhombic phase three absoprtion bands are predicted, the first one is close to that observed experimentally under electron irradiaton.     

一步水热合成Sm3+-SrTiO3/TiO2复合纳米纤维及可见光催化性能

以电纺TiO₂纳米纤维为基体和反应物,通过一步水热法,将SrTiO₃原位构筑在TiO₂纳米纤维表面的同时将稀土Sm³⁺掺入SrTiO₃中,合成了Sm³⁺-SrTiO₃/TiO₂复合纳米纤维光催化材料.利用XRD、XPS、FESEM和HRTEM等测试手段对样品进行了表征.以罗丹明B和对氯苯酚模拟有机污染物进行光催化降解.结果表明：稀土Sm³⁺掺杂进入SrTiO₃晶格取代Sr²⁺,在SrTiO₃禁带内形成杂质能级,拓宽了光谱响应范围;同时在SrTiO₃晶格内引入了缺陷位,成为电子的捕获中心,降低了载流子的复合几率;而SrTiO₃与TiO₂复合形成异质结,进一步提高了光生电子-空穴的分离程度,Sm³⁺-SrTiO₃/TiO₂复合纳米纤维表现出良好的可见光催化活性。     

Carrier control of β-FeSi2 by 1.2 MeV-Au ion irradiation

We have investigated effects of 1.2 MeV-Au⁺⁺ ion irradiation into β-FeSi₂ samples which are synthesized by different processes. Such a high energy Au⁺⁺ ion irradiation can be expected to induce Si and Fe vacancies in the β-FeSi₂ lattice. After recrystallization of the lattice, we found that one of the samples showed conversion from the initial p-type conduction to the n-type one. RBS analysis revealed that irradiated Au atoms even after recrystallization by thermal annealing were not included in the β-FeSi₂ layers. These results suggest that the Au atoms cannot contribute to conversion of the electrical conduction type observed. One possible explanation is that the highly induced Fe vacancy can play donor and its annihilation rate is much slower than that of Si vacancy that surely plays acceptor, so that carrier's compensation balance shifts toward n-type conduction. These results imply that vacancy-induction by high energy ion irradiation can be employed to control a conduction type of β-FeSi₂.     

Computer modelling of point defects in ABO3 perovskites and MgO

We present results for basic intrinsic defects: F-type electron centers (O vacancy which trapped one or two electrons) and hole polarons bound to Mg or K vacancy in ionic MgO and partly covalent KNbO₃ perovskite, respectively. We demonstrate that a considerable covalency of the perovskite chemical bonding makes the F-type centers therein much more similar to defects in partly-covalent quartz-type oxides rather than the conventional F centers in alkali halides and ionic MgO. Both one-site (atomic) and two-site (molecular) polarons are expected to coexist in KNbO₃ characterized by close absorption energies. Our calculations confirm existence of the self-trapped electron polarons in KNbO₃, KTaO₃, BaTiO₃, and PbTiO₃ crystals. The self-trapped electron is mostly localized on B-type ion due to a combination of breathing and Jahn–Teller modes of nearest six oxygen ion displacements. The relevant lattice relaxation energies are typically 0.2–0.3 eV, whereas the optical absorption energies 0.7–0.8 eV, respectively. According to our calculations, the absorption energy of a bound electron polaron in KNbO₃ by 0.1 eV exceeds that for the self-trapped electron polaron and equals 0.88 eV.     

Electron transfer effect in BaTiO3 and CaTiO3 due to Nb-doping

Using an advanced quantum-chemical method, a study of Nb-doping has been carried out in the BaTiO₃ and CaTiO₃ crystals. The lattice distortion due to the impurity presence is studied in a comparative manner for the two crystals and also considering different crystallographic phases. A new phenomenon of an extra electron transfer from the local energy level within the band-gap to the conduction band is found. This effect is discussed in terms of the available experimental data on electrical conductivity augmentation in these materials.     

A study on thermal emission of charges at Si3N4—GaAs interfaces after annealing in N2 and N2 + H2 mixtures

Si₃N₄—GaAs interfaces subjected to annealing in N₂ + H₂ mixture or pure N₂ atmosphere were investigated by a small-signal charge deep-level transient spectroscopy (Q-DLTS) method. The method measures the physical parameters of selective populations of the interface traps continuum. A dependence of the capture cross-section on activation energy was constructed for the continuum of interface states at the Si₃N₄—GaAs interface. The dependence shows an exponential character in the part of the gap ranging from 0.3 to 1.0 eV below the conduction band minimum. It was found that annealing in the temperature interval 400–450 °C reduces the zero-bias band bending by about 0.1 eV. At temperatures of 500 °C and more, degradation of the interface started; compared with annealing in pure N₂ ambient, annealing in an N₂ + H₂ mixture degraded the interface slightly more.     

Quantum chemical modelling of point defects in KNbO3 perovskite crystals

We present results of semi-empirical quantum chemical calculations for several perovskite KNb_xTa_1−xO₃ (KTN) solid solutions, as well as point intrinsic defects – F centers and hole polarons bound to K vacancy – in KNbO₃. Method of the intermediate neglect of the differential overlap (INDO) was combined with typically 320-atom supercells and atomic geometry optimization. Analysis of the optimized atomic and electronic structure has clearly demonstrated that several nearest Nb atoms substituting for Ta in KTaO₃ – unlike Ta impurities in KNbO₃ – reveal the self-ordering effect, which probably triggers the ferroelectricity observed in KTN. We predict co-existence of one-site (atomic) and two-site (molecular) polarons with close absorption energies (≈1 eV). When available, the INDO results are compared with ab initio calculations. The relevant experimental data are discussed.