Dielectric relaxation in YTTRIA-doped Ceria solid solutions

The nature of the defects present in Y₂O₃-doped CeO₂ is explored by means of dielectric relaxation, employing primarily the thermally-stimulated depolarization current (TSDC) technique. For Y₂O₃ concentrations ? 1 mole %, two relaxation peaks are observed. The lower-temperature peak shows a dielectric relaxation rate, τ^?1 which is $\text{1}\text{2}$ the relaxation rate of the corresponding anelastic relaxation. This proves that the peak is due to nn, 〈111〉 oriented, (YV_O) pairs, where V_O represents the oxygen vacancy. These pairs are positively charged defects which are compensated by an equal number of isolated Ý. The upper peak is a broad peak whose position varies with Y₂O₃ concentration in the same manner as the activation enthalpy for the ionic conductivity. The peak is not due to simple dipoles, but to relaxation of the array of alternately charged (YV_O) and Ý defects by the redistribution of oxygen-ion vacancies. A simple model, in which these defects form a superlattice containing wrong pairs, explains the essential features of the upper peak.     

Microscopic theory of the long-wavelength modes of two-component plasmas and ionic liquids: II. The longitudinal modes

The modification of the damping rate of the sound modes by Coulomb phenomena is demonstrated from first principles. The heat modes of one- and two-component systems of charged particles are shown to differ by a factor c_p/c_v. Microscopic expressions for the interspecies energy and longitudinal momentum relaxation frequencies are provided. The charge relaxation modes are shown to reduce in the limit of weak-coupling to a pair of plasma oscillations occurring slightly below the plasma frequency while being slightly damped even at infinite wavelength. In the opposite limit of strong-coupling the same pair of charge relaxation modes is shown to split into an interspecies momentum relaxation mode and an approximate hydrodynamic diffusion mode. An Einstein relation between the diffusion constant and the electric conductivity is also demonstrated. All expressions are obtained for arbitrary density and coupling.     

Epitaxial strain induced ferroelectricity in rocksalt binary compound: Hybrid functional Ab initio calculation and soft mode group theory analysis

We have studied the detailed mechanism of epitaxial strain induced ferroelectricity in rocksalt binary compound by ab initio calculation and soft mode group theory analysis. By applying compressive strain, cubic binary rocksalt (F_m3m) transforms into tetragonal (I_4/mmm) structure. With increasing compressive strain, tetragonal structure becomes unstable against spontaneous transformation to lower symmetry tetragonal structure (I_4/mm), evident both from ab initio calculation and from soft mode group theory analysis. For the tensile strain, phase transition sequence can be cubic binary rocksalt to tetragonal (I_4/mmm) and to orthorhombic structure (I_m2m). From ab initio calculation and space group analysis, we propose that the epitaxial strain induced ferroelectricity of rocksalt binary compound is the generic property.     

Pressure and temperature dependence of the electric field gradient (EFG) in CdS

Using the time differential perturbed angular correlation technique (TDPAC), the electric hyperfine interaction of¹¹¹Cd in the II-VI-semiconductor CdS was investigated. The results of the temperature and pressure dependence of the electric field gradient (EFG) are discussed. The binding energyE _b for¹¹¹In at a Cd lattice site and a Cd vacancy (¹¹¹In_Cd–V_Cd–pair) could be estimated to 340 meV±80 meV by means of an Arrhenius-Plot. The disappearance of the 79 MHz and 73 MHz frequencies under a pressure of about 30 kbar could be due to their vacancy character or to the beginning of the phase transition to rocksalt structure.     

General relationships for isovalent cation substitution into oxides with the rocksalt structure

Isovalent cation substitution into rocksalt oxides, MO, has been investigated using atomistic simulation. A strain related parameter, ε, is established that relates the size of a substitutional cation to the host lattice ion for which it has been substituted. This has allowed us to identify relationships between solution energy, defect volume and a strain parameter, which are general for any rocksalt oxide host lattice and as such are predictive for any combination of a divalent cation and rocksalt host lattice.     

Dielectric relaxation properties of tysonite-type solid solutions La1−xBaxF3−x

Dielectric relaxation properties of solid solutions La_1?xBa_xF_3?x (x ? 0105) have been studied by thermally stimulated depolanzation current (TSDC)- and a c. dielectric loss (DL) techniques.For x < 30 × 10^?3 the dielectric spectra show a relaxation peak which is ascribed to a simple associate of a substitutional dopant ion and a fluoride ion vacancy (Ba_LaV_F)^x in nearest-neighbour position, the vacancy being confined to the B sublattice For x values of about 1.3 × 10^?2 a relaxation peak appears which is tentatively attributed to a similar type of defect associate with the vacancy now confined to the A sublattice of the tysonite anion array One broad relaxation peak dominates the TSDC and DL spectra over the whole concentration range This peak is due to the relaxation of macroscopic space charge, i e ionic conductivity The low-temperature ion conductivity has been determined for several solid solutions, and extrapolates to the high-temperature conductivity determined previously with impedance spectroscopy Below liquid-nitrogen temperature three relaxations are observed, and ascribed to electronic transitions in cenum impurities. A computer programme has been developed to analyse TSDC relaxation peaks, taking dipole-dipole interactions into account Relaxation parameters and dipole concentrations are presented.     

EPR investigation of the trivalent chromium complexes in SrTiO3

The trivalent chromium centers were investigated by means of electron paramagnetic resonance (EPR) in SrTiO₃ single crystals grown using the Verneuil technique. It was shown that the charge compensation of the Cr³⁺-V_O dominant centers in octahedral environment is due to the remote oxygen vacancy located on the axial axis of the center. In order to provide insight into spin-phonon relaxation processes the studies of axial distortion of Cr³⁺-V_O centers have been performed as function of temperature. The analysis of the trigonal Cr³⁺ centers found in SrTiO₃ indicates the presence of the nearest-neighbor strontium vacancy. The next-nearest-neighbor exchange-coupled pairs of Cr³⁺ in SrTiO₃ has been analyzed from the angular variation of the total electron spin of S=2 resonance lines.     

Vibrational and electronic spectra of alkali halides doped with CrO42− ions

The i.r. spectra of alkali halides doped with FrO₄^2? ions are very sensitive to the concentration of background divalent impurities. Highly pure crystals containing CrO₄^2? ions show a 5 line i.r. spectrum in the v₃ stretch frequency region. One of these lines is attributed to CrO₄^2? ions in perfect crystal surroundings having T_d symmetry and the other 4 lines are attributed to C_s symmetry produced by a charge compensating anion vacancy in the nearest neighbour position. If the crystal contains a concentration of divalent cation impurity which is equal to or more than that of CrO₄^2? ions, a different spectrum characteristic of C_2x symmetry as well as the line corresponding to T_d symmetry are obtained. The crystals of ordinary purity contain all the three (C_s, C_2x and T_d symmetry) species and show eight line spectra. The changes in force constants calculated from the observed splitting of the energy levels agree with those evaluated from the known polarizability of the chromate ions.At 600°C, the vibration spectra of the highly pure crystals doped with CrO₄^2? ions show an increase in intensity of the line characteristic of T_d symmetry and the other lines practically disappear, because the anion vacancies move away from the neighbourhood of the CrO₄^2? ions at high temperatures. The energy of association of the vacancy with CrO₄^2? ions and the energy of migration of the vacancy are derived from the spectra obtained at different temperatures and with different heat treatments.The electronic spectra of the crystals show 3 bands (at 345, 270 and 240 nm for KCI). The lowest energy band has a fine structure with a spacing of about 800 cm^?1 at LAT. Since the transition involved is from a non-bonding to an antibonding state, the Condon parabola is shifted in the excited state and therefore the fine structure is attributed to a vibrational progression involving a totally symmetric frequency 800 cm^?1 in the excited state.     

空位所引起的晶体弹性常数的改变

认为具有少量空位的晶体是一个均匀的晶体,把完整与不完整晶体的区别,归结为势函数的修改。应用Morse函数推导出弹性常数c₁₁和c₁₂的变化率与空位浓度的关系。因为空位邻近的原子有松弛,考虑了由松弛而改变了的相互作用对弹性常数的影响。首先求得铝中空位的第一和第二最近邻的位移百分率各为1.61和-0.32,这里的正号和负号各表示向空位移进和移出。其次,进行了具体的计算,在铜晶体中,估计1％的空位浓度使弹性常数减小1.3—1.4％。然而,铝晶体中的空位对弹性常数的影响     

Excess entropy for a vacancy in a crystal

The following topics are discussed: (1) Within the harmonic theory a general expression is given for excess entropy of a localized detect in any crystal with cyclic boundary conditions and in the small perturbation limit. For a single vacancy defect Stripp and Kirkwood's formula is obtained, ΔS/k_B = d/2,d being the dimension of the space. For other localized defects the perturbation formula requires calculation of the unperturbed Green's matrix. (2) It is argued that the effect of free surface boundary conditions, as well as other surface changes, leaves ΔS effectively invariant. (3) Using the vacancy perturbation result for ΔS and Lawson's formula, a value for the vacancy formation volume is obtained only in terms of known measured parameters; this result is compared to experimental and other calculated values of ΔV_f. (4) If localized vibrational modes are present due to the vacancy, what effect these might have on ΔS, and, from this, what materials might be candidates for observing localized modes.     

Spin relaxation of quadrupole nuclei in paramagnetic and magnetically ordered insulators

The longitudinal and transverse spin relaxation through a (generally anisotropic) electron-nucleus interaction in paramagnetic and magnetically ordered insulators is theoretically studied for nuclei with a resolved quadrupole structure. Expressions are derived for the relaxation rates of both the transverse nuclear magnetization components when individual transitions are excited in the quadrupole structure and the total longitudinal nuclear magnetization component. These expressions are reduced to a form that contains the Fourier transforms of the time correlation functions only for the electron spins. Given the specific form of these correlation functions corresponding to different phase states of the electron spins and different origins of their fluctuations, the temperature dependences of the nuclear relaxation rates are ascertained in various cases, including those for dipole and isotropic hyperfine interactions. Calculations are performed for arbitrary electron and half-integer nuclear spins by taking into account the possible quadrupole splitting of the NMR spectrum without any restriction on the smallness of the ratio ?ω s/k_BT (ω_s is the resonance frequency of the electron spins). The derived expressions are compared with available experimental data on the longitudinal and transverse nuclear relaxation in colossal-magnetoresistance lanthanum manganites in the part of their phase diagram where the corresponding samples are either paramagnetic or magnetically ordered insulators and near the points of transition to an ordered state. Interpretations alternative to the existing ones are offered.     

Thermoelectric properties of tin clathrates under hydrostatic pressure

We report the temperature dependence of electrical resistance (R) and thermopower (S) of clathrate Cs₈Sn₄₄ under high pressure up to 1.2 GPa. We observe a reversible gap widening, prominent relaxation effect of R, irreversible increase of |S| under high pressure. We also find that the power factor S²σ (σ: electrical conductivity) reaches a maximum at pressure of 0.3 GPa. Comparison of the experimental results with band structure calculations suggests that the intrinsic vacancy in the clathrate structure of Cs₈Sn₄₄ plays an important role in transport properties under high pressure. Measurements on Cs₈Zn₄Sn₄₂, a clathrate which has defects other than vacancies, are compared with Cs₈Sn₄₄. The results indicate that replacing Sn by Zn has similar effect as the intrinsic vacancy on S.     

Electron structure of “oxygen vacancy” defect in SiO2

Electron structure calculations of the bulk defects, connected with oxygen vacancies in SiO₂ have been performed by the non-empirical discrete variational method. Two models of E′₁-center are analyzed, the first considering the unpaired electron in the planar SiO₃ fragment and the second in the pyramidal one (“asymmetrically relaxed vacancy”). The comparison of calculation results with ESR and optical data gives evidence in favour of the asymmetrical relaxation model.     

NMR studies of amorphous phosphorous

Linewidth, chemical shift, and relaxation time measurements have been made in amorphous phosphorous at 77 and 300 K. It is found that the chemical shift and T₂ measurements indicate that the short-range structure of amorphous phosphorous is similar to the short-range structure of black phosphorous. The T₁ measurements indicate that the spin-lattice relaxation is dominated by impurities, and that disorder mode relaxation is not effective in amorphous phosphorous, indicating either that the density of disorder modes in amorphous phosphorous is small or that the disorder mode coupling to the nuclear spin is weak.     

Half-metallic properties in rocksalt and zinc-blende M N (M=Na, K): A first-principles study

Pseudopotentials and plane-wave basis set method is used to investigate the electronic structure and magnetic properties for state-of-the-art zinc-blende and rocksalt M N (M=K, Na) alloys. We find that these compounds exhibit half-metallic characters with an integer magnetic moment of 2.00μ_B. The half-metallic properties result from a fully spin-polarization of s and p states. The origin of energy gap mainly comes from the hybridization both s and p states. Total energies calculations indicate the rocksalt phase is lower in energy than the zinc-blende one. The difference of total energy are about 0.035 Ry per formula unit for KN and NaN, respectively. For these compounds, Slater-Pauling curve M_t=(Z_t−4) (in μ_B unit) is obeyed between valence electrons and total magnetic moment. Meanwhile, we also find the preservation of half metallic characters when the lattice parameter is moderate compressed.     

Palladium-vacancy pairs in p-type and n-type germanium

Time differential perturbed angular correlation spectroscopy was performed for n-type and p-type germanium samples that had been recoil-implanted with the probe ¹⁰⁰Pd/¹⁰⁰Rh. Two different measurements with the detectors aligned along <100> and <110> crystallographic directions of germanium confirm that the palladium probe pairs with the nearest neighbour vacancy and not a dopant. The pairs interact with a coupling constant of υ _Q = 10.7(2) MHz and this agrees with what has been observed in undoped Ge. This further suggests that the palladium pairs with a vacancy in germanium, doped or un-doped. It was also observed that there are more Pd-V pairs in Ga-doped than there are in Sb-doped Ge. DFT calculations of the binding energies of the Pd-defect complexes support the experimental observations.     

Unstable equilibrium and radiation defects in solids

A new type of point defect in solids genetically related with Frenkel's pairs which appears when there exists the instability zone of pair vacancy (v) - interstitial atom (i). It is shown that, at large instability zones typical of ‘loose’ crystalline structures, equilibrium concentration of unstable pairs i-v may exceed the concentration of usual Frenkel's pairs. In the example of the semiconducting crystals In₂Te₃ and Ga₂Te₃ having loose structure it is experimentally demonstrated that equilibrium point defects cannot be preserved by specimen quenching and that irradiation by large doses of high-energy particles does not result in stable radiation defects formation.     

Oxygen vacancy model for the E1′ center in SiO2

An (oxygen)^- vacancy model of the E₁^≈ center in alpha quartz, featuring an asymmetric relaxation of the two silicons adjacent to the oxygen vacancy, is presented and analyzed. This model is shown to be consistent with both theoretical calculations and experimental hyperfine data, in contrast with any model previously proposed for the E₁^≈ center.     

Phase transformation and elastic behavior of MgX (X=S, Se, Te) alkaline earth chalcogenides

Pressure induced structural aspects of NaCl-type (B1) to CsCl-type (B2) structure in alkaline earth chalcogenides (AECs) magnesium chalcogenides (MgX; X=S, Se, and Te) are presented. An effective interionic interaction potential (EIoIP) with long-range Coulomb interactions and the Hafemeister and Flygare type short-range overlap repulsion extending up to the second neighbor ions and the van der Waals (vdW) interaction is developed. The vdW coefficients are evaluated following the Slater-Kirkwood variational method, as both the ions are polarizable. The present calculations have revealed reasonably good agreement with the available experimental data on structural transition (B1-B2 structure), the phase transition pressures P_t of 167 (MgS), 170 (MgSe), and 176 (MgTe) GPa as well the elastic properties. The calculated values of the volume collapses [ΔV(P)/V(0)] are also closer to their observed data. Further, the variations of the second and third order elastic constants with pressure have followed a systematic trend, which are almost identical to those exhibited by the observed data measured for other semiconducting compounds with rocksalt (B1) type crystal structure. The Born and relative stability criteria is valid in Mg monochalcogenides.     

Influence of the relative magnitude of the Jahn-Teller effect and level splitting in a cubic crystal field on the properties of the ground state of vacancy defects in semiconductors

The spatial structure of a vacancy and the properties of its electronic energy levels in a semiconductor with a lattice possessing point symmetry T _d are considered for an arbitrary relationship between the Jahn-Teller stabilization energy (associated with the F ₂ vibrational mode) and the t ₂-a ₁ splitting (Δ) caused by the cubic crystal field. The position of the minimum of the adiabatic potential and the distortion of the electronic density are calculated for the vacancy ground state for different relative values of Δ and coupling constants of the vacancy to the F ₂ vibrational mode. It is shown that, if the ground state of a carrier bound to a vacancy is a t ₂ state, the trigonal symmetry of the environment of the vacancy persists for any values of Δ, but the amount of displacements of atoms near the vacancy and the localization of the wave function of the bound carrier on the broken bond earmarked by the Jahn-Teller effect can depend heavily on Δ and are maximal at Δ → 0. This is also the case when the ground state of the vacancy is the a ₁ state, but the magnitude of Δ does not exceed a certain value, which is determined by the coupling constants and the elastic constant. The relation between Δ and the coupling constants is also shown to affect the properties of trigonal vacancy-shallow-donor complexes. For these complexes, calculations are performed of the dependence of the dipole direction determining the optical properties of the vacancy defect on the distortion of vacancy orbitals caused by the donor entering into the complex.