A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Thermodynamic properties and structure of oxyfluorides Rb<Subscript>2</Subscript>KMoO<Subscript>3</Subscript>F<Subscript>3</Subscript> and K<Subscript>2</Subscript>NaMoO<Subscript>3</Subscript>F<Subscript>3</Subscript>

According to the results of calorimetric and structural studies, the Fm{ie1202-1}m phase in K₂NaMoO₃F₃ remains stable at least to 100 K. No ferroelectric transformation assumed earlier has been revealed in a series of Rb₂KMoO₃F₃ samples prepared using various technologies. Only a phase transition of nonferroelectric origin has been observed near 195 K, and its thermodynamic characteristics have been determined. An analysis of the stability of the cubic structure of molybdenum fluorine-oxygen elpasolites-cryolites has been performed in the framework of the hypothesis on strengths of interatomic bonds. The barocaloric effect in Rb₂KMoO₃F₃ has been estimated.     

Micromechanics of polymer fracture

Small-angle x-ray scattering has been used to investigate the formation of embryonic submicroscopic cracks in polymers under a load. The main characteristics of crack formation in various loading regimes are analyzed. It is shown that there is a relation between the submicrocrack concentration and the deformation of the loaded polymer. The principal parameters of crack formation determining the strength properties of the polymer are found to be the transverse dimension of the initial submicrocracks relative to the loading axis, which is determined by the structural heterogeneity of the material, and the submicrocrack concentration in the prefracture state. The principles of the micromechanics of polymer fracture are formulated on the basis of the results of an analysis of the quantitative relationship between these parameters. The dominant role of the surface in the fracture process is demonstrated by comparing the parameters of crack formation in the interior and at the surface of the loaded polymer.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 5, pp. 792–801, September–October, 1974.     

On Automorphisms of Computability Potential Scales for n-Element Algebras

We prove that the filter of computability potentials of semiprimal algebras is preserved under an arbitrary automorphism of the computability potential scale.     

Molecular orientation and the tensile strength of polystyrene

The tensile strengths of uniaxially oriented polystyrene films with molecular orientation determined by IR spectroscopy are compared. Two different orientation methods involving different time and temperature regimes are employed. It is found that the tensile strength is a unique function of the degree of orientation of the molecular segments of the polystyrene carbon chain irrespective of the means used to orient the specimens and increases linearly with increase in the degree of molecular orientation of the polymer. The experimental dependence is in satisfactory agreement with the results of a theoretical analysis.A. F. Ioffe Physico Technical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 4, pp. 612–615, July–August, 1969.     

Atomic mechanism of fracture of solid polymers

Macromolecular chain scission under mechanical stress has been studied by infrared spectroscopy. The dependence of accumulation of chemical bond scissions on temperature T and uniaxial tensile stress σ has been investigated. The rate constant K for bond dissociation under mechanical stress has been found to obey the modified Arrhenius equation: K = K₀ exp{ ? (E_A ? ασ)/RA}. The quantitative connection between the rate constant for bond dissociation and mechanical lifetime τ has been established. Analysis of the experimental data indicates that the strength and mechanical lifetime of polymers is determined by the kinetics of mechanochemical scission of the main chains of polymer molecules.     

Luminescence and electronic excitations in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>: Ce<Superscript>3+</Superscript> crystals

This paper reports on a study of the luminescence emitted by Li₆Gd(BO₃)₃: Ce³⁺ crystals under selective photoexcitation to lower excited states of the host ion Gd³⁺ and impurity ion Ce³⁺ within the 100–500-K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10–15-fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd³⁺ ions and vibrational energy relaxation between the ⁶ I _j and ⁶ P _j levels. It has been shown that radiative transitions in Ce³⁺ ions from the lower excited state 5d ¹ to ² F _5/2 and ² F _7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.