Effects induced by 𝛄-irradiation on intraband transitions in Sr2+-doped ammonium zinc chloride crystals

Considerable changes were observed in the X-ray diffraction pattern of ammonium zinc chloride after doping with Sr²⁺ in different concentrations and after irradiating with γ rays at different doses. The effect of γ-radiation and Sr²⁺ content on optical parameters of (NH₄)₂ZnCl₄: x Sr²⁺ single crystals (x?=?0.00, 0.020, 0.039, 0.087 or 0.144?wt.%) has been investigated. The transmittance near the absorption edge of unirradiated crystals and crystals irradiated with different γ-doses has been measured, hence the absorption coefficient (α) was calculated. The values of α at room temperature increased under the influence of γ-irradiation. The rate by which α increases with photon energy just before the absorption edge is strongly inhibited by higher γ-doses. The type of intraband transition in (NH₄)₂ZnCl₄: x Sr²⁺ single crystals was found to be of the allowed indirect transition, and γ-irradiation had no effect on the type of transition. The values of the optical energy gap (E _g ) were calculated as a function of γ-dose. The effect of γ-irradiation was found to be more pronounced on samples doped with x?=?0.087 or 0.144?wt.% Sr²⁺. The results can be discussed on the basis of γ-irradiation-induced defects and Sr²⁺ concentration. A diagram representing probable transitions to the created bands due to irradiation could be constructed.     

用正电子湮没研究钙钛矿结构压电陶瓷中的点缺陷

用正电子湮没寿命谱技术对有意掺杂的或被杂质替代的钙钛矿结构压电陶瓷中的点缺陷进行了研究.掺杂了La³⁺的PbTiO₃，掺杂了Sr²⁺，Cd²⁺和La³⁺的Pb(Mg_1/3Nb_2/3)_0.375Ti_0.375Zr_0.25O₃，掺杂了Ca²⁺的BaT 关键词：     

A study of iso- and alio-valent cation doped Ag2SO4 solid electrolyte

2 SO₄. The solid solubility limits up to x≤3 mole% for monovalent, x≤5.27 mole% for divalent and x≤3.63 mole% for trivalent cation doped Ag₂SO₄ are set with XRD, SEM, IR and DSC techniques. A predominant dependence of conductivity on the ionic size of iso- and alio-valent cations is observed. In particular, the conductivity enhances in both α and β phases, despite having a lower ionic-size dopant cation (relative to that of Ag⁺) in the transition element cation doped Ag₂SO₄. Ca²⁺, Ba²⁺, Y³⁺ and Dy³⁺ doped samples show depature from the regular behaviour in the β-phase. The conductivity behaviour is discussed considering ionic size, valence and electronic structure of the guest cations. Received: 3 February 1997/Accepted: 27 May 1997     

Non-steady-state tunnelling recombination in insulating solids,controlled by defect diffusion and rotation

Abstract

The experimental data for KCl—Ag, KBr, NaCl—Ag and KCl—SO₄ crystals as well as Na₂O.3SiO₂—Tb³⁺ glass are summarized which demonstrate the delayed increase (decrease) of the intensity of the thermally stimulated tunnelling recombination of radiation defects after step-wise defrosting (frosting) of their mobility. Both theory and experiment presented here allow to distinguish the cases when the recombination kinetics is controlled by the rotation of an anisotropic defects or their diffusion respectively. It can be achieved studying the non-steady-state stages of kinetics. The effect of relative spatial defect distribution upon the kinetics under study is considered; it is argued that the {F, V _k } recombination in KBr occurs within spatially correlated pairs.     

Diffusion and solubility of SO2-4 in NaCl and KCl single crystals

We describe a new and very accurate method for measuring as a function of temperature the solubility of very slightly soluble salts (≈ 10^-5) in single crystals. The method is applied to the systems NaCl, Na₂SO₄ and KCl, K₂SO₄ and to the preparation of SO^2-₄ doped KCl single crystals. The parameters for diffusion of SO^2-₄ at infinite dilution into NaCl and KCl have been measured in the intrinsic range.     

Ab initio calculations of the geometry and electronic structure of point defects in ferroelectrics with a perovskite structure

Ab initio calculations of the optimized geometry and the electronic structure of lattice defects in incipient perovskite ferroelectrics SrTiO₃ and KTaO₃ are performed in the framework of the density functional theory. The results are presented for the Li⁺ impurity ion at the A site in the KTaO₃ and SrTiO₃ ferroelectrics; the Mn²⁺, Cd²⁺, Ca²⁺, Mg²⁺, and Zn²⁺ ions at the A site and the Mn⁴⁺ and Mg²⁺ ions at the B site in the SrTiO₃ compound; and the MN _Ti ²⁺ -V _O and Mg _Ti ²⁺ -V _O complexes in the SrTiO₃ ferroelectric. The results are obtained by the cluster method with allowance made for the structural relaxation initiated by the defect and, for nonisovalent substitutional impurities, with due regard for the charge and spin states of the defect. It is established that the Ca _Sr ²⁺ , Cd _Sr ²⁺ , Mn _Ti ⁴⁺ , and Mg _Ti ²⁺ ions have a stable central position, whereas the Li _K ⁺ ion in the KTaO₃ compound and the Li _Sr ⁺ , Mn _Sr ²⁺ , and Zn _Sr ²⁺ defects in the SrTiO₃ ferroelectric are off-center ions. The shape of the multiminimum adiabatic potential and the parameters of dielectric relaxators (activation barrier, dipole moment) for polar defects are obtained. The electronic impurity levels are determined for the Li _Sr ⁺ and Mg _Ti ²⁺ neutral defects.     

Crystal field investigations of Yb(C2H5SO4)3. 9H2O and Er: YA1G

Using the spectroscopically derived crystal field parameters for Yb(C₂H₅SO₄)₃. 9H₂O and Er³⁺: YA1G, the temperature dependence of Schottky specific heat, paramagnetic susceptibility, magnetic anisotropy and μ_eff has been calculated over a temperature range 5–400°K. The hyperfine interaction parameters for ¹⁷¹Yb³⁺, ¹⁷³Yb³⁺ and ¹⁶⁷Er³⁺ systems are also obtained and in turn used to estimate the nuclear specific heat. The nice agreement obtained for susceptibility and specific heat of Yb(C₂H₅SO₄)₃. 9H₂O at very low temperatures confirms the accuracy of CEF parameters employed and the neglect of exchange interaction. However, for Er³⁺: YA1G, the CEF parameters are adequate to explain the bulk thermal and magnetic properties but not the g-values.     

四方相BaTiO3缺陷性质的第一性原理计算

采用基于密度泛函理论的第一性原理方法,研究了BaTiO₃在四方相下的各种缺陷性质.计算结果表明,在富氧环境下,钛的中性氧空位、分肖特基缺陷2V^3-_Ti+3V²⁺_O形成能分别为最低;而当体系处在还原环境下时,氧空位逐渐成为主要缺陷,其形成能最低.由于四方相下存在较强的Ti—O键共价杂化,四方相下全肖特基缺陷V^2-_Ba关键词： 缺陷 第一性原理 3')" href="#">BaTiO₃     

Density functional theory study on two-peak emission of Eu activators in Sr2SiO4

The supercells of pure and Eu-doped Sr₂SiO₄ were theoretically analyzed by density functional theory (DFT) calculations to investigate the typical two-peak emission of Sr₂SiO₄:Eu²⁺, which originates from two different Sr²⁺ (or Eu²⁺) sites in the Sr₂SiO₄ host structure. The Perdew-Wang generalized-gradient approximation (GGA) functional and the double numerical plus d-functions (DND) basis set with effective core potentials (ECP) were employed in the calculations of electronic properties. The electron transfer between Eu²⁺ ions placed at two different crystallographic Sr²⁺ sites was understood based on the accurate assignment of deconvoluted peaks of the two-peak emission to their corresponding crystallographic sites. This study ought to be instructive as a basic guideline to improve the color chromaticity of Sr₂SiO₄:Eu²⁺ for use in white light emitting diodes (WLEDs).     

Electrical conduction in undoped and yttrium-doped Na2SO4

The electrical conductivity of undoped and yttrium-doped Na₂SO₄ has been measured by the ac two-terminal method at temperatures between 540 and 1075 K in air or in SO₂O₂SO₃ atmospheres. At high temperatures such as 1075 K, doping of 1.7 at% yttrium (Y) decreases the conductivity, which indicates that the Frenkel defect of sodium ions is pre-dominant in Na₂SO₄ and that the mobility of the interstitial ions is higher than that of the vacancies. However, the vacancy concentration is high enough to increase the conductivity in the samples doped with 2.0–3.8 at% Y. A further addition of yttrium decreases the electrical conductivity again, suggesting the appeareance of defect interactions such as ordering or clustering of the defects. The electrical conductivity of the undoped and the 4 at% Y-doped Na₂SO₄ is independent of the SO₃ partial pressure. The complex impedance analysis shows that the grain boundary resistance increases with sintering time. This is probably caused by a decrease in the contact area between grains during sintering.     

金红石相和锐钛矿相TiO2本征缺陷的第一性原理计算

采用第一性原理的计算方法, 分别研究了金红石相和锐钛矿相TiO₂各种缺陷态形成的类型, 以及几何结构、生长气氛和Fermi能级位置对缺陷形成能的影响, 从理论上预测产生点缺陷的实验条件. 重点是讨论带电点缺陷的形成能, 并对结果进行适当修正. 研究发现, 本征缺陷的类型和浓度对 TiO₂的性能有一定的影响: 在富O条件下, TiO₂容易形成V_Ti(Ti空位)缺陷; 在富Ti条件下, TiO₂的Ti_i⁴⁺和V_O(O空位)缺陷将大量出现, 形成Schottky缺陷.     

Luminescence investigations on sulfate apatite Na6(SO4)2FCl:RE (RE=Dy, Ce or Eu) phosphors

A new phosphor in the Cl-F system doped with Dy, Ce and Eu has been reported. Characterization of this phosphor using XRD, PL and TL techniques is described. Polycrystalline Na₆(SO₄)₂FCl:Dy; Na₆(SO₄)₂FCl:Ce and Na₆(SO₄)₂FCl:Eu phosphors prepared by a solid state diffusion method have been studied for their X-ray diffraction, photoluminescence (PL) and thermoluminescence (TL)characteristics. The PL excitation and emission spectra of phosphors were obtained. Dy³⁺ emission in the host at 475 and 570 nm is observed due to ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transition, respectively, whereas the PL emission spectra of Na₆(SO₄)₂FCl:Ce phosphor shows the Ce³⁺ emission at 322 nm due to 5d→4f transition of Ce³⁺ ion. In Na₆(SO₄)₂FCl:Eu lattice, Eu²⁺ as well as Eu³⁺ emissions are observed. The emission of europium ion in this compound exhibits the blue as well as red emission. The TL glow curves of the same compounds have the simple structure with a prominent peak at 150, 175 and 200 °C. TL response, fading, reusability and trapping parameters of the phosphors are also studied. The TL glow curves of γ-irradiated Na₆(SO₄)₂FCl sample show one glow peak indicating that only one set of traps is being activated within the particular temperature range each with its own value of activation energy (E) and frequency factor (s). The trapping parameters associated with the prominent glow peak are calculated using Chen’s half width method. The release of hole/electron from defect centers at the characteristic trap site initiates the luminescence process in these materials. The intensity of the TL glow peaks increases with increase of the added γ-ray dose to the samples.     

Defect structure of Cu2+ center in K2SO4‒Na2SO4‒ZnSO4 glasses

The spin Hamiltonian parameters (g factors g _//, g _⊥ and hyperfine structure constants A _//, A _⊥) of Cu²⁺ in K₂SO₄?Na₂SO₄?ZnSO₄ glasses are calculated from the high-order perturbation formulas of 3d⁹ ion in tetragonal octahedral sites. The calculated results are in agreement with the observed values. Since the EPR parameters are sensitive to the local structure of a paramagnetic impurity center, the defect structure of Cu²⁺ center in K₂SO₄?Na₂SO₄?ZnSO₄ glasses is estimated. The validity of results is discussed.     

A theoretical study of alkaline-earth cations in crystalline sodium chloride,potassium chloride and potassium bromide

Theoretical methods, based on Mott-Littleton techniques are used to investigate the defect structures of alkali-halide crystals doped with divalent ions. The systems studied are those having Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ as impurity ions in NaCl, KCl and KBr crystals. Our calculations find comparable stability of the (110) nearest-neighbour and (200) next-nearest-neighbour complexes whereas the (211) complex is found to be less stable. Significant trends in the variation of binding energy with dopant ion radius are predicted. We also consider the activation energies for the w₁, w₂, w₃ and w₄ type jumps which occur in the vicinity of the impurity ion. The results are, where possible, compared with experimentally determined defect energies and their implications in diffusion processes of M²⁺ impurities in the alkali halides are discussed.     

Photoelectrochemical properties of FTO/m-BiVO4 electrode in different electrolytes solutions under visible light irradiation

Photoelectrochemical properties of FTO/BiVO₄ electrode were investigated in different electrolytic solutions, potassium chloride (KCl) and sodium sulphate (Na₂SO₄), and under visible light irradiation condition. In order to accomplish that, an FTO/BiVO₄ electrode was built by combining the solution combustion synthesis technique with the dip-coating deposition process. The morphology and structure of the BiVO₄ electrode were investigated through X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. Photoelectrochemical properties were analyzed through chronoamperometry measurements. Results have shown that the FTO/BiVO₄ electrode presents higher electroactivity in the electrolyte Na₂SO₄, leading to better current stabilization, response time, and photoinduced current density, when compared to KCl electrolyte. Besides, this electrode shows excellent performance for methylene blue degradation under visible light irradiation condition. In Na₂SO₄, the electrode has shown higher degradation rate, 51 %, in contrast to 44 % in KCl, plus higher rate constant, 174?×?10^?4 min^?1 compared to 150?×?10^?4 min^?1 in KCl. Results presented in this communication leads to the indication of BiVO₄ thin films as alternate materials to use in heterogeneous photoelectrocatalysis, more specifically in decontamination of surface water.     

Schottky defects in cubic lattice of SrTiO3

Structural and electronic properties produced by formation of Schottky defects in cubic structure of SrTiO₃ crystal are investigated by means of a quantum-chemical simulation based on the Hartree-Fock methodology. The occurrence of Sr partial Schottky defect (V_Sr+V_O) and two types of Ti partial Schottky defects (V_Ti+2V_O) is modeled using a supercell containing 135 atoms. Vacancy-induced changes in the positions of their neighboring atoms are analyzed in light of the computed electron density redistribution in the defective region of supercell. The observed local one-electron energy levels in the gap between the upper valence band and the conduction band can be attributed to the presence of anion and cation vacancies.     

Luminescent properties of Sr2.5−3x/2Ba0.5SmxAlO4F oxyfluorides

Effective orange Sm³⁺-doped Sr_2.5Ba_0.5AlO₄F phosphors excited at 254 and 408 nm excitation were prepared by the solid-state method. The excitation and emission spectra of Sr_2.5?3x/2Ba_0.5Sm_xAlO₄F and Sr_2.5?3x/2Ba_0.5Sm_xAlO_4?αF_1?δ (x=0.001～0.1) based on photoluminescence spectroscopy are investigated. The defects in anion-deficient Sr_2.5?3x/2Ba_0.5Sm_xAlO_4?αF_1?δ (x=0.001, 0.01) are monitored by broad-band photoluminescence emission centered near 480 nm along with the orange emission transitions of Sm³⁺. CIE values and relative luminescent intensities of Sr_2.5?3x/2Ba_0.5Sm_xAlO₄F and Sr_2.5?3x/2Ba_0.5Sm_xAlO_4?αF_1?δ by changing the Sm³⁺ content (x=0.001～0.1) are discussed.     

Effect of strontium additive on thermoluminescence properties of (Ba1-xSrxSO4)99.8%: Eu0.2% nanophosphor

Nanophosphors of barium strontium sulfate complex (Ba_1-xSr_xSO₄)_99.8%:Eu_0.2% (0 ≤ x ≤ 1) were prepared through the chemical co-precipitation method at room temperature. Precipitated samples were characterized using X-ray diffraction (XRD), dynamic light scattering (DLS) and high resolution transmission electron microscope (HRTEM) techniques. The obtained XRD patterns from the prepared nanophosphate series (Ba_1-xSr_xSO₄)_99.8%:Eu_0.2% exhibit an orthorhombic structure with semispherical particle shape. The lattice parameters of (Ba_1-xSr_xSO₄)_99.8%:Eu_0.2% solid crystals change and the cell volume decreases with the increase of x value of strontium. The thermoluminescence (TL) glow curves induced by gamma rays of (Ba_1-xSr_xSO₄)_99.8%:Eu_0.2% series were recorded and compared. The substitution of Ba²⁺ by Sr²⁺ cations shift the trap centers in the host of (Ba_1-xSr_xSO₄)_99.8%:Eu_0.2% material to the higher temperature side. The TL glow curve (GC) of sample with x = 0.12, with grain size ranging between 13–31 nm, reveals that it has deep trap centers, and higher TL sensitivity. The different heating rates effect of the glow peaks of samples with x = 0, 0.12 and 1 showed that they follow the first-order kinetics. These samples have been studied and analyzed with the help of both T_stop experimental method, and the computerized glow curve deconvolution (CGCD) program. T_m–T_stop experiment indicates that there are three trapping levels in both (BaSO₄)_99.8%:Eu_0.2% and (SrSO₄)_99.8%:Eu_0.2% sulfate samples at 452, 489, 543 K and 487, 513, 530 K respectively, while five peaks at 458, 486, 499, 544 and 556 K in the complex GC of (Ba_0.88Sr_0.12SO₄)_99.8%:Eu_0.2%. These values are used as input for CGCD. The figure of merit (FOM) during fitting procedures is determined.     

Growth of BaxSr1−xSO4 nano-steps on barite (0 0 1) face

Crystal growth has been promoted in the fluid cell of an Atomic Force Microscope (AFM) by passing Ba-Sr-SO₄ aqueous solutions over barite (0 0 1) cleavage surfaces. Steps advance in structural continuity with the original barite (0 0 1) surfaces and two-dimensional nucleation occurs preferentially on the newly-formed terraces. The terraces are, on average, 7.5% lower than pure barite terraces. Since the ionic radius of Sr²⁺ is smaller that the ionic radius of Ba²⁺, the reduction of terrace height is consistent with an extensive incorporation of Sr²⁺ into the barite structure. Therefore, it can be considered that the newly-formed terraces have compositions corresponding to terms of the Ba_xSr_1−xSO₄ solid solution. A non-linear dependence of step rate on [SrSO₄] concentration in the solution (and therefore on supersaturation) has been found. The growth behaviour has been discussed by considering both the physicochemical properties of the Ba_xSr_1−xSO₄ solid solution-aqueous solution (SS-AS) system and a kinetic-based step growth model.     

γ-irradiation effect on the spectroscopic and electrical properties of K2SO4—Na2SO4 mixed system

Abstract

Measurements of electron paramagnetic resonance, infrared and electrical properties were carried out for the K₂SO₄—Na₂SO₄ mixed system before and after γ-irradiation. EPR measurements revealed the presence of a quartet of lines characterized by an isotropic g-value of 2.0034. These lines are mainly attributed to the formation of a SO^? ₃ center which results from the interaction of γ-rays with the sulfate ion. A decrease in the absorption intensity of the Infrared radiation was observed after γ-irradiation due to radiation damage in the sulfate group. The electrical conductivity, σ, was measured for the K₂SO₄—Na₂SO₄ system before and after γ-irradiation in the temperature range from 30 up to 430°C. A considerable decrease in the conductivity value accompanied by an increase in the activation energy was observed after γ-irradiation. The energy of formation of Frenkel defects was estimated to be 2.94eV. The current-voltage characteristics were measured at different temperatures in order to estimate the type of conduction in the samples. Isothermal annealing kinetics was investigated at different temperatures before and after γ-irradiation. The electrical conductivity decreases with increasing time of annealing and the annealing process is dominated by a unique rate process.