Aggregation and precipitation stages in NaCl:Pb2+ studied by UV absorption

UV absorption measurements during isochronal annealing of homogenised and as-grown single crystals show two distinct stages in NaCl:Pb²⁺ where number density of free impurities reduce. With the help of our earlier light scattering results the low temperature stage has been attributed to aggregation of impurity-vacancy dipoles whereas the stage appearing at the higher temperature is assigned to the precipitation process.     

Metastable precipitation kinetics in NaCl:Pb2+ crystals at room temperature

The RT precipitation kinetics have been studied for the metastable phase in NaCl:Pb²⁺ crystals by precise density measurements. The results obtained seem to be unaccountable in terms of the diffusion-controlled growth of constant or increasing number of particles. The observed process is concluded to be a coalescence (Ostwald ripening). At the late stage the experimental data fit the $\text{t}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}$ law for solute depletion predicted by the asymptotic coalescence theory of Lifshitz and Slezov. A diffusion coefficient of Pb²⁺ in the NaCl lattice and the particle size have been estimated by using the theory.     

Effect of M+2 ions on the luminescence of Pb+2 in NaCl

The effect of additional doping with M⁺² ions on the luminescence of NaCl:Pb⁺² has been systematically investigated. The data show that the same two emissions at 310 and 380 nm are observed for mixed Pb⁺²-M⁺² clusters as for Pb⁺² aggregates. Moreover, the presence of a M⁺² ion associated to a Pb⁺² ion in a mixed cluster shifts the 310 nm emission (predominantly observed for free Pb⁺²-vacancy dipoles) to 380 nm. The results are consistent with the Fukuda's model, involving an emission at 310 nm from tetragonal (T) minima in the adiabatic potential energy surface (APES) and another one from less symmetry (X) minima at 380 nm. The data for NaCl:Pb⁺²:Mn⁺², show that an excitation transfer is taking place from the X minima of the ³T_1u state of Pb⁺² to Mn⁺².     

Absorption spectra of NaCl: Pb2+ at the A-band region

The origin of the absorption spectrum of NaCl:Pb²⁺ at the A-band region has been investigated. Main peaks are observed at 265, 273 and 290mμ in an untreated sample. In addition to the effect of lead concentration and thermal treatments, the luminescence and excitation spectra associated to the various bands have been determined. It is concluded that the 290 mμ band should be a composition of a number of bands corresponding to different types of simple dipole aggregates. The 265 mμ band, as well as the structure appearing in the region 220–260 mμ, is consistent with the existence of PbCl₂ precipitates.     

Calcium precipitation in NaCl monitored by the Eu2+ -fluorescence

Taking into account that the optical spectroscopy of the Eu²⁺ ion is quite sensitive to the crystalline environment in which this ion is located, in the present investigation the fluorescence of a small concentration (8 ppmm) of divalent europium incorporated into a NaCl crystal which was also doped with ≈4500 ppmm of Ca²⁺ has been systematically investigated as a function of different thermal treatments in order to study the calcium-precipitation processes in the host NaCl. The data presented in this paper strongly suggest that the annealing of quenched samples at 200°C produces the incorporation of Eu²⁺ into the stable dihalide phase CaCl₂, as well as into the metastable precipitated CaCl₂-like plate zones parallel to the {111} planes of the NaCl matrix. This fact is associated with the increase in intensity of two overlapping emission bands peaking at 430 and 432 nm. On the other hand, the aging of quenched samples at room temperature produces the growth of three emission bands peaking at 400, 414, and 447 nm. The former two emission bands have been associated with Eu²⁺ embedded into calcium precipitates, the structures of these precipitates being different from that of CaCl₂, while the band at 447 nm has been ascribed to europium ions incorporated into the metastable precipitated CaCl₂-like plate zones parallel to the {310} planes of the NaCl matrix. Some of the characteristics of the different calcium second phase precipitates have been obtained by measuring the crystal field splitting (10 Dq) of the 4f⁶5d configuration of the divalent europium ions when they were located inside them. Values for the 10 Dq splitting were determined from the excitation spectrum of each of the emission bands associated with the different calcium-precipitated phases.     

An interpretation of the optical absorption spectra of Pb3+: KCl and Zn+: NaCl

The low energy optical absorption (O.A.) spectrum of a nsion placed in a local O_h symmetry is discussed within the molecular orbital (M.O.) picture. Such model predicts two dipole allowed charge-transfert transitions and one band, arising from the atomic ns np excitation, which qualitatively explains the experimental O.A. spectra of Pb³⁺ : KCl and Zn⁺ : NaCl. Each of the three observed bands in these cases is reasonably assigned and the possible effects - of the spin-orbit coupling on the ns np transition discussed. Suggestions concerning the interpretation of the O.A. spectrum - of X-irradiated Cd: NaCl are also reported.     

Influence of precipitation processes on the optical spectra of Pb2+ in alkali halides

The influence of the aggregation and precipitation processes of lead ions on optical (absorption and luminescence) spectra have been studied for KBr, KI and RbCl. Bands presumably associated with aggregates and precipitates have been identified and compared with those observed for NaCl and KCl. The thermal annihilation behavior of precipitate bands as well as the effect of precipitation on yield-stress have also been determined. As a consequence, a more complete scheme of the influence of aging processes on optical spectra of lead-doped alkali halides has now become available.     

ESR detection of pressure-induced structural change around Pb2+ and Cd2+ in soda borate glasses

The pressure-induced changes in the coordination structure of Cd²⁺ and Pb²⁺ were examined by measuring the ESR spectra of γ-ray-induced Cd⁺ and Pb³⁺ centers. The s-character of the magnetic electron in the centers increased on densification and was estimated from the analysis of the hyperfine absorption due to a ²⁰⁷Pb³⁺ and ¹¹¹Cd⁺ and ¹¹³Cd⁺. The increase in the s-character means that the strength of both Pb²⁺O and Cd²⁺O bonds was reduced upon densification and that the coordination number of Cd²⁺ and Pb²⁺ increased. The coordination sphere of Cd²⁺, which responds more sensitively than that of Pb²⁺ to the change in chemical environments, was more heavily compressed than that of Pb²⁺.     

EPR spectra of NaCl:V2+ and KCl:V2+ above room temperature

Measurements above 300 K of the temperature variation of EPR spectra in NaCl and KCl, doped with divalent vanadium, are analysed. A new isotropic spectrum in KCl:V²⁺, which appears above 500 K, due to vanadium in cubic surroundings, is reported.     

Positron annihilation in Pb2+ doped KCl single crystals

The angular distribution of annihilation photons (ADAP) has been measured in a pure KCl single crystal and in one doped with Pb²⁺ impurities to concentrations of 100 ppm, 550 ppm and 1050 ppm. In unannealed, impurity doped, crystals the intensity of the narrow component increases with increase of impurity concentration. The narrowing of the ADAP curves is explained by assuming trapping of positrons at impurity induced positive ion vacancies. On annealing, the result on the 550 ppm impurity doped crystal shows that there is a dissolution of impurity-vacancy complexes at nominal concentrations of impurity, and the result on the 1050 ppm crystal shows that there is precipitation of impurity for very high concentrations.     

Effect of hyperfine interaction on the luminiscence of Pb2+ and Bi3+ centers in alkaline-earth chalcogenides

Emission spectra of MgS, CaO, CaS, CaSe and SrS phosphors activated with a Pb²⁺ or a Bi³⁺ ion are measured at 4.2 K. By the analysis of the phonon structure of the ³A_1u → ¹A₁g emission band, it is found that the hyperfine interaction provokes the zero-phonon line on the emission band for the alkaline-earth chalcogenide phosphors activated with a Pb²⁺ or a Bi³⁺ ion.     

Neutron scattering in Ag+ and Na+ β-aluminas

Na⁺ and Ag⁺ β-alumina for various stoichiometries and for various hydration states have been studied by neutron scattering using incident energy of 0.8 meV with a resolution of 20 μeV in the 200–325° C temperature range. From the weak quasielastic signal (QES) observed for the non- stoichiometric (NS) Ag⁺ β-alumina a translational diffusion constant D_t has been estimated at about 10^-5 cm² s^-1 at 200°C. No such QES has been observed for stoichiometric Ag⁺ and Na⁺ compounds. The dehydration of Na⁺ β-alumina has been evidenced by the disappearance of the broad QES above 300°C; correlatively the low frequency inelastic spectrum of Na⁺ of β-alumina is changed.     

Influence of the precipitated phases of Pb++ and Sr++ on the f-coloring behavior of NaCl

The F-coloring efficiencies of NaCl:Pb⁺⁺ and NaCl:Sr⁺⁺ have been measured during stage I as a function of lead and strontium precipitation into the metastable and stable phases, the optical absorption spectrum of the doped crystals was analyzed after each annealing treatment. On the other hand, strontium precipitation was monitored using Eu⁺⁺ as an optical probe taking advantage of the fact that this ion can be incorporated into the Sr-precipitates producing emission bands characteristic of each type of precipitate in which it is embedded. The results indicate that the metastable and stable second phase precipitates of these ions contribute in a quite different manner to the coloring processes.     

White light generation of glass ceramics containing Ba_2LaF_7：Eu~（2＋）,Tb~（3＋） and Sm~（3＋） nanocrystals

The Eu 2＋ /Tb 3＋ /Sm 3＋ co-doped oxyfluoride glass ceramics containing Ba2LaF7 nanocrystals are prepared in the reducing atmosphere.The X-ray difiraction results show that Eu 2＋ ,Tb 3＋ and Sm 3＋ ions are enriched into the precipitated Ba2LaF7 nanophase after the annealing process.It deduces efficient energy transfers from Eu 2＋ to Tb 3＋ and Sm 3＋ and intenses warm white luminescence of the glass ceramics. Comparing with the glass,the luminescence quantum yield of the glass ceramics is also enlarged by about 3 times.This demonstrates the potential white light-emitting diode application of the glass ceramics produced in this letter.     

Phonon scattering by Ni3+ ions in Al2O3

Previous thermal conductivity measurements on Al₂O₃ + Ni single crystals have been extended down to 80 mK. The quantitative analysis shows that the lineshape of the low temperature resonant is determineded by random strains in the crystal, and gives a possible value for the ‘tunneling splitting’ 3Γ of 0.54 cm^?1.     

Time resolved spectroscopy of KCl:Pb2+ excited in the A absorption band

By using a single photon-time correlation technique, time resolved emission spectra of the three time components are investigated in KCl:Pb excited in the A absorption band. The results, obtained in the temperature range 80–300 K, are discussed on the basis of the available theoretical models and of the effect of the charge compensating vacancy.     

Transition moment variation in the A2Σ+ → X2Πi transition of HCl+, DCl+ and HBr+

Relative emission intensities of sixteen bands of HCl⁺ (A²Σ⁺ - X²Π_i), four bands of DCl⁺ (A²Σ⁺ - X²Π_i), and 5 bands of HBr⁺ (A²Σ - X²Π_i) have been made using both ion-beam excitation and microwave discharge sources. Intensities were determined by comparison with computer-generated spectra. Treatment of the data within the r-centroid approximation shows that in HCl⁺ the electronic transition moment decreases strongly at large $\text{r}{}_{\mathrm{v\prime v″}}$ [$\text{Re}\text{α}\text{exp}\text{(?3.6}\text{r}{}_{\mathrm{v\prime v″}}\text{)}\text{for 1.44}\text{A}\text{?}\text{< r}{}_{\mathrm{v\prime v″}}\text{< 1.82}\text{A}\text{?}$] but levels off at shorter $\text{r}{}_{\mathrm{v\prime v″}}$. DCl⁺ data agree quantitatively with HCl⁺. The variation in the HBr⁺ moment is similar, with $\text{R}\text{e}\text{α}\text{exp}\text{[?4.5}\text{r}{}_{\mathrm{v\prime v″}}\text{]}\text{for 1.58}\text{A}\text{?}\text{<}\text{r}{}_{\mathrm{v\prime v″}}\text{< 1.78}\text{A}\text{?}$.     

Optical properties of Cu+ centers in NaCl

Different aggregation-precipitation states of Cu⁺ have been characterized by absorption bands peaked at 305, 350 and 372–383 nm.The absorption bands at 372–383 nm, observed exclusively in the most doped crystal, have been associated with the Z₁₂, Z₃ excitons of CuCl microcrystals incorporated into the NaCl matrix Their positions shift to low energies with increasing concentration, as expected for a decrease in the stress over the precipitate.The Z1₁₂, Z₃ exciton bands of CuCl microcrystals precipitated in NaCl can be observed by the optical absorption spectrum without reaching saturation Therefore, this technique could be an alternative method to studies of CuCl thin-film depositions or reflectivity of CuCl single crystals.The red emission band observed at 600 nm is a long-lived emission (τ? 29 ms) at variance with the behavior reported for the Cu⁺ emission It is related to energy transfer processes from Cu⁺ to Mn²⁺.     

Yields of H+ and He+ at the energy for elastic scattering from H2+3He+ incident on Ag and Pd

Yields, Y, of H⁺ and He⁺ emerging specularly with the energy for single binary elastic collisions have been measured from polycrystalline Ag and Pd surfaces bombarded with mixed monoenergetic (300 < E₀ < 2600 eV) beams of H₂⁺³He⁺ impinging at an angle of 45° from the surface normal. The surfaces were exposed to H₂⁺ at a dynamic pressure of 8 × 10^?3 Pa (6 × 10^?5 Torr) during the measurements. The He⁺ yields from Pd are slightly larger than from Ag, and the H⁺ yields from Pd are 10 to 40 times as large as those from Ag. These results suggest that differences between Pd and Ag in the amount of hydrogen adsorbed and in the character of the hydrogen-metal bond may be responsible for the yield differences through shadowing by, and possibly the sputtering of, adsorbed hydrogen. The Y versus E₀ curves for all four systems have qualitatively the same singly peaked shape which implies that reactive and noble gas ions undergo similar neutralization processes during elastic surface collisions. The ratios of the yields from Ag and Pd do not exhibit the theoretically expected exponential dependence on collision time over the entire range studied, but at the lowest energies the ratios lead to estimates of the difference of neutralization constants which do agree with theory. The potential utility of the large difference in proton yields from Ag and Pd for studying the Ag-Pd alloy system is noted.     

EPR study on Mn2+-Cu2+ and Ni2+-Cu2+ mixed pairs

An EPR study at X- and Q-band frequencies has been made on mixed metal pairs Mn²⁺-Cu²⁺ and Ni²⁺-Cu²⁺ obtained by doping the dimer complex C₅H₅NO CuCl₂ · H₂O. Available experimental data have been confirmed and extended, particularly as regards the characteristic directions of the various magnetic tensors. The qualitative behaviour of the spectra and the somewhat singular values of the magnetic parameters can be well-described on the basis of a theoretical model in which a very strong “cosine” interaction between the total electronic spins is assumed. From the model some interesting new aspects emerge, concerning the relations between pairs and single-ion EPR parameters.