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     

PbWO4电子结构的密度泛函计算

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模拟计算了PbWO₄晶体的本征能级结构.发现价带主要由O2p轨道组成,含有部分W5d轨道;导带主要由W5d和O2p的轨道组成.发现导带底由Pb6p_1/2的狭窄能级占有.禁带宽度和价带宽度分别约为4.8和4eV.计算结果很好地解释了实验得到的反射谱,并从理论上分析了PbWO₄晶体蓝光的发光模型. 关键词： 密度泛函 电子结构 4')" href="#">PbWO₄     

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.     

笼状Au$lt;sub$gt;20$lt;/sub$gt;内掺$lt;i$gt;M$lt;/i$gt;$lt;sub$gt;13$lt;/sub$gt;($lt;i$gt;M$lt;/i$gt;=Fe，Ti)团簇磁性的密度泛函计算研究

采用密度泛函理论中的广义梯度近似方法,对M₁₃（M=Fe,Ti）以及M₁₃内掺Au₂₀团簇的几何结构和磁性进行了计算研究.结果表明：M₁₃和M₁₃内掺Au₂₀团簇的几何结构在0.006—0.05 nm误差范围内保持着I_h对称性.Fe₁₃团簇最低能态的总磁矩为44 μ_B,内掺到Au₂₀笼中后形成的Fe₁₃内掺Au₂₀团簇的最低能态总磁矩为38 μ_B,且Au原子与内掺Fe₁₃团簇之间存在着弱铁磁相互作用.Ti₁₃团簇在总磁矩为6 μ_B时能量最低,掺入Au₂₀笼后形成的Ti₁₃内掺Au₂₀团簇最低能态总磁矩是4 μ_B,内表面12个Ti原子与表面Au壳之间是弱铁磁相互作用,而与中心Ti原子之间是弱反铁磁相互作用.由于Au₂₀笼状外壳的影响,Fe₁₃内掺Au₂₀和Ti₁₃内掺Au₂₀团簇中Fe₁₃和Ti₁₃的磁矩比无金壳的Fe₁₃和Ti₁₃团簇的磁矩分别减少了6.81 μ_B和2.88 μ_B. 关键词： 几何结构 磁性 密度泛函理论     

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     

Eu_mSi_n(m=1～2,n=1～8)团簇结构和性质的密度泛函理论研究

在密度泛函理论的框架下,采用广义梯度近似（GGA）研究了Eu₂Si_n（n=1～7）团簇的基态几何结构,系统计算了平均结合能E_b、二阶能量差分△₂E、最高占据轨道（HOMO）与最低未占据轨道（LUMO）之间的能隙,并与已有的EuSi_n（n=2～8）团簇相关数据作对比分析.研究表明:单Eu原子比双Eu原子掺杂的Si团簇具有更高的稳定性,EuSi₃、EuSi₆、Eu₂Si₄团簇较相应邻近团簇结构稳定;Eu_mSi_n（m=1～2,n=l～8）团簇的能隙随着团簇总原子数的增加呈现振荡变化,态密度分析得到能隙振荡变化的原因是S原子的s轨道与Eu原子的p轨道发生了杂化.     

密度泛函理论研究Zrn(n=2—16)团簇的基态结构及其稳定性

从第一性原理出发，利用密度泛函理论中的广义梯度近似(GGA)对Zr_n(n=2—16)团簇进行了结构优化、能量和频率计算.在充分考虑自旋多重度的前提下，对每一具体尺寸的团簇，得到了多个平衡构型，并根据能量高低确定了团簇的基态结构.综合团簇的结合能、离解能、二阶能量差分以及团簇的最高占据轨道(HOMO)和最低未占据轨道(LUMO)间的能隙可知Zr₂，Zr₅，Zr₇，Zr₁₃ 关键词： n团簇')" href="#">Zr_n团簇 密度泛函理论(DFT) 基态结构 自旋多重度     

Pd-Y微团簇的结构与性质研究

在相对论有效原子实势近似下，用密度泛函理论方法，对Pd_n(n=2,3,4)，Y_n（n=2,3,4）,Pd_nY(n=1,2,3,4)，PdY_n(n=2,3,4)，Pd₂ Y_n(n=2,3,4)团簇的各种可能的几何构型进行全优化计算,得到它们的基态结构和 光谱性质.结果表明，由于 Jahn-Teller效应， Pd₄和Y₄的基态结 构为C_s构型，P d₃和Y₃为C_2v 构型；混合微团簇Pd₃Y，Pd ₄Y，PdY₃，PdY₄ 和 Pd₂Y₄基态为C_s构型.最后计算了团簇的能级分布和最 高占据轨道与最低空轨道之间的能级间隙,分析了团簇的化学活性. 关键词： Pd-Y团簇 有效原子实势 密度泛函     

钇小团簇的结构和电离势的计算

采用密度泛函DFT中的B3LYP方法，选择LANL2DZ双ζ基组，优化并得到了Y_n=(n=2—8)小团簇的基态平衡结构，同时计算出其电离势.结果表明，钇原子之间形成团簇最稳定的结构是倾向于平均配位数最大，其电离势没有“奇-偶”振荡或“幻数”效应，表明Y_n团簇光致电离开始主要发生在Y原子局域化的4d轨道上的电子而不是在5s上.对Tomasz提出的钇团簇电离势的解析式进行合理地修正，修正后电离势解析式的计算值与实验值更接近. 关键词： Y团簇 密度泛函 平衡几何结构 电离势     

F,Y双掺钨酸铅晶体的发光性能和微观缺陷

通过透射光谱、光产额(LY)和光致发光等发光性能测试，研究了F,Y双掺钨酸铅(PbWO₄ ，简称PWO)晶体的发光性能，并利用热释光曲线和正电子湮没寿命谱对F,Y双掺PWO晶 体中的缺陷种类和变化进行了分析. 结果表明：与未掺杂晶体相比，双掺样品在350nm附近 的透过率大大提高，吸收边向短波方向移动约30nm，光致发光谱中出现位于350nm的发光峰 ，双掺样品的LY(100ns内)为未掺杂PWO的2.7倍左右.晶体中主要存在的缺陷为(WO_{3)^{-关键词： F Y双掺钨酸铅闪烁晶体 高光产额 热释光 正电子湮没寿命谱}}     

掺Gd3+,Y3+对PbWO_4低温热释光的影响

通过热释光方法研究了PbWO₄(PWO),PWO:Y³⁺,PWO:Gd³⁺多晶粉末及PWO,PWO:Y单晶的低温(<300K)热释光现象.多晶粉末中,掺杂Y³⁺或Gd³⁺都会大大降低甚至消除200K附近的热释光峰,同时产生新的热释光峰,分别位于125和150K(掺Y掺Gd).这表明掺三价离子除了起到电荷补偿作用以减少Pb³⁺,O^-浓度外,还可以产生新的陷阱能级.对于PWO:Y单晶,掺杂Y³⁺可以消除253K的热释光峰,即消除较深(～0.89eV)的陷阱,但PWO单晶中较浅的陷阱(～0.42eV)对应130K热释光峰仍然存在,对此进行讨论,它最可能源于氧空位缺陷.根据Pb³⁺,Gd³⁺,Y³⁺的电子库仑势不同,在PWO晶体中替代Pb²⁺后形成的电子陷阱深度有别(E_Pb>E_Gd>E_Y),从而解释了相应的热释光峰值温度的不同 关键词： 4')" href="#">PbWO₄ Y和Gd掺杂 热释光 陷阱     

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     

+3价离子掺杂钨酸铅晶体发光性能和微观缺陷的研究

通过透射光谱、x射线激发发射光谱（XSL）的测试，研究了Bridgman法生长的几种不同+3价离子掺杂钨酸铅晶体的发光性能，并利用正电子湮没寿命谱（PAT）和x光电子能谱（XPS）的实验手段，对不同钨酸铅晶体的微观缺陷进行研究.实验表明，不同的+3价离子掺杂，对钨酸铅晶体发光性能的改善不同，并使得晶体中正电子俘获中心和低价氧的浓度发生不同变化.其中掺镧晶体的正电子俘获中心和低价氧浓度均上升，而掺钇和掺铋晶体的正电子俘获中心和低价氧浓度均下降，掺锑晶体则出现了正电子俘获中心浓度上升、低价氧浓度下降的情况.提 关键词： 钨酸铅晶体 +3价离子掺杂 正电子湮没寿命谱 x光电子能谱     

含铅空位的PbWO4晶体光学性质及其偏振特性的研究

利用完全势缀加平面波局域密度泛函近似，计算了含铅空位的PbWO₄（PWO）晶 体的电 子结构，模拟计算了复数折射率、介电函数及吸收光谱的偏振特性. 比较含铅空位的PWO晶 体与完整的PWO晶体的吸收光谱及其偏振特性，得到与铅空位相关的吸收光谱及其偏振特性 ，计算结果与实验结果基本相符. 计算得到的含铅空位的PWO晶体的光学偏振特性反映了PWO 晶体的结构对称性. 计算结果表明PWO晶体中350，420，550和680 nm的吸收带的出现与PWO 晶体中铅空位的存在直接相关. 关键词： 4晶体')" href="#">PbWO₄晶体 电子结构 光学性质 铅空位     

First-principles study on the origin of optical transitions to be associated with F colour centers for PbWO4 crystals

The electronic structures of PbWO₄ crystals containing F type color centers with the lattice structure optimized are studied within the framework of the fully relativistic self-consistent Direc–Slater theory, using a numerically discrete variational (DV-Xα) method. The calculated results show that F and F⁺ centers have donor energy level in forbidden band. Their optical transition energy are 1.84 eV, 2.21 eV, respectively, which corresponds to the 680 nm, 550 nm absorption bands. It predicts that the 680 nm, 550 nm absorption bands originate form the F and F⁺ centers in PbWO₄ crystals.     

Thermal annealing behavior of luminescence of Gd3+-doped PbWO4 single crystal in air atmosphere

The results of photoluminescence measurements on as-grown PbWO₄:Gd³⁺ show that Gd³⁺ acts as a sensitizer of luminescence in the PbWO₄ lattice. Effects of sequential annealing in air atmosphere on luminescence of PbWO₄:Gd³⁺ were investigated by means of X-ray excited luminescence and photoluminescence. The annealing experiments indicate that the essence of the annealing in air atmosphere is the exchange of oxygen components between PbWO₄:Gd³⁺ crystal and the environment to improve the defects in the lattice. Annealing at a temperature of about 640 °C could further enhance the luminescence of PbWO₄ and depress it at a higher temperature. However, PbWO₄:Y³⁺ and PbWO₄:La³⁺ crystals do not show any luminescence enhancement in the same annealing process. A tentative mechanism of luminescence enhancement due to annealing is suggested. PACS 74.62.Dh; 81.40.Tv; 78.55.-m; 82.80.Pv; 78.70.En     

Luminescence characteristics of Eu3+ activated borate phosphor for white light emitting diode

In this paper, the Sr₃Y₂ (BO₃)₄:Eu³⁺ phosphor was synthesized by high temperature solid-state reaction method and the luminescence characteristics were investigated. The emission spectrum exhibits one strong red emission at 613nm corresponding to the electric dipole ⁵D₀--⁷F₂ transition of Eu³⁺ under 365nm excitation, this is because Eu³⁺ substituted for Y³⁺ occupied the non-centrosymmetric position in the crystal structure of Sr₃Y₂ (BO₃)₄. The excitation spectrum indicates that the phosphor can be effectively excited by ultraviolet (254nm, 365nm and 400nm) and blue (470nm) light. The effect of Eu³⁺ concentration on the red emission of Sr₃Y₂ (BO₃)₄:Eu³⁺ was measured, the result shows that the emission intensities increase with increasing Eu³⁺ concentration, then decrease. The Commission Internationale del'Eclairage chromaticity (x, y) of Sr₃Y₂(BO₃)₄:Eu³⁺ phosphor is (0.640,0.355) at 15 mol% Eu³⁺.     

Thermodynamics stability,electronic structures and spectroscopic properties of defects and Ce3+ ions in Y2O3

Thermodynamic properties, electronic structures and spectroscopic properties of defects and Ce³⁺ in Y₂O₃ are studied by using the hybrid density functional theory associated with multi-reference configuration interaction ab-initio calculations. Thermodynamic transition energy levels of the easily generated oxygen vacancies in the host are analyzed according to HSE06-calculated formation energies, which may be conducive to interpretations of the persistent luminescence (PersL) of Y₂O₃-based phosphors. Besides, the locations of impurity states (caused by V_O and Ce³⁺) in energy bands are obtained from derived density of states. Moreover, energies and oscillator strengths of 4f₁ → 5d₁₋₅ transitions of Ce³⁺ ions (at Y1 and Y2 sites) calculated from the CASSCF/CASPT2/RASSI−SO method agree reasonably well with experimental excitation spectra of Y₂O₃: Ce³⁺ phosphors, achieving the assignment of excitation spectra. The presented calculations can be applied to identify luminescent centers in Ce³⁺-doped phosphors and reveals possible native defects and their roles in the PersL of phosphors.     

First-Principles Study on the Electronic Structures for Y^3＋：PbWO4 Crystals

The possible defect models of Y^3＋：PbWO4 crystals are discussed by defect chemistry and the most possible substituting positions of the impurity Y^3＋ ions are studied by using the general utility lattice program （GULP）. The calculated results indicate that in the lightly doped Y^3＋ ：PWO crystal, the main compensating mechanism is [2Ypb^＋ ＋ VPb^2-], and in the heavily doped Y^3＋ ：PWO crystal, it will bring interstitial oxygen ions to compensate the positive electricity caused by YPb^＋, forming defect clusters of [2Ypb^＋ ＋Oi^2-] in the crystal. The electronic structures of Y3＋ ：PWO with different defect models are calculated using the DV-Xα method. It can be concluded from the electronic structures that, for lightly doped cases, the energy gap of the crystal would be broadened and the 420nm absorption band will be restricted; for heavily doped cases, because of the existence of interstitial oxygen ions, it can bring a new absorption band and reduce the radiation hardness of the crystal.     

Mechanism of Th, Zr doping in PbWO4 crystals

To check the doping mechanism of trivalent ion doping in PbWO₄ (PWO) and verify the formation of dipoles [2M_Pb³⁺-V_Pb], the researches on an investigation of tetravalent ion (Th⁴⁺, Zr⁴⁺) doped PWO was conducted by dielectric loss spectroscopy, thermoluminescence and optical absorption spectroscopy. The doping mechanism of tetravalent ion doping is similar to that of trivalent ion doping, while the dipole defects should be formed more easily due to the convenience for the two defects V_Pb and M_Pb⁴⁺ to get close to each other. However, in the case of Zr⁴⁺ doping, Zr was found to enter interstitial sites because of its small ionic radius, with the introduced charges compensated by V_Pb far away and hence does not exert an obvious influence on the performance of this material. Thus, no dielectric relaxation was found in the DS (dielectric spectra) experiment.