Thermally Stimulated Electron–Hole and Ionic Processes in Zinc Oxide Crystals

The principal results of the investigation of thermally stimulated electron–hole and ionic processes in hydrothermal and gas-phase ZnO single crystals preexcited at low temperatures, based on simultaneous study of photo-EPR and thermoluminescence (TL), are presented. The nature of the traps determining the TL peaks at 17, 24, 40, 53, 90–110, 140–150, and 160–200 K is discussed. In particular, it has been established that the lithium paramagnetic centers (Li_Zn ⁺–O^I) play the role of hole traps in ZnO giving green and red TL in the temperature range 160–200 K and, in the case of association with small-sized donors, also TL in the temperature range 90–110 K. The other traps are electronic in character, and in the presence of acceptor lithium in the crystals, they form yellow-orange TL. Optical quenching of TL has been evaluated, and it has been found that there is a difference E 0.75 eV between the thermal and optical energies of ionization of lithium acceptors. Irreversible ionic processes associated with the healing of cationic vacancies at T 360–420 K have been revealed.     

EPR and Thermoluminescence in ZnO Single Crystals with Anionic Vacancies

The radiation defects created in hydrothermal ZnO–Li single crystals by irradiating them with electrons, protons, and highenergy ions have been investigated. The anionic vacancies (Fcenters) in ZnO are established to be the centers of radiationless recombination of the charge carriers with a photoionization energy of 2.3 eV (a signal of the photoEPR with the gfactor for the F⁺center: g = 1.9948 and g = 1.9963). The anionic vacancies in the form of the F and F⁺ states are a good reference of the electron and hole processes. The [F_Li]^×centers that correspond to the oxygen vacancies localized near the point defects Li_Zn are detected. In the temperature range 530–660 K, ZnO crystals display thermally stimulated processes such as the healing of anionic vacancies (530–630 K) and the disappearance of the [F_Li]^×centers (610–660 K).     

Quadrupole relaxation enhancement and polarisation transfer in DMSO solution of [Bi(NO3)3(H2O)3]*18-crown-6 in solid state

ABSTRACT

¹H spin-lattice relaxation studies have been performed for pure [Bi(NO₃)₃(H₂O)₃]*18-crown-6 in powder and its solution in dimethyl sulfoxide (DMSO). The experiments have been carried out in the frequency range of 10?kHz–30?MHz and the temperature range of 240–277?K; at 277?K the solution is already frozen. The ¹H relaxation of pure [Bi(NO₃)₃(H₂O)₃]*18-crown-6 has been interpreted in terms of three dynamical processes. Quadrupole Relaxation Enhancement effects have been observed in the frozen DMSO solution of [Bi(NO₃)₃(H₂O)₃]*18-crown-6. The specific mechanisms of the ¹H spin-lattice relaxation enhancement have been discussed distinguishing between effects caused by time independent (residual) and fluctuating ¹H-²⁰⁹Bi dipole-dipole interactions.     

Effects of high-temperature annealing on magnetic ordering in thin films of Bi2O3-CaO-Fe2O3

Magnetic properties of rf sputtered (0.5–x) Bi₂O₃-x CaO-0.5 Fe₂O₃ (x=0–0.5) have been studied through magnetization and ferromagnetic resonance (FMR) measurements. Films prepared in a mixed oxygen-argon atmosphere are amorphous and paramagnetic. Samples annealed in air at temperatures of 700–1000 K show a ferrimagnetic behavior even though X-ray diffraction data for the films do not indicate the precipitation of any crystalline ferrimagnetic compounds. The room-temperature saturation magnetization 4M and the uniaxial anisotropy field H _u , decrease with increasing x. The Curie temperature and the gyromagnetic ratio increase with increase in the concentration of CaO. Studies on the effects of sputtering atmospheres on magnetic parameters show that films sputtered in oxygen-rich atmospheres have a large 4M and H _u , and a relatively small and FMR line-width. Ordered amorphous clusters are suggested to give rise to the observed ferrimagnetic character in the annealed films.     

Superconductivity and magnetic properties of Nb—Al phases: Nb2Al and NbAl3

Magnetic properties of tetragonal phases in Nb—Al system have been investigated down to 0.42°K. It has been found that the-phase (Nb₂Al) and the intermetallic compound NbAl₃ are superconductors with superconducting transition temperature 0.74°K and 0.64°K, respectively. Magnetic susceptibility of the investigated phases does not depend on temperature within the range 4.2°K–300°K, and is equal =(1.0±0.1) × 10^–6 emu)/g for Nb₂Al, and =(0.9±0.03) × 10^–6 emu/g for NbAl₃.On leave of absence from Institute for Solid State Physics of Bulgarian Academy of Sciences, Sofia, Bulgaria     

A new distance scale of planetary nebulae

A new scale of distances to planetary nebulae is proposed. This scale is free of the common a priori assumptions about the evolution of the nebulae and exciting stars. The recently found regression relationship between radius and temperature of the central star is employed. As the initial data for distance finding, we use the radio flux density of the nebulae and the stellar magnitude of the nucleus, or one of these parameters and the central star temperature. The distances to 75 planetary nebulae have been calculated, and their linear dimensions and hydrogen atom concentrations have been determined. It is found that the ionized masses of the planetary nebulae range from 10^–3 M to 1.419M.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 6, pp. 522–542, June, 1995.     

Infrared Spectral Study of Crown Ether Molecular Complexes with 4-Nitroaniline

Abstract

The IR spectra of the crystalline complexes of 4-nitroaniline with crown ethers were studied, viz., 18-crown-6, benzo-18-crown-6, dibenzo-18-crown-6, dicyclohexano-18-crown-6 and dibenzo-24-crown-8. The hydrogen bonding interaction was found to be predominant in these complexes. The strongest among them was the one formed with the most flexible crown ether dicyclohexano-18-crown-6, in which both the N-H bonds of 4-nitroaniline were shown to be equivalently hydrogen bonded to the crown ether oxygens. The shift of the NO₂ absorption bands indicated that the resonance form (^?C₂N-C₆H₄=N⁺H₂) is favored upon complexation with the larger ring size dibenzo-24-crown-8, while for the other complexes, the resonance form (C₂N-C₆H₄-NH₂) is favored; a fact which was attributed to geometrical factors.     

Gap exponents for percolation processes with triangle condition

A study is made of the gap exponents for percolation processes with the triangle condition in the subcritical region. It is show that the gaps are given by _t =2 fort=2, 3,. Scaling theory predicts thatP _p (¦C ₀¦S(p))–(p _c –p) andE _p (1/¦C ₀¦; ¦C ₀¦S(p))–(p _c –p)³, whereS(p) is the typical cluster size. It is found that (p _c –p)P _p (|C ₀S(p) ^1–)(p _c –p)^1–2 and (p _c –p)³E _p (1/|C ₀|;|C ₀|S(p) ^1–))(p _c –p)^3–4.     

Superconductivity in the Bi - Sr - Cu - O system

During the investigation of the system Bi–Sr–Cu–O a novel family of superconducting oxides, close to the composition Sr₂Bi₂Cu₂O₇₊ has been isolated, with a midpoint critical temperature ranging from 7 K to 22 K. The X-ray diffraction and high-resolution electron microscopy studies have shown its relationships with perovskite and demonstrated its bidimensional character. The possible relationships of this lamellar oxide with Aurivillius phases is discussed.     

Optical and Photoelectric Inhomogeneity of CdTe:V Crystals

The optical and photoelectric properties of CdTe:V crystals with the doping impurity concentration N _V = 5·10¹⁸–5·10¹⁹ cm^–3 are investigated and the possibility of their use as a photorefractive material is considered. As is seen from the spectra of optical transmission, the crystals of both types possess high transparency (50–65%), which for CdTe:V specimens with N _V = 5·10¹⁹ cm^–3 decreases sharply and in the range 12–14 m does not exceed 5%, whereas for CdTe:V crystals with vanadium concentration of 5·10¹⁸ cm^–3 such a value of transmission remains unchanged up to 25 m, implying a good optical quality of the latter crystals and their possible application in the spectral range 1.06–1.25 m in modern fiber-optic communication lines.     

Simulation of Transferred Electron Devices with Linearly Graded Composition of 3-5 Threefold Semiconductor in Active Zone

Two-level, displaced-maxwellian approach model of transfer electron effect in variband semiconductors has been created. With the help of the model research of TED based on variband semiconductor compounds In _x(z) Ga _1–x(z) As, Al _x(z) As, InP _1–x(z) Ga _1–x(z) As _x(z) has been done. Composition of the semiconductors x(z) in the active zone of TED depends linearly on the coordinate. It has been proved that work of TED with variband active zone is defined by dependence of relaxation frequency of electrons concentration in -valley upon coordinate. If the frequency is decreasing function dipole domains transit in the diode, but if the frequency is not decreasing function, so accumulation layers transit in the diode. Critical generation frequencies, power and frequencies characteristics of TED's based on semiconductor compounds In _x(z) Ga _1–x(z) As, Al _x(z) As, InP _1–x(z) Ga _1–x(z) As _x(z) have been defined.     

Temperature dependence of anisotropy in monocrystalline lithium ferrite films

A study of crystallographic and uniaxial anisotropy in monocrystalline Li-ferrite films in the temperature range 4.2–550°K is presented. The experimental results K₁(T) agree well with calculations based on the one ion model with crystalline field coefficients of a_A=–2.77 ·10^–2 cm^–1, a_B=3.34 · 10^–2 cm^–1. An experimental function K_u(T) is obtained which does not contradict the assumption that anisotropic stresses are responsible for the development of uniaxial anisotropy in Li-ferrite films.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 113–116, August, 1973.     

Photoconductivity of highly excited A IV B VI thin films

We report the results of picosecond photoconductivity measurements in photosensitive electrolytically deposited PbS and vacuum evaporated PbTe polycrystalline films. We determine Auger recombination to be the prevailing carrier recombination mechanism in highly excited PbTe and PbS films and found Auger coefficients _A5×10^–28 cm⁶ s^–1 for PbTe and _A5.3×10^–29 cm⁶ s^–1 for PbS for carrier concentration changes N>10¹⁸ cm^–3. The results indicate that the low mobility values are controlled by intergrain carrier scattering. We have studied the thermal annealing influence on picosecond photoconductivity of the films.     

IR Spectra and Structure of Immunoactive 8–Azasteroids in Model Compounds

The IR spectra of biologically active molecules of 8-azasteroids and model compounds in the region of C=O, C=C, C--N, and C--H vibrations (1800–1400 cm^–1) have been studied in detail. The structure of compounds containing isostructural and isoelectronic --acyl----aminovinylcarbonyl and --acyl----alkoxyvinylcarbonyl fragments have been analyzed. Interpretation of the structure of the IR spectra and of the specificity of the manifestation of the vibrational modes under consideration in the molecules of a particular class of 8-azasteroids is given. The electronic structure of the molecules under investigation complies with the model of mesomeric tautomerism of the aminovinyldicarbonyl fragment.     

Infrared properties of quartz fibres

The IR properties of micron-size amorphous SiO₂ fibres have been investigated between 400 cm^–1 and 1400 cm^–. For the single scattering size and shape effects have been measured and compared with theory. The observed diffuse reflection of quartz wool can be explained qualitatively by the Kubelka-Munk theory for multiple scattering.reported at the 5th International Conference on Infrared and Millimeter Waves, October 6–10, 1980     

Thermodynamic studies of successive phase transitions in BaZnGeO4 and a new solid phase below 186.1 K

The successive phase transitions of BaZnGeO₄ have been studied on meltsolidified samples. A new solid phase (named phase VI) has been found below 186.1 K in samples of large particle size (diameter:D0.1 mm). The higher temperature crystalline phase V can be supercooled easily down to liquid helium temperature. On heating, however, it transforms into phase VI above 95 K in a slow exothermic process. Heat capacities have been measured by adiabatic calorimetry between 14 and 300 K. The enthalpy and entropy of the V–VI phase transition are 187.1 Jmol^–1 and 0.971 J K^–1 mol^–1, respectively. The corresponding data for the IV–V phase transition at 199.8 K are 229.3 J mol^–1 and 1.168 JK^–1 mol^–1. The phase VI does not appear in samples of smaller particle size (D0.1 mm).     

Measurement of the normal thermal diffusivity of a dielectric film by a pulsed photoacoustic method

A method of determining the thermal diffusivity of a thin film of a transparent dielectric in the direction normal to the surface has been developed. It is based on excitation of reflecting dynamic gratings. The effectiveness of the method is checked experimentally with the example of a thermally oxidized submicron SiO₂ film on a silicon substrate. The temperature dependence of in the range 290–420 K is measured. The possibilities of setting up thermal measurements of films tens of nanometers thick and its problems are discussed.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 793–797, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Spectral‐Luminescence Characteristics of Polyphenyls and Polyacenes in the Gas Phase in Electron‐Beam Excitation

The spectra of electronenergy loss, excitation functions, and fluorescence spectra in excitation of the vapor of polyphenyls and polyacenes by electron beams of different energies are determined. The influence of successive complication of the molecules under study on these spectralluminescence characteristics is tracked. Unlike the optical absorption spectra, in the spectra of electronenergy loss of all the substances studied one observes a band which is related to the singlettriplet transition S ₀–T ₁. The transitions up to S ₀–S ₅ are recorded in excitation of the molecules by highenergy electrons, including the region of vacuum ultraviolet. From the functions of fluorescence excitation the authors have determined the excitation thresholds that correlate with the energies of the S ₁ levels, except for pyrene in which the S ₀–S ₁ transition is forbidden and does not show up not only in photon excitation but also in electronbeam excitation, although the intercombination forbiddenness in the latter case is removed and the S ₀–T ₁ band is observed.     

Instability of the molecular structure of monobenzoporphin to the alternation of the macrocycle bond lengths and its manifestation in the electronic spectra

Quantum-chemical calculations of the geometric structure of the molecules of monobenzoporphin (H₂ MBP) and monobenzoporphin with methyl and ethyl substituents in the five-member rings (H₂MBPm) have been carried out by the restricted and unrestricted Hartree-Fock methods with the AM1 Hamiltonian (AM1 RHF and AM1 UHF methods). The calculation of the above-indicated molecules by the AM1 RHF method without restrictions on their symmetry has given, for them, a planar structure with an alternation of the lengths of the bonds along the 18-member azacyclopolyene and the symmetry C _1h for their aromatic part. The calculation of the transitions to the excited electron Q states in such a structure by the CNDO/S method has shown that these states are characterized by large hypsochromic shifts (~3000–4000 cm^–1 ) relative to the Q levels of porphin (H₂P), which is in contradiction with the experimental data, according to which these shifts are bathochromic and comprise = –330 cm ^–1 and = –750 cm^–1. Optimization of the geometry of the H₂ MBP and H₂MBPm molecules by the AM1 UHF method gives, for them, a structure with equal lengths of the bonds along the 18-member azacyclopolyene with a symmetry differing insignificantly from the D _2h symmetry; elements of the structure with a lower symmetry and an alternation of the lengths of the bonds are retained in the condensed pyrrolenine and benzene rings. The calculation of the shifts of the Q levels in the H₂MBPm molecule of this geometry relative to the analogous levels in H₂P has shown that they are bathochromic and equal to = –520 cm^–1, and the RHF calculation with optimization of the geometry of the molecule and restrictions on the effective symmetry D _2h of the 18-member azacyclopolyene has given = –350 cm^–1 and = –430 cm^–1. The restrictions imposed on the C ₂ symmetry of the H₂MBP molecules by the RHF method are inadequate to equalize the lengths of the bonds along the 18-member azacyclopolyene. The calculations of the energy of the B levels of the monobenzoporphyrins considered also lend credence to their geometric structure with equal lengths of the bonds along the 18-member azacyclopolyene.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 712–721, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Magnetic behaviour ofCuAuFe alloys

The magnetic properties of Cu_99-xAu_xFe₁ alloys (x=12, 37 and 50.7 ats) have been investigated over the temperature range 4.2–70 K using low field AC magnetic susceptibility measurements and mössbauer spectroscopy. All alloys exhibit spin glass behaviour at low temperature with freezing temperatures T_f-5.7–7.6 K. Results of analysis of the high temperature (T3 T_f) Curie-Weiss behaviour are compared with those obtained from analysis of the broadly distributed 4.2 K Mössbauer spectra.