Effect of the spin and valence states of cobalt ions on the kinetic properties of Ho1 ? xSrxCoO3 ? δ and Er1 ? xSrxCoO3 ? δ compounds

The electrical conductivity and Seebeck effect in ceramics based on cobaltites Ho_{1 − x} Sr _x CoO_{3 − δ} (x = 0.65, 0.75, 0.85, 0.95) and Er_{1 − x} Sr _x CoO_{3 − δ} (x = 0.75, 0.85, 0.95) with a perovskite-like structure have been investigated in the temperature range T > 77 K. All the compounds under study are characterized by the variable-range-hopping conductivity with the temperature dependence of the electrical resistivity corresponding to the Mott law. It has been found that, in the Ho_0.35Sr_0.65CoO_{3 − δ} compound, thermally excited Co³⁺ ions contribute to the electrical conductivity with an increase in temperature to 250 K. The Seebeck coefficient of the systems studied decreases as the strontium concentration and temperature increase. It has been shown that, for an adequate explanation of this behavior, proper allowance must be made for the splitting of the 3d levels, as well as for the charge disproportionation of the cobalt ions.     

Time-resolved luminescent spectroscopy of YAG:Ce single crystal and single crystalline films

The peculiarities of the luminescence and energy transfer from YAG host to the emission centers formed by the Y_Al antisite defects and Ce³⁺ ions have been studied in YAG:Ce single crystals, grown from the melt by modified Bridgman method in Ar and CO₂ + H₂ atmospheres, and YAG:Ce single crystalline film, grown by liquid phase epitaxy method, using the comparative time-resolved luminescent spectroscopy under excitation by synchrotron radiation in the range of fundamental adsorption of this garnet.     

Doping of KDP single crystals with cerium: Growth and optical properties

The features of doping of KDP crystals with cerium ions and organocerium complexes with alizarin complexon and arsenazo III have been investigated. It is established that “direct” doping by introducing cerium salts into the initial solution cannot be implemented. The effect of organometallic complexes of cerium on the crystal growth has been studied. Organocerium complexes predominantly enter the prismatic or pyramidal growth sectors. It is shown that the complex arsenazo III + Ce blocks the growth of the prismatic sector. Cerium-doped KDP crystals exhibit a photoluminescence band peaking at the wavelength λ_max= 350 nm.     

Potassium and ammonium dihydrogen phosphates activated with thallium: Growth and luminescence and scintillation properties

The conditions for the growth of potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) crystals with concentrations of the thallium activator in the initial solution of 0.01, 0.1, and 1.0 wt % are investigated. It is shown that the character of incorporation and distribution of thallium in the KDP and ADP lattices is limited, apparently, by the difference in the ionic radii of K⁺, NH ₄ ⁺ , and Tl⁺ cations and the charge state of prismatic {100} and pyramidal {101} growth planes. Doping of KDP and ADP with thallium (to 0.1 and 1.0 wt %, respectively) does not deteriorate the structural quality of these crystals. The dependence of the lattice parameters a and c on the thallium impurity concentration is investigated. The absorption bands of thallium in the KDP:Tl⁺ and ADP:Tl⁺ crystals peak at 218 and 215 nm, respectively, while the photoluminescence band peaks at 280 nm for both types of crystals. The relative light yield upon excitation of scintillations by α particles (Pu²³⁹) and β particles (Bi²⁰⁷) is measured.     

Active media based on SiO2 matrices with incorporated molecules of rhodamine dyes

SiO₂ matrices with incorporated molecules of Rhodamine 6G and Rhodamine 800 laser dyes were synthesized by the sol?Cgel method. The spectral properties of the matrices were studied. It was found that the incorporation of Rhodamine 6G molecules into the xerogel network decreased their association. The influence of the molar ratio of water and formamide on the transparency of the samples was studied. Laser emission of SiO₂ matrices activated with Rhodamine 800 was obtained in the near infrared spectral range. The values of laser spectrum halfwidth of SiO₂ matrices with Rhodamine 6G and Rhodamine 800 are 2?C6?nm. A well-defined structure of the laser emission spectra of the matrices at microsecond pumping duration testifies to their relatively high optical quality.     

Anomalous X-ray transmission through sapphire crystals

The triple-crystal X-ray diffractometry of anomalous X-Ray transmission is used to investigate sapphire single crystals grown in protective gaseous media with the help of the horizontal directional-crystallization method. The dependences between the integral intensities of anomalously transmitted and Bragg-Laue reflected X-ray beams (T _i and R _i , respectively) and the thickness of the samples under study, which enable us to determine regions of thin- and thick-crystal approximation, have been obtained at 2.52 ≤ μt ≤ 18.9. The integral coefficient μ _i of interference absorption and the integral characteristic y _i are calculated in the dynamic thick-crystal approximation. It is demonstrated that dislocations are distributed mainly within the small-angle boundaries of a grown sapphire crystal. It is ascertained that the parameter μ _i is highly sensitive to the dislocation density and the pattern of the dislocation distribution in a crystal.     

Dielectric properties of ZnSe crystals grown from melt

The frequency and temperature dependences of the real and imaginary parts of the permittivity of ZnSe crystals grown from melt have been measured in the low-frequency range. It has been found that the crystal samples cut from different parts of the ingot exhibit different properties depending on their distance from the ingot origin. The difference in the properties is explained by the dominant influence exerted on the polarization by point defects, the formation of which is associated with the deviation of the composition from stoichiometry, as well as by residual impurities and stresses in the crystals.     

Production of Y3Al5O12 and Y2O3 nanopowders for optical ceramics

The physical and technical conditions for reproducible production of nanodispersed yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) and yttrium oxide (Y₂O₃) powders by chemical coprecipitation have been investigated. It is established that the obtained YAG nanopowders have enhanced reactivity, which significantly decreases the temperature range of interaction in the Y₂O₃-Al₂O₃ system in comparison with ceramic synthesis. It is shown that vacuum heat treatment may lead to reversible transformation of the YAG crystal structure from cubic to tetragonal.     

Random replicators with high-order interactions

We use tools of the equilibrium statistical mechanics of disordered systems to study analytically the statistical properties of an ecosystem composed of N species interacting via random mutual interactions, as well as via deterministic self-interactions of order p>/=2. We show that the main effect of increasing the order of the interactions among the species is to make the system less competitive, in the sense that the fraction of extinct species is greatly reduced. In addition, we find that for p>2 there is a threshold value which gives a lower bound to the concentration of the surviving species, preventing then the existence of rare species and, consequently, increasing the robustness of the ecosystem to external perturbations.     

Temperature dependences of the optical characteristics of ZnMgSe single crystals

The temperature dependences of the refractive index of wide-gap Zn_{1 − x} Mg _x Se (x ∼ 0.5) single crystals are measured. It is established that, in the temperature range 300–530 K, the value of dn/dT for such crystals grown by the vertical Bridgman method is 9.31 × 10⁻⁵ at λ = 0.63 μm and 5.29 × 10⁻⁵ at λ = 10.6 μm. The obtained values of the coefficients dn/dT for hexagonal Zn_{1 − x} Mg _x Se single crystals are close to the corresponding values for cubic ZnSe single crystals and are much lower than those for hexagonal CdS. Examples of practical application of single crystals of the ZnMgSe substitutional solid solution as a thermostable material for polarization optical elements in the IR range are given.