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1.
The absorption and MCD spectra of the 3A2g1Eg transition of CdI2:Ni2+ at about 12,500 cm?1 have been measured. The unusual vibronic fine structure is explained by the coupling of the phonons of the CdI2 lattice to the electronic states of Ni2+. The dispersion curves for the acoustical and optical branches in the Brillouin Zone of CdI2 are deduced.  相似文献   

2.
《Journal of luminescence》1987,37(3):123-131
We have investigated the luminescence and absorption spectra of doped and undoped ZrO2-Y2O3 and MgO crystals at room- and low temperatures. The crystals used are partly doped with the transition metals Ni, Co, Cr and the rare earth Pr. The emission spectra were obtained under laser excitation at different wavelengths. The observed optical emission and absorption bands of the MgO crystals doped with Ni, Co and Cr correspond to transitions between spin-orbit split crystal field levels of the transition metals. Luminescence and absorption bands of undoped yttria-stabilized zirconia (YSZ) crystals are due to color centers, absorption bands of the doped YSZ correspond to the well known transitions of the Ni2+, Co2+ and Pr3+ ions, respectively. The emission spectra of the doped YSZ obtained under various laser excitations can be explained by an energy transfer process between the color center and the doping materials. The influence of annealing on the absorption and emission of Pr3+/Pr4+ is investigated.  相似文献   

3.
Results of comprehensive research into optical and luminescent-kinetic characteristics of europium-doped cadmium iodide crystals excited by nitrogen laser radiation, α-particles, and x-rays are presented. Crystals under study have been grown by the Bridgman–Stockbarger method. The doping EuCl3 admixture was introduced into the charge in quantities of about 0.05 and 1.0 mol%. Impurity absorption detected in the near-edge region of the crystals is interpreted as part of the Eu2+ ion long-wavelength band associated with fd-transitions. The cation impurity and matrix defects in CdI2:Eu2+ crystals create complex centers responsible for emission with a maximum in the 580–600-nm region. The short component in the luminescence decay kinetics of weakly-doped crystal excited by α-particles and x-ray photons is due to the exciton emission characteristic of CdI2. The slow component in the scintillation pulse results from recombination of charge carriers followed by creation of exciton-like states on the defect-impurity centers. Laser or x-ray excitation induces light-sum accumulation on the trapping levels at a depth of 0.2–0.6 eV that is mainly related to matrix microdefects. Trapping centers associated with the chlorine impurity are observed in the heavily-doped crystal. Photostimulated luminescence at 85 K arising at the electron stage of the recombination process is caused by recombination of electrons released from F-type centers with holes localized near the activator. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 358–364, May–June, 2009.  相似文献   

4.
In this paper we demonstrate that two independent methods of calculations (DFT based ab initio and semi-empirical crystal field theory) can be used to form a complementary picture of the optical and electronic properties of the doped host and impurity ion. The crystals considered in the present paper are: (i) YAlO3:Ce3+ and (ii) two dominant phases of TiO2—rutile and anatase. As an example, detailed calculations of the band structure and crystal field energy level scheme of YAlO3:Ce3+ are reported. From the analysis of the band structure and density of states, the character of the YAlO3 energetic bands and positions of the Ce impurity energy levels were established. It was also shown how the ab initio methods can be used for calculations of the structural properties of solids under elevated pressure. Taking the two dominant phases of TiO2 as an example, it was demonstrated how the elastic properties can be extracted from the calculated unit cell’s volume at different pressures. Particular attention was paid to the microscopic effects of crystal field, which were evidenced by the pressure-induced changes of the structure and shape of distribution of the Ti 3d electrons density of states. It was demonstrated how the difference in crystal structure of the anatase and rutile phases leads to remarkable difference in microscopic crystal field effects, which was explained by different Ti-O distances in both phases. In addition, the pressure dependence of the band gaps for anatase and rutile was investigated. It was shown that the hydrostatic pressure leads to the band gap narrowing in anatase and band gap widening in rutile, with pressure coefficients +0.00681 eV/GPa for rutile and −0.0088 eV/GPa for anatase.  相似文献   

5.
We analyze the optical characteristics of crystals within the framework of a model representation that presupposes the entry of the background impurity of iron ions in two valence states, Fe3+ and Fe2+, into regular lattice nodes. The spectral manifestation of intrinsic defects of vacancy-type structure depending on the conditions of crystal growth and heat treatment is comprehensively investigated. With allowance for the specific features of the technology applied, we have tested the technique suggested in scientific periodicals for bleaching ZnWO4 and CdWO4 crystals by doping with nonisovalent ions, in particular, with Sb and Ag impurities. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 211–216, March–April, 2000. The work was carried out with support from INTAS-UKRAINE within the framework of the project INTAS-UA-95-0166.  相似文献   

6.
It has been shown using atomic-force microscopy that the PbI2 impurity is embedded in the CdI2 crystal lattice in the form of nanocrystalline inclusions. The model of a high-energy cation exciton related to the 3 P 2 state of a free Pb2+ ion has been considered for the impurity absorption (excitation) band at 3.23 eV. The resonance narrow photoluminescence bands with the split absorption band at 3.12 and 3.20 eV have been compared with the emission of a free Frenkel exciton. It has been demonstrated that, in the temperature range 25–45 K, there arises a self-trapped exciton state, and the main role in its formation is played by the bending vibrations of the CdI2 crystal lattice. The potential barrier separating the self-trapped state from the free exciton is 23 meV. The photoluminescence band at 2.4 eV is assigned to the emission of the self-trapped high-energy cation exciton of PbI2 in the CdI2 crystal lattice.  相似文献   

7.
The local environment and the charge state of a nickel impurity in cubic Ba0.8Sr0.2TiO3 are studied by XAFS spectroscopy. According to the XANES data, the mean Ni charge state is ~2.5+. An analysis of the EXAFS spectra and their comparison with the results of first-principle calculations of the defect geometry suggest that Ni2+ ions are in a high-spin state at the B sites of the perovskite structure and the difference of charges of Ni2+ and Ti4+ is mainly compensated by distant oxygen vacancies. In addition, a considerable amount of nickel in the sample is in a second phase BaNiO3 ? δ. The measurements of the lattice parameter show a decrease in the unit cell volume upon doping, which can indicate the existence of a small amount of Ni4+ ions at the B site.  相似文献   

8.
Present study reports the structural, optical and dielectric properties of Ni substituted NdFe1−xNixO3 (0 ≤ x ≤ 0.5) compounds prepared through the ceramic method. X-ray diffraction (XRD) confirmed an orthorhombic crystal structure of all the samples. Both unit cell volume and grain size were found to decrease with an increase in Ni concentration. Morphological study by Scanning electron microscope (SEM) shows less porosity with Ni substitution in present system. From UV–vis spectroscopy, the optical band gap was found to increase with Ni doping. This observed behavior was explained on the basis of reduction in crystallite size, unit cell volume and its impact on the crystal field potential of the system after Ni substitution. The dielectric properties (?′ and tanδ) as a function of frequency or temperature, and the ac electrical conductivity (σac) as a function of frequency have been studied. Hopping of charge carriers between Fe2+ → Fe3+ ions and Ni2+ → Ni3+ ions are held responsible for both electrical and dielectric dispersion in the system. Wide optical band gap and a very high dielectric constant of these materials promote them to be a suitable candidate for memory based devices in electronic industry.  相似文献   

9.
应用晶体场理论和不可约张量算符方法构造了3d2/3d8态离子在C3v对称晶场中包含自旋-轨道相互作用、自旋-自旋相互作用、自旋-其它轨道相互作用和其它轨道-其它轨道相互作用四种微观磁效应的45阶可完全对角化的能量哈密顿矩阵.利用该矩阵,计算了V3+∶α-Al2O3和Ni2+∶α-Al2O3晶体的光谱精细结构、晶体局域结构和零场分裂参量,研究了掺入两种互补态离子Ni2+和V3+对同种晶体的光谱精细结构、晶体局域结构和零场分裂参量的影响,理论计算值和实验值相符.研究发现:掺杂没有改变晶体的光谱精细结构和能级分裂条数,但改变了能级间距|掺杂也没有改变晶体的对称性,但使晶体局域结构发生了一定程度的畸变| Ni2+∶α-Al2O3晶体局域结构的伸长畸变量大于V3+∶α-Al2O3晶体,键角的变化量小于V3+∶α-Al2O3晶体.  相似文献   

10.
The optical absorption and EPR spectra of Ni2+-VAg centres in the AgCl:Ni2+ and AgBr:Ni2+ systems have been investigated theoretically on the basis of the complete energy matrices including the electron–electron repulsion interaction, the ligand field interaction, the spin–orbit coupling interaction, and Zeeman interaction. Because the charge compensation forms a silver ion vacancy (VAg) which makes the attractive force acted on the each ligand ion different, it was determined that the Ni–X (X = Cl, Br) distance next to VAg is shorter than others for both AgCl:Ni2+ and AgBr:Ni2+ systems in the tetragonal symmetry. Besides, it was found that the local lattice structure of (NiX6)4? clusters in AgCl and AgBr crystals exhibit a compression distortion. This compression distortion may be ascribed to the fact that the Ni2+ ion has a smaller ionic radius and more effective charge than the Ag+ ion.  相似文献   

11.
Currents of thermogradient polarization and depolarization of an electret state that arises in a photochromic crystal CdI2: Ag during one-side cooling of the sample in the dark from 325 to 90 K in the presence of a temperature gradient directed along the crystallographic axis C 6 have been found. At 90 K, the crystal polarized in the thermogradient electromotive force field is characterized by the photosensitivity in the near-edge, impurity, and infrared spectral regions. It is revealed that the electret state in the CdI2: Ag crystal is also formed at room temperature during photolysis under irradiation of the samples by integrated light from a xenon lamp. Models of photosensitive centers formed upon doping of the CdI2 crystal from the melt by the Ag+ impurity and during the occurrence of thermo- and photostimulated chemical reactions are proposed. The mechanism of the photochromic effect, including the change in the charge state of silver impurity ions, is considered.  相似文献   

12.
In this paper we presented structural and spectroscopic study of zinc silicate powder samples doped with divalent nickel and cobalt ions. Results of the Rietveld structural refinement, combined with optical spectroscopic study and theoretical crystal field calculations, are presented and discussed. X-ray diffraction studies were performed to establish reliable structure of the doped samples; in this way the interionic distances and chemical bond angles in Zn2SiO4:Co2+ and Zn2SiO4:Ni2+ were calculated and are reported for the first time. The room temperature reflection spectra of the prepared samples were measured in a spectral region from 4000 to 50000 cm?1. The exchange charge model of crystal field has been applied to analyze the experimental spectra and assign all observed details in the spectra to the corresponding electronic transitions between the Co2+ and Ni2+ crystal field energy levels. The only input information for the model calculation was the experimentally obtained structural data, which were used for the calculations of the crystal field parameters with subsequent diagonalization of the crystal field Hamiltonian for both ions. Agreement between the calculated and experimentally detected energy levels of impurity ions was good. On the basis of the crystallographic and crystal field studies it was established that there exists a systematic trend of preferential occupation of one out of two possible crystallographic sites (namely, Zn2) for both impurity ions.  相似文献   

13.
CaF2 crystals doped with Yb3+ ions have been studied by electron paramagnetic resonance (EPR) and optical spectroscopy. EPR spectra of paramagnetic centers (PCs) for cubic (Tc) and tetragonal (Ttet) symmetries were identified. Empirical energy level diagrams were established and crystal field parameters were determined. Information on the CaF2∶Yb3+ phonon spectra was obtained from the electron-vibrational structure of the optical spectra. The crystal field parameters were used to analyze the crystal lattice distortions in the vicinity of the Yb3+ ion. Within the framework of a superposition model, it is established that four F ions located symmetrically with respect to the fourfold axis from the side of the ion-compensator approach the impurity ion and deviate from the PC axis. The second set of four fluorine ions also approaches the Yb3+ ion and the PC axis. The ion-compensator F also approaches considerably the impurity ion.  相似文献   

14.
Based on the density functional pseudopotential method, the electronic structures and the optical properties of CdI2 doped with Cu are investigated in detail. The calculation results indicate that the defect of Cu(Cd) exists steadily with a certain solubility. For the Cu doped CdI2, the new highly localized impurity bands induced by Cu 3d states lie just across the Fermi energy at the top of the valence band. The doping of Cu induces reduction of band gap of CdI2; red shifts are revealed in both the imaginary part of dielectric function and the absorption spectra corresponding to the change in band gaps. Moreover, the study of the reflection spectrum and the loss function shows that the doped Cu is responsible for the increased reflection peak intensity and the red shift of the plasma resonant frequency of CdI2.  相似文献   

15.
The experimental and theoretical studies of the optical properties of pure α-ZnAl2S4 and α-ZnAl2S4:V3+ crystals were carried out. The ab initio and crystal field calculations of the structural and optical properties of α-ZnAl2S4:V3+ were compared with the corresponding experimental data. It was shown that the lowest vanadium 3d states are located at about 1.36 eV above the valence band's top. The complete energy level scheme of the α-ZnAl2S4:V3+ system, which includes the host's electronic band structure and impurity ion's energy levels, was suggested on the basis of the performed calculations.  相似文献   

16.
The local crystal structures and electronic structures of LiMxFe1-xPO4 (M = Co, Ni, Rh) are studied through first-principles calculations. The lattice constants and unit cell volumes are smaller for the Co and Ni doped materials than for pure LiFePO4, while larger than for the Rh doped material. The local structures around M atoms in the doped materials are studied in details. The total density of states (DOS) and atomic projected DOS (PDOS) are all calculated and analysed in detail. The results give a reasonable prediction to the improvement of electronic conductivity through Fe-site doping in LiFePO4 material.  相似文献   

17.
A detailed analysis of the energy level structure of the six-fold coordinated Cr3+ ion in the chromium oxide Cr2O3 is performed using the exchange charge model of the crystal field theory. Parameters of the crystal field acting on the Cr3+ optical electrons are calculated from the crystal structure data for the [CrO6]9− impurity center. The energy levels obtained are compared with the experimental absorption spectra for the considered crystal; a good agreement with experimental data is demonstrated. One possible explanation for the ultraviolet p1 absorption band is proposed based on the results of crystal field calculations.  相似文献   

18.
Analysis of the energy-level scheme and absorption spectrum of the Ni2+ ion in MgAl2O4 was performed. The recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multi-electron (DV-ME) method [K. Ogasawara, et al., Phys. Rev. B 64 (2001) 115413) was used in the calculations. The method is based on the numerical solution of the Dirac equation; no phenomenological parameters are used in the calculations. As a result, complete energy-level scheme of Ni2+ and its absorption spectra were calculated, assigned and compared with experimental data on the ground and excited state absorption spectra. Numerical contributions of all possible electron configurations into the calculated energy states were determined. By performing analysis of the molecular orbitals population, numerical contributions of the oxygen 2p- and 2s-orbitals into the 3d molecular orbitals were determined.  相似文献   

19.
Recently ultrabroadband infrared solid state lasers based on a new vibronic material Cr2+:ZnSe x S1–x were demonstrated [1–3]. Cr2+ ion substitutes the metal ion (tetrahedral sites), the crystal field of the solid solution is responsible for large inhomogeneous broadening of Cr2+ electron states. The crystal field can be reconstructed by investigation of lattice dynamics — optical phonon parameters and dielectric function in IR. We paid special attention to investigation of vibrational and infrared spectroscopic properties of ZnSe x S1 ? x crystals. A very interesting and somewhat unexpected result of these studies was the existence in the crystals of effective S-Se dipoles, which generate an additional deep dynamically charged level in the forbidden gap of the semiconductors. The results of the first-principles calculations of both the phonon structure and the electron localization in ZnSe x S1–x crystals as well as acceptor levels in Cr2+: ZnSe crystal are discussed.  相似文献   

20.
The optical absorption spectra (d-d transition bands) and covalent effect of Ni2+ ions in octahedral sites of Ca3Sc2Ge3O12 crystal have been investigated by the full energy matrix based on the two spin–orbit coupling parameters model. The bond length of octahedral site is Ri?=?2.19 Å, which can be determined by the cubic crystal-field parameter and optical spectral data. The lattice distortion of the Ni2+ center in Ca3Sc2Ge3O12 crystal is also obtained from the calculations. In addition, the result has shown that the covalent effect of Ni2+ ion in the octahedral site of Ga3Sc2Ge3O12 is obvious and cannot be ignored. The calculated d-d transition bands agree well with that of the experimental findings, suggesting that the present methods can explain reasonably the optical spectral data and covalent effect of 3d8 ions in octahedral lattices.  相似文献   

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