EPR of radiation defects in LiBaF 3 crystals

EPR spectra of LiBaF 3 crystals have been investigated after X-irradiation at RT. A spectrum consisting of approximately 35 nearly equidistant EPR lines has a strong angular dependence on the line intensities. The spectrum is caused by a hyperfine interaction (hfs) of a spin S =1/2 with neighbouring groups of nuclei. The observed large number of hfs lines required Li nuclei being in the first shell and fluorine nuclei in the more distant second shell. We analysed the spectrum in the F -centre model, taking reduced hfs values of the F -centre in LiF and found qualitative explanation of the number of hfs lines. The angular dependence of the line intensities could be explained by an anisotropy of the g -tensor with its main axis along the [1 v 0 v 0] axis of the crystal.     

EPR of Pb3+ ion in LiBaF3 crystals

Complex EPR spectra of paramagnetic centers Pb³⁺ formed in LiBaF₃:Pb²⁺ crystals under X-ray irradiation are studied in the temperature range of 10–150 K. It is shown that lead ions substitute Ba²⁺ ions in the LiBaF₃ crystal and are in the cubic-octahedral 12-fold environment of the fluorine ions. The hyperfine structure constants describing the observed spectrum are determined and parameters of superhyperfine interaction with the nearest fluorine ions are estimated.     

First-principles study of orbital ordering in cubic fluoride KCrF_3

Comprehensive first-principles calculations are performed to provide insight into the intriguing physical properties of the ternary cubic fluoride KCrF3. The electronic structures exhibit a prominent dependence on the effective local Coulomb interaction parameter Ueff. The ground state of the cubic phase is a ferromagnetic （FM） half-metal with Ueff equal to 0, 2, and 4 eV, whereas the insulating A-type antiferromagnetic （A-AFM） state with concomitant homogeneous orbital ordering is more robust than the FM state for Ueff exceeding 4 eV. We propose that the origin of the orbital ordering is purely electronic when the cooperative Jahn-Teller distortions are absent in cubic KCrF3.     

First-principles study on electronic states of SrWO4 crystals containing F-type color centers

The electronic structures of the SrWO₄ crystals containing F-type color centers are studied within the framework of the fully relativistic self-consistent Dirac–Slater theory using a numerically discrete variational (DV-X^α) method. The calculations indicate that either F or F⁺ center has donor energy level within the forbidden band. The electronic transition energies from the two donor levels to the bottom of the conduction band are 1.82 eV and 2.28 eV corresponding to the 685 nm and 545 nm absorption bands, respectively. It is, therefore, concluded that the 545–685 nm absorption bands are originated from the F and F⁺ center in SrWO₄ crystal respectively.     

铜铁镁三掺铌酸锂晶体的第一性原理研究

利用基于密度泛函理论的第一性原理,研究了Cu:Fe:Mg:LiNbO3晶体及对比组的电子结构和光学特性.研究显示,单掺铜或铁铌酸锂晶体的杂质能级分别由Cu 3d轨道或Fe 3d轨道贡献,禁带宽度分别为3.45和3.42 eV;铜、铁共掺铌酸锂晶体杂质能级由Cu和Fe的3d轨道共同贡献,禁带宽度为3.24 eV,吸收峰分别在3.01,2.53和1.36 eV处;Cu:Fe:Mg:LiNbO3晶体中Mg^2+浓度低于阈值或高于阈值(阈值约为6.0 mol%)的禁带宽度分别为2.89 eV或3.30 eV,吸收峰分别位于2.45 eV,1.89 eV或2.89 eV,2.59 eV,2.24 eV.Mg^2+浓度高于阈值,会使吸收边较低于阈值情况红移;并使得部分Fe^3+占Nb位,引起晶体场改变,从而改变吸收峰位置和强度.双光存储应用中可选取2.9 eV作为擦除光,2.5 eV作为读取和写入光,选取Mg^2+浓度达到阈值的三掺晶体在增加动态范围和灵敏度等参量以及优化再现图像的质量等方面更具优势.     

First-principles study on electronic structures of BaWO4 crystals containing F-type color centers

The electronic structures of BaWO₄ crystals containing F-type color centers are studied within the framework of the fully relativistic self-consistent Dirac-Slater theory, using a numerically discrete variational (DV-Xα) method. It is concluded that F and F⁺ color centers have donor energy level in the forbidden band. The optical transition energies are 2.449 and 3.101 eV, which correspond to the 507 and 400 nm absorption bands, respectively. It is predicted that 400-550 nm absorption bands originate from the F and F⁺ color centers in BaWO₄ crystals.     

He原子掺杂铝材料的第一性原理研究

采用基于第一性原理的平面波超软赝势方法,结合广义梯度近似(GGA),计算了铝及铝晶胞间隙位置掺入He原子后体系的几何结构、电子结构、总体能量和电荷布居值.计算结果表明:随着氦在金属铝中逐渐形成,铝晶胞体系会发生晶格畸变,但总的趋势是He在铝体系的八面体位置的晶格畸变小于其在四面体位置的晶格畸变.He在铝晶胞八面体和四面体间隙的杂质形成能分别为1.3367 eV和2.4411 eV.由此可知,He在铝晶胞中最稳定位置是八面体间隙位置.同时,文中还从原子尺度层面分析了He原子在铝晶胞中的占位及其键合性质,讨论 关键词： 铝材料 第一性原理 形成能     

Annealing of color centers in LiBaF 3

Results of the glow rate technique to analyze the activation energy of thermostimulated annealing of X-ray created F -type color centers in LiBaF 3 crystals are presented, showing pure and containing oxygen centers. It is shown that depending on the impurity composition two alternative mechanisms are involved in the annealing of color centers. It is proposed that either the anion vacancy governed migration of F -centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine ( F i ) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F -type centers are responsible for the recombination of radiation defects above RT.     

Mössbauer study of various fluoride glasses containing iron fluoride

(PbF₂, ZnF₂, AlF₃, or ZrF₄) - (BaF₂ and CaF₂) - FeF₃ glasses with 20 or 30 mol%FeF₃ contents were studied by ⁵⁷Fe Mössbauer spectroscopy. The Mössbauer spectrum at room temperature for the PbF₂-based glass is composed of one doublet due to Fe³⁺, whereas those for the ZnF₂?, AlF₃?, and ZrF₄-based glasses, two doublets due to Fe³⁺ and Fe²⁺. Both the iron ions have octahedral F coordination in the high spin states. The Fe²⁺/(Fe²⁺ + Fe³⁺) value in glass increases with the basicity of base glass.     

Ab initio study of electronic structures of BaMoO4 crystals containing an interstitial oxygen atom

The electronic structures of the perfect BaMoO₄ and BaMoO₄ crystals containing an interstitial oxygen atom situated at an appropriate position with the total energy being the lowest are studied within the framework of the density functional theory with the lattice structure optimized. The calculated results reveal that the interstitial oxygen atom situated at two different interstitial sites would combine with formal lattice oxygen ions forming molecular ions in two different ways, and the interstitial oxygen atom would cause visible range absorption band peaked at about 320 nm.     

NiAl和Cr材料中H原子间隙的第一性原理计算

通过第一性原理赝势平面波方法研究了氢原子在B2-NiAl和Cr合金中的占位以及对键合性质的影响. H在NiAl的富Al和富Ni八面体间隙杂质形成能分别为-2.365和-2.022eV,而在四面体间隙不稳定. H在Cr的八面体和四面体间隙的杂质形成能分别为-2.344和-2.605eV. H在NiAl中的最稳定位置为富Al八面体间隙,而在Cr中的最稳定位置是四面体间隙. 可以预期H在B2-NiAl/Cr体系中主要占据Cr 的四面体间隙位置. 通过分析原子结构、电荷集居数、价电荷密度以及态密度,讨论了H对B2 关键词： NiAl/Cr双相合金 H原子 第一性原理     

Ionic,electronic and ion-diffusion controlled relaxation processes in CaF2, BaF2 and LiBaF3 crystals

Abstract

The ionic, electronic and anion-diffusion controlled thermally stimulated relaxation (TSR) processes at 80—700 K in CaF₂ BaF₂ and LiBaF₃ crystals (X-ray irradiated or non-irradiated) have been investigated by means of ionic conductivity, ionic thermally stimulated (TS) depolarization current (TSDC); as well as current (TSC), luminescence (TSL) and bleaching (TSB) techniques. Above 250—290 K broad and overlapping anion TSDC peaks and correlated TSB stages are detected. The TSB kinetics is initiated and controlled by anion detrapping and interaction with the localized charges, i.e., the anion-diffusion controlled TSR processes take place in fluorides. The TSL and TSC data for LiBaF₃ indicate that the lifetime and drift of electrons at 80—250 K is very small because of deep retrapping. The main TSL peaks at 132K, 170K and 220 K are caused by V_k center detrapping and hole-diffusion controlled tunnel recombination within pairs like {D_n e^?…V_k }.     

Co掺杂BiFeO3的第一性原理研究

采用密度泛函理论结合投影缀加波（PAW）方法,研究了具有钙钛矿结构的BiFeO₃材料及对BiFeO₃进行B位Co元素替代掺杂得到的BiFe_075Co_025O₃材料的磁结构、电子结构、能带结构.结果表明：Co的掺入不破坏原有的钙钛矿结构,对材料铁电性影响不大;掺杂导致原有的G型反铁磁序发生变化,形成了亚铁磁序的磁结构,材料的铁磁性有了很大提高;然而,Co杂质的掺入使材料的绝缘性有所减弱. 关键词： 第一性原理计算 3')" href="#">Co掺杂BiFeO₃ 铁磁性     

First-principles study of color centers in PbMoO4 crystals

Electronic structures of PbMoO₄ crystals containing Mn ion impurities located at Pb²⁺ sites 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 Mn²⁺ ions have donor energy levers in the forbidden band, which may correspond to the yellowish color absorption band of PbMoO₄ as-grown in air. The new-formed Mn³⁺ ions have acceptor energy levers in the forbidden band, which may relate to the photochromic effect in PbMoO₄ crystal.     

Theoretical study of divalent samarium defects in lanthanum fluoride crystals

The results from a theoretical study of the electron structure of an impurity rare-earth Sm²⁺ defect in a LaF₃ crystal are presented. The electron energy levels of the rare-earth impurity defect and the transitions between them are studied using the multiconfigurational CASSCF/CASPT2 method. The absorption spectrum obtained during the calculations is consistent with the experimental data. Based on our model, we can state definitively that a vacancy on an anion sublattice serves as a charge compensator for a divalent ion.     

四方相BaTiO3铁电性的第一性原理研究

在广义梯度近似下,利用超软赝势对立方相和四方相BaTiO3晶胞中Ti原子沿c轴位移时体系的能量、原子间电子云重叠布局数和各原子上的净电荷等进行了自洽计算.结果显示,当Ti原子沿c轴位移0.012nm时,四方相BaTiO3体系能量最低,其自发极化强度为0.261C/m2,该结果与实验数据相符合;同时表明,O原子的2p轨道和Ti原子的3d轨道的杂化是BaTiO3晶体出现铁电性的重要原因.     

3d过渡金属掺杂锐钛矿相TiO2的第一性原理研究

采用基于密度泛函理论的平面波超软赝势方法研究了纯锐钛矿相TiO₂及掺杂3d过渡金属TiO₂的几何、电子结构及光学性质. 计算结果表明掺杂能级的形成主要是掺杂过渡金属3d轨道的贡献，掺杂能级在禁带中的位置是决定TiO₂吸收带边能否出现红移的重要因素. Cr，Mn，Fe，Ni，Co，Cu掺杂使TiO₂的吸收带边产生红移，并在可见光区有一定的吸收系数； Sc，Zn掺杂使TiO₂的吸收带边产生蓝移，但在可见光区有较大的吸收系数；掺V不但使TiO₂的吸收带边产生红移，增强了在紫外光区的光吸收，而且在可见光区有非常大的吸收系数.