Modes of vibrations due to (U-center + additive anion impurities) in KC1—II

Defect modes due to U-centers in potassium chloride containing additive anion impurities have been computed by Green's functions techniques. Local vibrations due to U-centers in alkali halides have cubic symmetry, but when one of the twelve nearest neighbour anions is replaced by an additive impurity, the site symmetry of the system reduces from O_h to C_2v and gives rise to new vibrational modes. The phonon Green's functions have been analysed according to the irreducible representation of the point group symmetry, pertaining to the substitutional impurity. We have considered the vibrations of the U-center, additive anion impurity and their nearest neighbours i.e. 36-dimensional defect space. Using group theory, symmetry coordinates were constructed and the 36 x 36 dimensional matrix was block diagonalised into various irreducible representations. The computed local mode frequencies have been conpared with the experimental measurements. Reasonably good agreement is found between them. The computed results for resonant mode frequencies are also displayed.     

Modes of vibrations due to (U-center + additive cation impurities) in alkali halides

Local and resonant modes due to hydride ions in various alkali halides containing additive cation impurities have been computed by the Green function technique. Local vibrations due to U-centers in alkali halides have O_h symmetry. When one of the six nearest neighbour cations is replaced by an additive impurity, the site symmetry of the system is lowered from O_h to C_4v. The phonon Green functions matríx is analysed according to the irreducible representation of the point group symmetry pertaining to the substitutional impurity. We have considered the vibrations of the hydride ion and all its six nearest neighbours. Analytical expressions have been derived for various modes of vibrations. Using group theory the 21 × 21 matrix has been block diagonalized into various irreducible representations. The effect of mass changes and the changes in short-range force constants have been taken into account. The computed results of the localized modes have been compared with the available experimental results. Good agreement has been found. Theoretical results on resonant modes are also displayed, which will be of use in future experiments on these systems.     

Local vibrations in fcc crystals with two-parametric substitutional impurities

The threshold parameters of defects (the mass defect and the relative change in the force constants) are determined at which local vibrations start to occur in an fcc crystal with substitutional impurities. The characteristics of local vibrations are investigated, and the influence of the defect parameters on the frequency of local vibrations and their decay rate with distance from the impurity atom is analyzed. The frequencies and the intensities of local vibrations are calculated for the nearest neighboring atoms of an impurity, which, combined with the impurity atom, form a defect cluster.     

Substitutional impurity in the graphene quantum dots

The process of formation of the localized defect states due to substitutional impurity in sp²-bonded graphene quantum dot is considered using a simple tight-binding-type calculation. We took into account the interaction of the quantum dot atoms surrounding the substitutional impurity from the second row of elements. To saturate the external dangling sp² orbitals of the carbon additionally 18 hydrogen atoms were introduced. The chemical formula of the quantum dot is H₁₈C₅₁X, where X is the symbol of substitutional atom. The position of the localized levels is determined relative to the host-atoms (C) ε_p energies. We focused on the effect of substitutional doping by the B, N and O on the eigenstate energies and on the total energy change of the graphene dots including for O the effect of lattice distorsion. We conclude that B, N, and O can form stable substitutional defects in graphene quantum dot.     

Dynamique de reseau et frequences des modes de vibration pour des impuretes substitutionnelles dans InP,GaP et ZnS

The model used is a rigid-ion model with an effective ionic charge including general interactions for nearest and next nearest neighbours and long range Coulomb interactions. It provides a good fit with available neutron data and with IR absorption results for InP. In this model, no hypothesis is made apriori on the interatomic forces and the eleven parameters given by the model are used. A mathematical model which employs a Green's function technique in the mass defect and the nearest neighbour force constant defect approximation is used to calculate the lattice dynamics of the imperfect crystal. The frequencies of the local modes, the gap modes and the band modes, are given for isolated substitutional impurities. The same calculation is achieved for GaP and ZnS and the results are compared with IR data.     

Estimation of the recoil-free fraction of a57Fe atom present as a dilute impurity in copper,gold and palladium

The temperature variation of the recoil-free fraction of the Mössbauer⁵⁷Fe atom present as a dilute impurity in metallic hosts of various masses has been estimated by employing the double-time Green function technique at the high temperature limit. The changes in the force constants due to the⁵⁷Fe atom present as a substitutional impurity in Cu, Au and Pd have been evaluated from the available calculated values of hostfrequency moments and effective host-impurity force constant ratios using neutron dispersion data to estimate theoretically the recoil-free fraction for the resonant nucleus.     

Effect of non-magnetic impurities on the observation of superconducting gap excitations by raman scattering

The effect of small concentrations of non-magnetic substitutional impurities (characterized by changes in mass, nearest neighbour force constants and local electron-phonon interaction) on the observability of superconducing (sc) gap excitations in 2H-NbSe₂ by Raman scattering is considered within the theoretical framework of Balseiro and Falicov (bf). The phonon self-energy when evaluated forq=0 using the Einstein oscillator model for the host phonons is seen to have a pole corresponding to the impurity mode besides the gap excitation one around 2Δ previously obtained bybf. A splitting of the impurity mode into two in thesc state is predicted. An enhancement occurs in the strength of thesc gap excitation peak whenever it is close to the impurity mode. The data from the experiments of Sooryakumar, Klein and Frindt on impure samples of 2H-NbSe₂ are discussed in the light of the present calculation. Preliminary results of this paper were presented by one of us (snb) at the International Conference on Phonon Physics at Indiana University, Bloomington, USA, in August-September 1981.     

Infrared absorption of various isolated impurities in GaAs

We have calculated the infrared absorption at the local mode frequency for various isolated impurities such as B, Al, Mg, Si, C, and P in GaAs. To calculate the local mode frequency we take into account the effects from impurity mass defect and local force constant shifts and use the symmetry properties of the lattice. To calculate the absorption we consider contributions to the dipole moment from impurity and nearest neighbour charges and from charge migration effects. We suggest a physical model which accounts for impurity and nearest neighbour charges and polarizabilities.     

Features of Electronic Structure of Intermetallic Compounds CeNi<Subscript>4</Subscript>M (M = Fe,Co, Ni,Cu)

The evolution of the electronic structure of CeNi₄M (M = Fe, Co, Ni, Cu) intermetallics depending on the type of nickel substitutional impurity is explored. We have calculated band structures of these compounds and considered options of substituting one atom in nickel 3d sublattice in both types of crystallographic positions: 2c and 3g. The analysis of total energy self-consistent calculations has shown that positions of 2c type are more energetically advantageous for single iron and cobalt impurities, whereas a position of 3g type is better for a copper impurity. The Cu substitutional impurity does not change either the nonmagnetic state of ions or the total density at the Fermi level states. Fe and Co impurities, on the contrary, due to their considerable magnetic moments, induce magnetization of 3d states of nickel and cause significant changes in the electronic state density at the Fermi level.     

Electronic structure of BN nanotubes with intrinsic defects NB and BN and isoelectronic substitutional impurities PN, AsN, SbN, InB, GaB, and AlB

The effect of intrinsic defects and isoelectronic substitutional impurities on the electronic structure of boron-nitride (BN) nanotubes is investigated using a linearized augmented cylindrical wave method and the local density functional and muffin-tin approximations for the electron potential. In this method, the electronic spectrum of a system is governed by a free movement of electrons in the interatomic space between cylindrical barriers and by a scattering of electrons from the atomic centers. Nanotubes with extended defects of substitution N_B of a boron atom by a nitrogen atom and, vice versa, nitrogen by boron B_N with one defect per one, two, and three unit cells are considered. It is shown that the presence of such defects significantly affects the band structure of the BN nanotubes. A defect band π(B, N) is formed in the optical gap, which reduces the width of the gap. The presence of impurities also affects the valence band: the widths of s, sp, and p_π bands change and the gap between s and sp bands is partially filled. A partial substitution of the N by P atoms leads to a decrease in the energy gap, to a separation of the Ds(P) band from the high-energy region of the s(B, N) band, as well as to the formation of the impurity Dπ(P) and Dπ*(P) bands, which form the valence-band top and conduction-band bottom in the doped system. The influence of partial substitution of N atoms by the As atom on the electronic structure of BN nanotubes is qualitatively similar to the case of phosphorus, but the optical gap becomes smaller. The optical gap of the BN tubule is virtually closed due to the effect of one Sb atom impurity per translational unit cell, in contrast to the weak indium-induced perturbation of the band structure of the BN nanotube. Introduction of the one In, Ga or Al atom per three unit cells of the (5, 5) BN nanotube results in 0.6 eV increase of the optical gap. The above effects can be detected by optical and photoelectron spectroscopy methods, as well as by measuring electrical properties of the pure and doped BN nanotubes. They can be used to design electronic devices based on BN nanotubes.     

Metastable <Emphasis Type="Italic">X</Emphasis> centers in cadmium telluride single crystals

A model is proposed for metastable DX centers formed through Jahn-Teller distortion of the crystal lattice of cadmium telluride, i.e., through the displacement of a D _Cd residual donor impurity atom (where D is a Group III element of the periodic table) to the region of a nearest neighbor interstice. The configuration-coor-dinate diagram for a V _Cd-D _i associated defect is constructed with due regard for the tetrahedral and hexagonal positions of interstitial atoms. The Stokes shift, n-type conductivity, location of the Fermi level, specific features of photoluminescence, and some other effects are explained in terms of the configuration-coordinate diagram. The results of experimental investigations of the energy spectrum of DX centers in cadmium telluride single crystals are in agreement with available theoretical data.     

Local magnetic ordering of Fe impurities in Pd2MnSn

Mössbauer effect of Fe⁵⁷ embedded as very dilute substitutional impurities in Pd₂MnSn was studied. The impurities are seen to replace the three elements in the alloy. Although the Curie temperature of the alloy is 189K, well below the room temperature, the Mössbauer spectrum recorded at room temperature consisted of two distinct 6-finger magnetic hyperfine spectra and a single unsplit line. One of the 6-finger patterns which corresponds to an internal magnetic field ofH _int=?375 kOe is inferred to arise due to local magnetic coupling of the localized magnetic moments of Fe impurities at the Pd sites with those of the 4 Mn first nearest neighbours of the Fe impurities. The other 6-finger pattern which corresponds to an internal magnetic field ofH _int=?335 kOe is inferred to arise due to the local magnetic coupling of the localized magnetic moments of the Fe impurities at the Sn sites with those of the 6 Mn second nearest neighboours of the Fe impurities. The difference in the internal magnetic fields observed at the Pd and Sn sites in the alloy could be understood qualitatively, on the basis of RKKY theory, as arising due to the different conduction electron polarization contributions to the net internal magnetic field at the Fe impurity sites. The results of the measurements suggest that the localized magnetic moments of Fe⁵⁷ impurities at Pd and Sn sites are antiferromagnetically coupled with the moments of their neighbouring Mn atoms.     

Electronic structure of alkali-metal impurities in InP

The electronic structure of the substitutional alkali metal impurities in InP has been studied by MSX_αSCF method. The shift of the t^DBH₂-level for different impurities is determined. In none of the cases the impurity level is shifted out of the band gap. Li and Na impurities with their shallow acceptor levels behave as p-type semiconductors whereas K impurity with its deep level behave somewhat like a semi-insulator.     

Impurity–defect emission from undoped Cd<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Zn<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>Te single crystals near the fundamental absorption edge

Shallow impurity–defect states in undoped Cd_1–xZn_xTe (x ～ 3–6%) single crystals have been studied using low-temperature photoluminescence measurements. It has been found that the effect exerted by zinc is mainly reduced to a rigid shift of all the specific features associated with the exciton radiation, which made it possible, with a high (～0.3 meV) accuracy, to measure the band gap and the zinc concentration in solid solutions. Hydrogen-like donors with the ground-state energy of ～14 meV and four types of acceptors with average activation energies of 59.3 ± 0.6 meV, 69.6 ± 1.5 meV, 155.8 ± 2.0 meV, and 52.3 ± 0.6 meV have been identified in all the crystals studied. Based on a comparison with the results of the analysis of the impurity background and the data available in the literature on impurity–defect emission in undoped CdTe, the first three acceptors can be assigned to the substitutional impurities Na_Cd, P_Te, and Cu_Cd, respectively. The most shallow acceptor (52.3 ± 0.6 meV) is a complex defect in which there is a nonstandard excited level separated by only 7 meV from the ground level. This level is formed apparently due to the removal of degeneracy, which is characteristic of T_D acceptors, by the low-symmetry potential of the complex defect.     

Electronic structure of substitutional nitrogen impurity in TiO 2 studied by the β-NMR method

In order to study the local electronic structure of nitrogen impurity in rutile TiO₂, we have measured double-quantum NMR spectra of short lived β-emitter ¹²N(I = 1, T _1/2 = 11 ms) implanted into a rutile single crystal by means of the β-NMR technique. The resonance line obtained at room temperature is well accounted for by the second order shift due to the quadrupole interaction at the oxygen substitutional site. The spectrum at 25 K has shown the other lines than the central diamagnetic line shifted by 10?15 kHz to both sides, which has been already shown in the previous data obtained with a different crystal orientation and an external field. The present results supports the existence of a paramagnetic state formed by the substitutional nitrogen impurities.     

Hyperfine interactions of111Cd in ion-implanted face centered cubic cobalt

The magnetic-dipole and electric-quadrupole hyperfine interactions of¹¹¹Cd probes in fcc cobalt after implantation of radioactive¹¹¹In⁺ ions have been investigated by PAC measurements with fast BaF₂ detectors. Six different sites of the probe atoms could be distinguished and characterized by their hyperfine parameters and annealing behaviour. Besides the substitutional site, three sites are assigned to In-vacancy complexes which are formed athermally in the implantation process or by thermally-activated trapping of lattice defects in annealing stage III. The remaining two sites are attributed to In located in stacking faults or hcp regions of the host. Systematic trends of impurity hyperfine fields in defect sites become evident from a comparison with other impurity-host combinations. The temperature dependence of the magnetic hyperfine fields has been measured between 20 K and 390 K. Large differences found for the various sites are discussed.     

The influence of substitutional imperfections on the electronic structure of refractory compounds. SP-element impurities in titanium nitride

Using the SCF MO LCAO (self-consistent MO LCAO method in the Mulliken-Wolfsbergelmholz approximation) cluster method and the Green's function method in the ASA-KKR (Korringa-Kohn-Rostoker method in the atomic spheres) approximation, the effect of substitutional impurities (O, C, B, Be, He) on the electronic structure of titanium nitride has been studied. The distributions of the local electronic states of impurity centers and the location of impurity levels in the total energy spectrum of the crystal have been calculated. A detailed discussion is presented of how the entire system of chemical bonding near a defect and the partial interatomic interactions vary as a function of the nature of the impurity atom. The results obtained are employed to interpret the available X-ray emission spectra of ternary systems, and also in treating problems of the mutual solubility of refractory binary phases and heterogeneous catalysis with their participation.A quantum chemical simulation of possible hydrogen atom positions in the nitride structure has been carried out. Hydrogen has been found to occupy preferentially tetrahedral interstices of the TiN lattice.     

Electronic properties of anatase TiO2 doped by lanthanides: A DFT+U study

The effects of mono-doping of 4f lanthanides with and without oxygen vacancy defect on the electronic structures of anatase TiO₂ have been studied by first-principles calculations with DFT+U (DFT with Hubbard U correction) to treat the strong correlation of Ti 3d electrons and lanthanides 4f electrons. Our results revealed that dopant Ce is easy to incorporate into the TiO₂ host by substituting Ti due to its lower substitutional energy (∼−2.0 eV), but the band gap of the system almost keeps intact after doping. The Ce 4f states are located at the bottom of conduction band, which mainly originates from Ti 3d states. The magnetic moment of doped Ce disappears due to electron transfer from Ce to the nearest O atoms. For Pr and Gd doping, their substitutional energies are similar and close to zero, indicating that both of them may also incorporate into the TiO₂ host. For Pr doping, some 4f spin-down states are located next to the bottom of the conduction band and narrow the band gap of the doping system. However, for Gd doping, the 4f states are located in deep valence band and there is no intermediate band in the band gap. The magnetic moment of dopant Gd is close to the value of isolated Gd atom (∼7 μB), indicating no overlapping between Gd 4f with other orbitals. For Eu, it is hard to incorporate into the TiO₂ host due to its very higher substitutional energy. The results also indicated that oxygen vacancy defect may enhance the adsorption of the visible light in Ln-doped TiO₂ system.     

氧离子辐照二氧化钛单晶后的结构与磁性

氧离子辐照二氧化钛单晶可以诱发其铁磁性．辐照后在室温下也能观察到二氧化钛的铁磁性,且对温度依赖性较小．结合X射线衍射实验、卢瑟福背散射/沟道实验、拉曼散射实验谱、电子自旋共振实验谱、超导量子干涉仪实验、单位原子随沟道位移实验,测定了晶格的损伤随辐照流强的增加而增加．发现在氧离子辐照二氧化钛时出现了Ti³⁺替代氧空位(O_V)的缺陷复合体,即形成Ti³⁺-OV复合体．这种缺陷复合体导致了局部(TiO_6-x)的拉曼模式的伸展．说明了Ti³⁺结合一个未成对的3d电子是二氧化钛局部铁磁性的起源.     

On the nature of a lattice vibrational mode at a frequency of 135 cm−1 in Hg1 − x Cd x Te alloys

The vibrational spectrum of a cadmium impurity atom in the HgTe crystal has been calculated using the microscopic theory of lattice dynamics in the approximation of a low impurity concentration. Within this theory, the behavior of the local and quasi-local modes induced upon substitution of the lighter Cd atom for the Hg atom in the region of the zero or very low one-phonon density of states in the HgTe crystal has been considered. It has been found that, apart from the local mode at a frequency of 155 cm^?1, the calculated vibrational spectra exhibit a weak (but clearly pronounced) feature at a frequency of 134 cm^?1, which coincides with the experimentally observed vibrational mode (the “minicluster” mode) at a frequency of 135 cm^?1 in the Hg_{1 ? x} Cd _x Te (x = 0.2–0.3) alloys at 80 K.