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.     

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.     

Ab initio calculations of electronic structures of SrMoO4 crystals containing F and F color centers

The electronic structures of SrMoO₄ crystals containing F and F⁺ color centers with the lattice structure optimized are studied within the framework of the fully relativistic self-consistent Dirac–Slater theory, using a numerically discrete variational (DV-Xα) method. From the calculation, it is concluded that F and F⁺ color centers have donor energy level in the forbidden band. The electronic transition energies from the donor level to the bottom of the conduction band are 1.855 eV and 2.161 eV, respectively, which correspond to the 670 nm and 575 nm absorption bands. It is predicted that the 670 nm and 575 nm absorption bands originate from the F and F⁺ centers in SrMoO₄ crystals.     

Thermally activated structural changes in KCl crystals containing color centers

Structural changes taking place in the temperature interval 300–750K in KCl crystals additively colored in Na vapor are studied by spectrophotometric and light scattering methods in conjunction with crystal lattice constant measurement.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 39–45, September, 1984.     

First-principles study of LiBaF3 crystals containing interstitial fluoride

The positions of the interstitial fluoride atoms in LiBaF₃ crystal are simulated by GULP. It is found that the formation energy is low when interstitial fluoride atoms at the center of the edge of the cube formed by Ba ions. The electronic structures and absorption bands for the perfect LiBaF₃ crystal and the LiBaF₃ crystal containing interstitial fluoride have been calculated using density functional theory code CASTEP. It is predicted that the interstitial fluoride atom combine with a formal lattice fluoride ion forming H center causes the 320 nm absorption band.     

碘化铯晶体中电子型色心的电子结构研究

运用以密度泛函理论为基础的相对论性离散变分方法(DV-Xα)模拟计算了完整的和含有F心、F+心以及F2心的碘化铯(CsI)晶体的电子结构,得到了含F心和F+心以及F2心的CsI晶体电子态密度分布以及它们可能产生的光学跃迁模式.计算结果表明,含F心和F2心的CsI晶体的禁带宽度明显变窄,F心和F2心的能级都出现在禁带中并且作为施主能级位于导带底部,利用过渡态理论计算得到其能级向Cs的5d轨道发生光学跃迁,能量跃迁值分别为1.69eV和1.15eV,该结果与实验结果完全一致,F+心没有能级出现在禁带中.计算结果从理论上成功地解释了碘化铯晶体经过辐照后电子型色心所产生的吸收带起源问题.     

Transient color centers in GGG crystals

Electron pulse induced absorption and their decay kinetics have been investigated in samples of GGG crystals with different starting absorption spectra. It is shown that for all samples there appears a wide transient absorption (TA) band with two maxima in the region 14,000-17,000 v cm m 1 and 22,000-26,000 v cm m 1 . TA decay kinetics measurements in 14,000 v cm m 1 and 22,000 v cm m 1 are two-exponential (with half-time order several tens and several hundreds ns). Analyzing the obtained results, we can suppose that low and high energy TA bands are connected with the F + (or O m ) and F transient color centers (TCC) respectively.     

Photoinduced color centers creation in superionic crystals RbAg 4 I 5

Abstract

A new phenomenon of a reversible photoinduced coloration caused by light irradiation is discovered and investigated in superionic RbAg ₄ I ₅ crystals. The reversible photoinduced absorption is found to be a result of irradiation by light with wavelengths in the region from 420 nm to 450 nm. The proposed mechanism of the discovered effect is associated with ambipolar diffusion of screened by mobile ions optically excited electronic carriers. The processes of color centers creation in superionic crystals RbAg ₄ I ₅ due to additive coloring in iodine vapours, ionic implantation and γ-ray irradiation are considered.     

Laws governing the creation of electronic color centers in LiF crystals acted on by pulsed irradiation

The processes of creating and transforming electronic color centers in an LiF crystal irradiated with a nanosecond electron pulse are investigated using pulse spectrometry with nanosecond resolution for times in the range 10⁻⁸ to 10⁵ sec. It is shown that the thermally activated mechanism of forming Frenkel pairs in the 12–200 K range consists of successively creating exciton states, as the temperature rises, having different degrees of spatial separation of the electron and hole components. It is concluded that the structure of self-trapped excitons evolves as a function of temperature and time, and that this evolution commences for any alkali halide crystal with the creation of self-trapped excitons ofD _2h point symmetry at 4 K. It is established that the interaction of electronic excitations with electronic color centers changes the properties of both the electronic excitations themselves and the color centers. In a crystal containing neutral electronic centers there is a fall in the yield of self-trapped excitons and Frenkel pairs and an increase in the contribution of the radiative channel for loss of the irradiation energy by the color centers. A mechanism is proposed for exciting luminescence of electronic color centers. It is established that short-lived irradiation-induced states exist, in particular a change in the spin state or in just the energy state of a center in the irradiation field, and that the appearance of these states changes the efficiency and directivity of the charge evolution of the electronic color centers. State Architectural Building Academy, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 57–75, November, 1996.     

First-principles study on electronic structure of LiFePO4

The electronic structure and band structure of olivine LiFePO₄ and its virtually delithiated product FePO₄ are compared based on first-principles calculations. It is found that Li intercalation mainly impacts the electronic and band structures of the Fe atom. Total static energy calculations indicate that it is more difficult for lithium to transfer in LiFePO₄ than in FePO₄.     

Radiative color centers in doped sapphire crystals

Experimental and theoretical results of investigation of the spectral properties of sapphire (α-Al₂O₃) single crystals are analyzed. Crystals grown under different conditions, both nonactivated and activated with impurity ions of the iron group (Cr, V, Mn, Fe, Co, Ni), primarily with chromium ions, were investigated before and after irradiation with photons and high-energy electrons. It is shown that the impurity composition of crystals significantly affects the radiative and optical properties and the character of changes in the electronic state of impurities.     

Experimental study of Raman scattering by NaI crystals containing F centers

Experimental Raman spectra of Na I containing F-centers are described and compared to the calculated spectra, in the off-resonance approximation. The position of the strong resonant mode, almost of A_1g symmetry, is interpreted using a decrease of 67% in the longitudinal force constant between the F-center and its nearest neighbours and of 20% between the nearest and the fourth neighbours.     

IR-spectroscopy of crystals containing Jahn-Teller impurity centers

Using a novel IR-technique, the low-temperature spectra of CdTe, ZnTe and Cd_0.95Mn_0.05Te crystals, doped by Jahn-Teller Co²⁺ impurity ions, were studied in the spectral range from 1 to 400 μm. The absorption spectrum regions and the energy positions of ZPL, corresponding to the transitions ⁴A₂(⁴F)→⁴T₁(P), ⁴T₁(F), ⁴T₂(F) and ²G were detected. The parameters of the spin-orbit interaction λ, the crystal field parameter D_q, the parameters of electron-electron interaction B and C were obtained. It was found, that Jahn-Teller interaction differently affects the parameter λ for ⁴A₂(F)→⁴T₁(F) and ⁴T₂(F) transitions and, consequently, the energy positions of ZPL. The electron-vibronic spectra were compared with phonon spectra.     

Spectra and color centers of strontium titanate crystals

The results of studying the spectral properties of strontium titanate single crystals (grown under different conditions), both pure and activated by iron-group or lanthanide ions, are presented. The data on the main color centers and the formation of systems of ordered crystallites from 10⁻⁷ to 10⁻⁹ m in size on the sample surface after treatment in plasma are obtained by optical and X-ray spectroscopy, cathodoluminescence, electron microscopy, and atomic-force microscopy.     

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.     

Energy transfer among color centers in LiF crystals

In colored LiF crystals there are many absorption and emission bands which cover a wide region of the spectrum from 200 up to 1 300 nm without breaking the continuity. In the frame of these favourable conditions we have performed some experiments of energy transfer among various color centers. The results indicate the existence of efficient exchanges of radiative energy among several bands by using only one exciting wavelength. In particular, the emissions of the F₃ ^- and F₂ ^- color centers centered at 900 and 1 100 nm, respectively, have been observed by pumping at 672 nm completely outside their absorption bands. Received 25 June 2001 and Received in final form 16 July 2001     

Electronic color centers in SrF2-Na crystals

It was shown that optical bleaching of M _A ⁺ color centers at 80 K in SrF₂-Na crystals causes the core of an M _A ⁺ -center to transform into the V _a ⁺ Me ⁺ V _a ⁺ configuration, in which all three point defects are arranged diagnonally in the cube cell. Reirradiation of an optically bleached crystal by x-rays generates F _D centers in it: V _a ⁺ Me ⁺ V _a ⁺ + e ^? → V _a ⁰ Me ⁺ V _a ⁺ ≡ F _D. The F _D → M _A ⁺ transformation in SrF₂-Na crystals proceeds at T = 135 K, in contrast to the F _A → M _A ⁺ transformations, which take place at T > 200 K.     

m-BiVO_4与t-BiVO_4的电子结构和光学性质的第一性原理研究

采用第一性原理的密度泛函理论赝势平面波方法,计算了单斜m-BiVO_4与四方t-BiVO_4的电子结构和光学性质.计算结果表明:m-BiVO_4为间接带隙半导体,禁带宽度为2.171 e V,t-BiVO_4为直接带隙半导体,禁带宽度为2.644 e V;m-BiVO_4与t-BiVO_4均可吸收紫外光及可见光,m-BiVO_4还可以吸收部分红外光.