Luminescence studies on zeolite crystals

Observations and results on studies of photoluminescence in zeolites are reported. Excitation and emission spectra for natrolite, scolecite and thomsonite are illustrated. Peak positions in spectra are summarised in tabular forms. It is shown that principal emission band in natrolite and heulandite increases with increase in iron percentage in them. Absence of this situation in stilbite is explained in terms of Na⁺ or K⁺ centre killers. It is established that photoluminescence observed in the present case is due to de-alumination of zeolites under study.     

The effect of divalent impurity state on the microhardness of NaCl crystals

The microhardness and bulk density variations with annealing temperature have been measured in NaCl crystals doped with Ca²⁺, Cd²⁺, Ni²⁺, and Pb²⁺. Both characteristics are not affected by coherent metastable precipitation while incoherent precipitation produces either hardening or softening depending on the impurity phase type (stable or metastable). The results support the suggestion that aggregates and coherent precipitates are cut through by dislocations whereas incoherent precipitates are by-passed via the Orowan looping mechanism.     

Luminescence during release of pressure in X-ray irradiated alkali halide crystals

Emission of light is found during application as well as release of uniaxial pressure in X-ray irradiated alkali halide crystals. As unpinning of dislocations can not take place, however, dislocations can move in backward directions during release of the pressure; the emission of light during release of pressure gives a direct support that the interaction of moving dislocations with F-centers is responsible for the emission of light during deformation of colored alkali halide crystals.     

Luminescence of GeO2 glass, rutile-like and α-quartz-like crystals

The luminescence of GeO₂ rutile-like crystals was studied. Crystals were grown from a melt of germanium dioxide and sodium bicarbonate mixture. Luminescence of the crystal was compared with that of sodium germanate glasses produced in reduced and oxidized conditions. A luminescence band at 2.3 eV was observed under N₂ laser (337 nm). At higher excitation photon energies and X-ray excitation an additional band at 3 eV appears in luminescence. The band at 2.3 eV possesses intra-center decay time constant about 100 μs at 290 K and about 200 μs at low temperature. Analogous luminescence was obtained in reduced sodium germanate glasses. No luminescence was observed in oxidized glasses under nitrogen laser, therefore the luminescence of rutile-like crystal and reduced sodium germanate glass was ascribed to oxygen-deficient luminescence center modified by sodium. The band at 2.3 eV could be ascribed to triplet-singlet transition of this center, whereas the band at 3 eV, possessing decay about 0.2 μs, could be ascribed to singlet-singlet transitions. Both bands could be excited in recombination process with decay kinetics determined by traps, when excitation realized by ArF laser or ionizing irradiation with X-ray or electron beam. Another luminescence band at 3.9 eV in GeO₂ rutile-like crystal was obtained under ArF laser in the range 100-15 K. Damaging e-beam irradiation of GeO₂ crystal with α-quartz structure induces similar luminescence band.     

The association energy of aggregates of divalent impurities and vacancies in NaCl-type crystals (point charge approximation)

The formation and association energies for the different aggregates of divalent metal impurities and cation vacancies are derived by considerations of the sums over all lattice vectors. This gives a simple formula in the point charge approximation if an infinite lattice is assumed. Because of the neglection of the polarization of the ions this method yields a higher value for these energies, as a comparison with the literature for the case of dipoles shows. But nevertheless it is a first possibility for a correct estimation of these energies also for higher aggregates.     

Structure-property relationship for complex-substituted molybdates and tungstates of the scheelite family

A high-precision X-ray diffraction study of crystals with the nominal compositions (Na_0.5Y_0.495Er_0.005)WO₄ and (Na_0.5Gd_0.25La_0.25)MoO₄ and the subsequent refinement and analysis of their compositions showed that an increased number of vacancies at the tungsten sites contributes to the cracking of crystals. It was also found that an associate—a color center (a vacancy at the molybdenum site and an electron localized near the vacancy)—gives the crystals a yellow color. The formation of regularly oriented isostructural domains with coherent boundaries as a bulk defect was observed for the first time for crystals of the scheelite family.     

The crystallisation of alkaline-earth metal tungstates from sodium tungstate melts

This paper presents studies on the crystallisation of alkaline-earth metal tungstates by slow cooling of saturated solutions in sodium tungstate melts at 700 to 1000 °C. Solubility-temperature diagrams were prepared for this temperature range. The effects of variation in the initial crystallisation temperature, cooling rate and metal salt structure on crystal number and size were investigated. Final crystal lengths increased with increase in the initial crystallisation temperature and decreased with increasing rate of cooling. Crystals grown from sodium tungstate melts at any temperature were generally smaller than those grown from lithium chloride melts (at the same temperature): they were similar in size to crystals grown from the metal chloride melts but crystallised at temperatures 150 ° to 250 °C lower.     

Luminescence of GaN single crystals prepared by heating a Ga melt in Na–N2 atmosphere

Colorless transparent prismatic crystals (0.5‐2.0 mm long) and hopper crystals (1.0‐2.5 mm long) of GaN were prepared by heating a Ga melt at 800°C in Na vapor under N₂ pressures of 7.0 MPa for 300 h. The photoluminescence (PL) spectrum of a prismatic crystal at 4 K showed the emission peaks of neutral donor‐bound exciton (D⁰‐X) and free exciton (X_A) at 3.472 eV and 3.478 eV, respectively, in the near band edge region. The full‐width at half‐maximum (FWHM) of (D⁰‐X) peak was 1.9 meV. The emission peaks of a donor–acceptor pair transition (D⁰‐A⁰) and its phonon replicas were observed in a lower energy range (2.9‐3.3 eV). The emission peaks of the D⁰‐A⁰ and phonon replicas were also observed in the cathodoluminescence (CL) spectrum at 20 K. The (D⁰‐X) PL peak of a hopper crystal at 4 K was at 3.474 eV (2.1 meV higher), having a FWHM of 6.1 meV which was over 3 times larger than that of the prismatic crystal. A strong broad band with a maximum intensity around 1.96 eV was observed for the hopper crystals in the CL spectrum at room temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simulation of a defect region in KDP crystals doped with divalent iron ions. Comparison of defects induced by di-and trivalent metals

The crystal structure of Fe^II-doped KDP crystals was simulated with the use of specially developed partly covalent potentials. Different variants of introduction of impurity into the structure were analyzed. The M1 position was found to be more favorable for both isolated divalent and trivalent metal ions. Upon optimization, the coordinates of the Fe^II ion are (0.25, 0.44, 0.125). The Fe^II ions can aggregate to form “clusters” energetically more favorable than isolated defects. It seems that Fe^III ions cannot form aggregates.     

Structure sensitivity of relaxation phenomena in NaCl and KCl crystals doped with some divalent impurities (II) effect of plastic deformation

By studying: 1/the role of dislocations in the origin of the C-ITC-peak in undeformed crystals, 2/ the effect of plastic deformation upon the behaviour of this peak, and 3/ the recovery of the deformation-induced changes of the basic characteristics of this peak (H^C, T_M^C) it has been confirmed that dislocation lines surrounded by the Cottrell-like atmospheres can be considered as polarizable entities related with the high-temperature dielectrical response of as cleaved NaCl and KCl crystals doped with divalent impurities. The Maxwell/Wagner model of polarization effects in heterogeneous dielectrics satisfactory accounts for the majority of the experimental data. The quantitative analysis of the results obtained yields some additional informations concerning the interaction- and motion-parameters of the defects considered.     

The effect of electron irradiation on the luminescence of cadmium and lead tungstates

It is shown that the intensity of luminescence of cadmium tungstate crystals changes nonmonotonically under electron irradiation. At low irradiation doses, an increase in the luminescence intensity can be observed. However, with an increase in the irradiation dose, the luminescence intensity decreases. The effect observed can be explained by the competition of two processes: increase in the number of luminescence centers and nonradiative recombination under the action of irradiation.     

Luminescence characteristics of undoped and Eu‐doped GdCa4O(BO3)3 single crystals and nanopowders

Single crystals of GdCa₄O(BO₃)₃ (GdCOB) pure and doped with Eu concentration of 1 and 4 at% were grown by the Czochralski and micropulling‐down methods. The distribution of Eu ions in GdCOB crystals was uniform. The substitutions of Eu³⁺ in Gd, Ca(1) and Ca(2) cation sites and eventually formation Eu²⁺ have been investigated. The spectroscopic properties of crystals are compared with the properties of nanopowders obtained by sol‐gel method. Radioluminescence spectra of undoped GdCOB crystal show the characteristic emission of Gd³⁺ at about 312 nm, whereas this emission dramatically decreases in Eu‐doped crystals upon X‐ray excitation, as well as in Eu‐doped nanopowders excited in vacuum ultraviolet (VUV) region. The VUV excitation in the range 125‐333 nm for Eu‐doped samples leads to strong emission in red coming from the ⁵D₀ multiplet of Eu³⁺, only. In the photoluminescence decay kinetics of 312 nm emissions substantial shortening and departure for single exponential decay in Eu‐doped samples is clearly observed. Higher Eu doping results in further acceleration of the decay. In undoped GdCOB crystal, the lifetime of the Gd^{3+ 6}P_7/2 multiplet is 2.79 ms. The Eu^{3+ 5}D₀ decay kinetics monitored at 613 nm are rather constant. Numerical fitting of fully exponential curves, reveals lifetimes 2.7 ms for nanopowder and 2.5 ms for single crystal. The results suggest that this material may be used as a red phosphor in plasma display panels in nanopowder form because of strong excitation band of Eu³⁺ luminescence in the 160‐200 nm regions. Contrary to nanopowder sample, such an excitation band, attributed to the Gd³⁺–O^2– charge transfer was not observed in crystal obtained by the micropulling‐down method. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure sensitivity of relaxation phenomena in NaCl and KCl crystals doped with some divalent impurities [I] ITC-spectra characteristic of the as-cleaved state

ITC spectra of NaCl and KCl crystals doped with europium, lead, and nickel (in a wide concentration range) have been measured at temperatures between 120 and 400 K. It has been shown that the spectra taken for as cleaved samples are usually very rich. The origin and behaviour of the high-temperature peaks (above the IV-dipole peak) have been discussed in frames of the SILLARS theory of Maxwell/Wagner-type polarization effects in heterogeneous dielectrics. It is suggested that the peak positioned near room temperature is related with any type of vacancy rich precipitates formed within otherwise homogeneous crystal lattice. The second peak, positioned at still higher temperatures, is a Maxwell/Wagner effect due to dislocations surrounded by impurity-related imperfections in a fashion of the so called Cottrell atmosphere.     

Specific features of twinning in divalent nitrates

The etch patterns similar to those found previously for strontium nitrate Sr(NO₃)₂ were revealed on the surface of barium nitrate Ba(NO₃)₂ and lead nitrate Pb(NO₃)₂. The geometry of these patterns is studied. It is suggested that these etch patterns correspond to the translation twins occurring in divalent nitrates at the Pm3 → Pa3 phase transition.     

一维结构中的发光特性

具有低维结构尤其是一维结构的物质往往具有较特殊或更优异的性质.本文研究了具有一维结构的发光物质的发光特性与微观结构之间的关系,得到了某些规律,即在一维结构中,只有存在相对较短的M-O键才可能引起电荷迁移产生发光;在该化合物中M应为变价元素且低价态具有发光性质;一维键上的能量传输往往具有很高的发光猝灭浓度和可能存在发光中间体.     

Thermodynamic analysis of solute-solvent interactions in the solubility-temperature relations of alkaline-earth metal tungstates and sodium tungstate

This paper presents a study of thermodynamic analysis of the solubility-temperature phase diagrams for solutions of calcium, strontium and barium tungstate in sodium tungstate melts in the temperature range 660 to 1200 °C. At temperatures 1000 °C and above, the solutions were ideal but below 1000 °C the solutions became non-ideal and the non-ideality increased with decreasing temperature. At any mole fraction concentration of the solute the excess free energies of mixing and the activity coefficients increased in the order CaWO₄ > SrWO₄ > BaWO₄, whereas the excess chemical potentials decreased in the order CaWO₄ < SrWO₄ < BaWO₄.     

Effect of Fast Electron Irradiation and Annealing on the Luminescence in GaAs(Zn) crystals. Irradiation-induced 1.26 and 1.39 eV Emission Bands

The effect of 2.2 MeV electron irradiation and subsequent annealings on the photoluminescence in zinc-doped p-type GaAs crystals is studied and analyzed. Rather strong emission bands peaked at hv_m (77 K) near 1.26 eV (induced by electron irradiation) and 1.39 eV (induced by annealing of irradiated crystals) are observed. Evidence is presented that the 1.26 and 1.39 eV emission bands occur due to radiative electronic transitions in As_iZn_Ga and V_AsZn_Ga pairs induced by irradiation and annealing of irradiated crystals, accordingly. The observed variations in the intensities of the 1.26 and 1.39 eV emission bands upon irradiation and subsequent annealings of GaAs(Zn) crystals are explained in terms of irradiation and annealing-induced variations in the amount of 1.26 and 1.39 eV radiative centres resulting from: a) the effective interaction of mobile radiation-induced defects in the arsenic sublattice with zinc atoms leading to the formation of As_iZn_Ga and V_AsZn_Ga pairs; b) the thermal dissociation of As_iZn_Ga and V_AsZn_Ga pairs on individual components.     

AlInGaN合金的发光机制研究

用变温光致发光谱和时间分辨光谱研究了AlInGaN合金的发光机制.实验结果表明,发光强度随时间并不是呈指数衰减关系,而可以用伸展指数哀减函数来描述,表明材料中存在明显的无序.形成这种无序的原因是In组分不均产生的微结构(如量子点).伸展指数衰减函数中弥散指数β不仅不随温度变化,在250K也不随辐射能量变化,表明载流子的弥散过程由局域态之间的跳跃所主导.进一步实验表明局域态在250K时仍然表现出零维特性.     

The crystallisation of alkaline-earth metal tungstates (molybdates,chromates, and sulphates) from metal chloride melts

This paper presents studies on the crystallisation of alkaline-earth metal tungstates, molybdates, chromates and sulphates by slow cooling of solutions in lithium chloride and alkaline-earth metal chloride melts at 600° to 1100°C. Solubility — temperature diagrams were prepared for this temperature range. The effects of solute — solvent interaction, crystallisation temperature range and rate of cooling on crystal form and size were investigated. Final crystal size increases with reduced rate of cooling and with increasing crystallisation temperature; barium sulphate crystallisation from lithium chloride melt is anomalous.