Z-like-centers in vanadium doped NaCl crystals

The absorption, thermostimulated luminescence (TSL) and electron emission (TSE) of prepared and X-irradiated NaCl crystals doped with vanadium were studied. From the TSE-, TSL- and thermo-optical bleaching (TOB-) curves it is suggested that the new 647 nm absorption band and the new TSE- and TSL-peaks at about 480–490 °K are due to Z-centers, i.e. to the aggregates ofF-centers with dipole complexes (divalent vanadium ions and cation vacancies).     

On relaxation phenomena in filamentary NaCl crystals

The internal friction and reversible creep of filamentary sodium chloride crystals are investigated. An internal friction peak, associated with the reorientation of the impurity-vacancy complexes in an elastic field is detected. The activation energy of the process is determined. The correlation between the internal friction level and the size of the filamentary crystal, as well as its electric charge, are established. The relaxation time of the reversible creep of filamentary crystals in a 10 kV/cm electrical field is estimated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 101–104, April, 1973.     

Defect centers in antimony doped NaCl crystals

The existence of both Sb²⁺ and Sb³⁺ in NaCl has been established by a series of correlated experiments like optical absorption, EPR, Electrical conductivity and dielectric loss. Sb²⁺ gives an optical band at 212 nm and in an arbitraryorientation gives an EPR signal whose direction dependence studies indicate three magnetically inequivalent sites. A vacancy occupying a second neighbour site with respect to the impurity ion satisfactorily, explains the observed spectra. The association energy of the impurity ion has been found to be 0·56 eV from ionic conductivity studies. Dielectric loss measurements give two prominent loss peaks which have been assigned to Sb²⁺ and Sb³⁺ on the basis of their temperature variation and other correlated experiments. Radiation damage reduces the Sb³⁺ content while Sb²⁺ is not affected.     

Comment on the model of the Mg centres in NaCl crystals

The structure models of the Mg₂ centre (substitutional Mg⁺ ion associated with a vacancy pair) and the Mg₄ centre (Mg⁰ atom situated in a vacancy pair) produced in NaClMg crystal by X-irradiation and subsequent F- and thermal bleaching are discussed. The given models explain without any contradiction all the experimental observations.     

Thermostimulated currents in n-type CdTe single crystals

Measurements of thermostimulated currents on n-type CdTe crystals yield activation energies of traps which can be identified with previously known localized levels. The diffusion of Cu and In into the crystals can be proved by means of the presence of corresponding trap levels.     

Ionic conductivity and cation selfdiffusion in NaCl crystals doped with calcium or cadmium

Measurements of the concentration dependences of dc conductivity and Na⁺ diffusion in NaCl + CaCl₂ and NaCl + CdCl₂ monocrystals have shown that the heats of association of both divalent admixtures into complexes with cation vacancies are very close (0·28±0·02 eV.) The Stasiw-Teltow model of association has been found suitable for concentrations of calcium not exceeding 0·1 mole-% within the whole temperature range of 150 to 720°C. This model gave the value of 0·284 eV for the enthalpy of association of Ca⁺⁺-complexes, the value of 0·74 eV for the enthalpy of migration of free vacancies and a negligibly low entropy of association of these complexes. The observed contribution of the complexes to selfdiffusion was in accordance with the model of association into tightly bound complexes. The contribution was, however, higher than could be expected for simple rotation of the associated vacancy round the admixture in the nearest neighbour positions.     

Low temperature ESR spectra of nickel — doped NaCl crystals

Paramagnetic resonance of Bridgman-grown NaCl crystals nominally doped with divalent nickel was studied over the temperature range from 4 K to 300 K. The broad isotropic EPR line with g = 2.12 ÷ 2.23 and ΔH _pp = 280 ÷ 780 G was detected at temperatures higher than 130 K for samples of various thermal history. The origin of these spectra was attributed to aggregated and precipitated Ni²⁺ ions. At temperatures lower than 40 K another ESR signal was registered. It was an anisotropic one, the intensity of which strongly depends upon the temperature. This spectrum results from substitutional Ni⁺ ions with tetragonally distorted surrounding along a S100T direction. The angular dependence of the ESR line positions was fitted to an appropriate spin-Hamiltonian with the g-factor components g _∥ and g _⊥ equal to 2.86 and 2.10, respectively. The temperature-dependence of the signal intensity at temperatures between 4 and 110 K has been interpreted in terms of a transition from the static to dynamic Jahn-Teller effect.     

Thermostimulated surface autosegregation in bismuth ferrite single crystals

The chemical and phase composition and morphological features of the surface nanostructure have been studied by the methods of high-resolution scanning electron microscopy, X-ray microprobe analysis, and atomic force microscopy in bismuth ferrite single crystals. This structure was formed as a result of the thermostimulated surface segregation after annealing in air or vacuum at the pressure of 10⁻⁴ Pa. It has been experimentally found that, at temperatures less than 500°C, Bi₂F₄O₉ nanoparticles were formed due to the selective diffusion of iron atoms to the surface. Starting from 300°C in vacuum and 450°C in air, the segregating atom type changed and nanophases with high bismuth concentration (sillenites Bi_{26 − x} Fe _x O₃₉ and Bi₂O₃ appeared in some regions. The partial orientation of new phases has been observed in some surface regions. A probable mechanism of the described phenomenon that represents a combination of selective intrinsic mass-transport of atoms from the bulk to the surface and their thermal evaporation has been discussed.     

Thermostimulated surface autosegregation in bismuth titanate single crystals

High resolution scanning electron microscopy (HRSEM), atomic force microscopy (AFM), and electron microprobe X-ray analysis (EPMA) were used to study the morphology and local phase composition of (001) faces in single crystals of bismuth titanate Bi₄Ti₃O₁₂ formed as a result of thermostimulated surface segregation (TSAS). One possible mechanism for this phenomenon, generated by the selective internal mass transfer of the matrix’s own atoms to the surface in competition with the processes of selective component evaporation, is discussed.     

Magnetically induced Freedericksz transition and relaxation phenomena in nematic liquid crystals doped with azo-dyes

A static and dynamic investigation was performed on liquid crystal cells containing pure nematics and nematics doped with an azo-dye (Methyl Orange). It was found that the critical field for magnetic Freedericksz transition was decreased in samples containing the “trans” isomer and increased in those containing the “cis  ” isomer. Changes in the relaxation time _τA${\tau }_A$, _τB${\tau }_B$ intervening when switching on/off the magnetic field were also noticed. A theoretical model was elaborated to explain these phenomena.     

Thermostimulated currents in p-type cadmium telluride single crystals

Ssveral samples of CdTe were measured in a temperature range of –190C to 100C. The activation energies of the levels and their cross-section were determined. With some samples it was found that for more rapid heating the maxima of the curves of thermostimulated currents are shifted towards lower temperatures. Altogether three trap levels were found.     

Thermostimulated conductivity of synthetic semiconductor diamonds doped with boron during synthesis

The properties of diamond as a new material for semiconductor electronics have been studied insufficiently at present. One of the most powerful techniques for study of impurity centers is the method of thermostimulated conductivity (TSC).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 130–132, September, 1978.In conclusion, the authors thank Yu. M. Eotner for providing the synthetic semiconductor diamond specimens for measurement.     

Thermostimulated luminescence of CdF2:Eu crystals

Using the method of fractional thermostimulated luminescence (FTSL), the temperature dependence of the mean activation energy of recombination processes in CdF₂:Eu³⁺ crystal was obtained. After thermal annealing of the crystal, thermostimulated luminescence peaks were identified. Anomalously low frequency factor (s=10⁷ s ^–1) of the recombination processes can be explained by the dependence of the resonance energy transfer probability on intercentre distance.The authors are very grateful to Dr. E. Kotomin for valuable comments and Dr. C. Paracchini for supply of CdF₂:Eu crystals.     

Thermostimulated luminescence and extended afterglow of NaI-Tl crystals

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 56, No. 2, pp. 259–263, February, 1992.     

The colouration by 60 Co-gamma rays of NaCl and KCl crystals doped with europium ions

Pure and europium doped NaCl and KCl crystals have been irradiated with 60 Co gamma rays and with ultraviolet light. The dose was 10 v kGy and temperatures during irradiation were ambient and liquid nitrogen. The effects of irradiation are determined using optical absorption and thermoluminescence. The role of the europium dopant is compared for the two materials. Factors investigated include the temperature of irradiation, the concentration of the dopant and the state of impurity aggregation and precipitation. A link is indicated between impurity precipitates and UV stimulated thermoluminescence.