Symmetry properties of the lattice vibrations of cadmium diarsenide crystals

The symmetry of the normal vibrations of crystals with the CdAs₂ lattice is established. Selection rules are derived for the infrared absorption and Raman scattering of light; also derived are the selection rules for the inelastic scattering of slow neutrons.     

Photoluminescence of calcite crystals of different origins

We have studied the photoluminescence of calcite crystals. In the blue region of the photoluminescence spectrum of calcite crystals obtained from Siberia (Russia) and from Saaremaa Island (Estonia), three strongly overlapping luminescence bands due to intrinsic defects are observed. Luminescence due to impurities in the crystals are hardly detectable. The experimentally measured time dependence of the luminescence intensity for the indicated luminescence bands is compared with the dependences obtained as a result of a calculation based on a proposed model for the luminescence center. Better agreement between experiment and calculation is achieved if the model of the luminescence center includes a metastable level with electron ejection energy of 4 meV; the characteristic time for the radiative transition is 1.3 nsec. Studying the time dependence of the luminescence at different wavelengths within the indicated bands allows us to conclude that the photoluminescence (three bands) is due to one type of luminescence center. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 498–501, July–August, 2006.     

Thermally and optically stimulated radiative processes in LiBaF3 crystals

In LiBaF₃ crystals both valence–core transitions (5.4–6.5 eV) and so-called self-trapped exciton luminescence (about 4.3 eV) are important for practical application. Here, we present a study of 4.3 eV luminescence under photo- and thermostimulation after X-irradiation of undoped LiBaF₃ crystals at various temperatures. Optically stimulated luminescence as a result of electron recombination with both self-trapped holes and holes localized at some defects, were observed after X-irradiation below 130 K and that of electron recombination with defect-localized holes was observed after X-irradiation above 130 K. The spectra of thermo-stimulated luminescence (TSL) contain a broad band about 4 eV related to the electron (high-energy side) or hole (low-energy side) recombination depending on TSL peak temperature.     

Photoluminescence of AgGaTe2 single crystals

The temperature dependence of the photoluminescence spectra of AgGaTe₂ single crystals obtained by the Bridgman-Stockbarger method is investigated within the temperature range 10-300 K. The emission bands associated with donor-acceptor recombination and free and bound excitons are detected. The bonding energy of the free exciton and the forbidden gap of the crystals at 10 K are calculated. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 222–224, March–April, 2000. This work is partially sponsored by the British Royal Society, INTAS (grant No. 634), and EPSRC (grant GR/L 62757).     

Electron spin resonance studies of VO2+ in cadmium maleate dihydrate single crystals

Electron spin resonance studies were carried out on VO²⁺ in cadmium maleate dihydrate single crystals at 303 K. The spin hamiltonian parameters were evaluated. The results lead us to conclude that like Cu²⁺, VO²⁺ enters the lattice interstitially.     

Creep of cadmium single crystals

The creep of cadmium single crystals has been investigated in the temperature range of 78 to 297 K. It has been shown that the transient creep of Cd single crystals is described by a time dependence of the creep strain rateå t ^–m wherem is a function of the resolved shear stress and temperature. The activation areaA* depends on the resolved shear stress,, A ^{* –r} wherer=1 for room temperature andr 1·4 for 204 and 78 K. The activation enthalpy does not practically depend on the resolved shear stress in the whole region of the resolved shear stress measured and with increasing temperature it increases roughly withT ². For temperatures above 200 K the nonconservative jog motion is proposed to be the dominant thermally activated process.The authors are very grateful to Dr. P.Kratochvíl for valuable discussions.     

Photoluminescence of LLGG:Cr crystals under high pressure

We present photoluminescence spectra of La_2.7Lu_2.29Cr_0.01Ga₃O₁₂ and La_2.32Lu_2.59Cr_0.02Ga_3.07O₁₂ doped with Cr³⁺ obtained at high hydrostatic pressure up to 220 kbar, applied in a diamond anvil cell at 20 K and room temperature. In both materials we have obtained a pressure-induced ⁴T₂–²E electronic cross-over. On the basis of the low-temperature R line luminescence at pressures above 100 kbar we have distinguished two dominant Cr³⁺ sites: and β, existing in both materials, and one minor site δ, that exists only in La_2.32Lu_2.59Cr_0.02Ga_3.07O₁₂. The pressure-induced shifts of the R₁, R_1β and R_1δ lines as well as the pressure shift of the broad band related to the ⁴T₂→⁴A₂ transition in both materials have been estimated.     

Photoluminescence of PbFCl and PbFBr single crystals

The luminescence properties of single crystals of PbFCl and PbFBr at 4.2 K under ultraviolet irradiation are presented for the first time. In PbFCl three and in PbFBr four emission bands have been observed. The red bands are ascribed to Pb⁺ centres. Direct exciton recombination is absent in both compounds.     

Photostability of luminescent water-soluble cadmium selenide nanocrystals with chemical surface modification

We have developed a method for solubilization of hydrophobic CdSe/ZnS nanocrystals of the core/shell type, obtained by high-temperature synthesis in coordinating organic solvents. The method is based on chemical modification of the surface of the nanocrystals with hydrophilic organic mercapto compounds. We have observed that long-chain mercaptoundecanoic acid molecules effectively protect the surface of CdSe/ZnS nanocrystals in water, increasing (compared with short-chain molecules) the photostability of the nanocrystals. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 506–509, July–August, 2006.     

Silver diffusion in cadmium sulfide single crystals

An experimental study was made of the penetration of silver by the thermal-diffusion method into cadmium sulfide. The silver was deposited on the samples by thermal evaporation in vacuum. Optical and electrical-probing methods were used to study the rates of surface and bulk diffusion at various temperatures. The temperature dependence of the diffusion coefficient was established for the purity used. After the penetration of the silver impurity, the low-resistivity ( 0.1 · cm), nonphotosensitive cadmium sulfide samples displayed high resistivity ( 10⁶-10⁸ · cm) and photosensitivity.Translated from Izvestiya VUZ. Fizika, No. 7, pp. 12–16, July, 1970.     

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.     

Photoluminescence spectra of the AgGaTe2 single crystals doped with hydrogen

The photoluminescence spectra of single crystals of the ternary AgGaTe₂ compound obtained by the Bridgman-Stockbarger method are studied before and after bombardment by the 100-eV hydrogen ions of dose 10¹⁵ cm^?2. The spectra were detected in the temperature range from 10 to 300 K at various powers of laser excitation. The spectra exhibit emission bands related to the donor-acceptor recombination, free, and bound excitons. The intensity of these bands in crystals doped with hydrogen is higher than that in undoped crystals. This is explained by the passivation of defects in a crystal lattice by hydrogen. The binding energy of a free exciton and the band gap in AgGaTe₂ crystals are calculated at 10 K.