R. Capelletti

High-efficiency SiO2:Ce3+ glass scintillators

We present the effect of a rapid thermal treatment (RTT) at high temperature (1800 °C) on the radio-luminescence properties of Ce-doped SiO2 glasses prepared by the sol–gel method and previously densified at 1050 °C. Cerium concentrations ranging from 0.05 up to 1 mol % were considered. We found that, for all concentrations, the RTT induces a strong increase of the Ce3+ radio-luminescence efficiency; the x-ray-induced luminescence intensity of the SiO2:0.1% Ce is about twice that of Bi3Ge4O12. The decay time of the scintillation response, evaluated as ≈50 ns, is not affected by RTT. Infrared absorption measurements indicate that the luminescence increase cannot be related to significant release of OH groups during RTT. The conversion of Ce4+ ions into Ce3+ ions can also be ruled out since an increase of about 20% of the intensity of the 4.8 eV optical absorption band related to Ce4+ was observed after RTT. The occurrence of dissolution of rare-earth aggregates is suggested.

Superposition-model analysis of rare-earth doped BaY 2 F 8

The energy level schemes of four rare-earth dopants (Ce 3+ , Nd 3+ , Dy 3+ , and Er 3+ ) in BaY 2 F 8 , as determined by optical absorption spectra, were fitted with a single-ion Hamiltonian and analysed within Newmans Superposition Model for the crystal field. A unified picture for the four dopants was obtained, by assuming a distortion of the F m ligand cage around the RE site; within the framework of the Superposition Model, this distortion is found to have a marked anisotropic behaviour for heavy rare earths, while it turns into an isotropic expansion of the nearest-neighbours polyhedron for light rare earths. It is also inferred that the substituting ion may occupy an off-center position with respect to the original Y 3+ site in the crystal.

Spectroscopic properties of ZnWO4:Fe single crystals

Abstract The incorporation and charge of iron in ZnWO4:Fecrystals were investigated by absorption spectrometry. The UV bands at 350 nm and the visible band at 460 nm have been found to be related to Fe3+ and Fe2+ ions respectively. Both ions are mainly in substitutional Zn positions. The infrared absorption at 3445 cm−1 is related to Fe3+ OH complexes. The OH− ions compensating the excess charge of Fe3+ are probably situated in interstitial positions.

Growth of SnO2 nanocrystals controlled by erbium doping in silica

We investigate the effects of erbium doping on SnO(2) nanoclustering in Sn-doped silica. Vibrational spectroscopy data from Raman and infrared absorption measurements show nanostructuring effects on the SnO(2) nanophase. Ultraviolet absorption spectra evidence a gap shift ascribable to size-dependent quantum confinement, also suggesting a role of erbium doping in determining cluster sizes and the amount of localized states on the nanophase boundary. Transmission electron microscopy confirms and details the spectroscopic data. As a result of these measurements, we find that the nanocrystal size distribution becomes narrower, increasing the erbium concentration, while the density of localized states at the nanocrystal surface decreases. The distribution of erbium ions among the possible environments is then examined through simultaneous spectroscopy of luminescence excited by nanocrystal-to-erbium energy transfer and the absorption of nanocrystal luminescence by erbium ions. This analysis shows that erbium behaves as an extrinsic nucleation centre of the SnO(2) nanophase at low doping levels, whereas at high concentrations it modifies the matrix, hindering the growth of SnO(2) crystals and passivating the interface.

Spectroscopic investigation and crystal field modelling of Dy3+ and Er3+ energy levels in yttrium aluminium borate (YAB) single crystals

High resolution (0.04 cm−1) absorption spectra of Dy3+-doped YAl3(BO3)4 (YAB) single crystals were measured by Fourier transform spectroscopy in the spectral (2000–23 000 cm−1) and temperature (9–300 K) ranges. Samples with nominal 1 and 4% Dy/Y molar ratios were studied. Dy3+ transitions from the 6H15/2 ground state to the 6H13/2, 6H11/2, 6H9/2+ 6F11/2, 6H7/2+ 6F9/2, 6H5/2, 6F7/2, 6F5/2, 6F3/2, and 4F9/2 excited states were analysed. A small (~3.3 cm−1) splitting between the first two sublevels of the 6H15/2 ground manifold was detected. The experimentally determined energy levels were fitted with a single-ion Hamiltonian and the crystal-field parameters were obtained. The same procedure was applied to analyse the previously published high resolution spectra of YAB:Er3+, leading to a reliable unified picture for the two dopants.

Basic spectroscopic properties of bismuth tellurium oxide, Bi2TeO5

Optical absorption measurements in the visible and infrared and reflectivity measurements in the visible and ultraviolet are reported for single crystals of bismuth tellurium oxide (Bi2TeO5). Strong dependence of these measurements on the light polarization is found for this recently grown non-linear material. Deviation from the Urbach rule (see Appendix) is believed to result from non-stoichiometry; approximate values of the Urbach parameters are given for the [0 1 0] polarization.

Vibrational spectroscopy of OH-related groups in Ce3+- and Gd3+-doped silicate glasses

FTIR spectroscopy was applied in the 500–14,000 cm � 1 range to detect OH vibrational modes (stretching, bending, combination, first and second overtone) in Ce 3+ - and Gd 3+ -doped silicate glasses, prepared by means of the sol–gel technique and aimed to applications as scintillators. The presence of OH groups, belonging either to silanol (Si-OH) or to H2O should be limited, since they could cause radiative emission quenching. The samples, doped with Ce 3+ or Gd 3+ (0.05–3% m.f.), were submitted to thermal treatments at 4501C, 7501C, and 10501C, to remove water and organic groups for producing a dense glass. A large amount of H2O and Si-OH is still present in samples annealed at 4501C and 7501C, while only a reduced quantity of Si-OH (evaluated as o1% m.f.) is detected after annealing at 10501C, i.e. in the final dense glasses (for all concentrations of both dopants). r 2002 Elsevier Science B.V. All rights reserved.

Linkage isomerism of two benzoato-copper(I) complexes containing bis(diphenylphosphino)methane bridging ligands. Crystal structures, TSDC measurements and reactivities of [Cu2(μ-dppm)2(O2CPh)2] and [Cu2(μ-dppm)2(O2CPh)(μ-O2CPh)]·H2O

Here we report the molecular structures and reactivities of the two complexes [{Cu(O2CPh)dppm}2] (A) and [{Cu(O2CPh)dppm}2]·H2O (B), containing isomeric dibenzoato copper(I) moieties doubly bridged by dppm ligands (dppm=bis(diphenylphosphino)methane). In the isomeric unit [{Cu(O2CPh)dppm}2] of B, the two metal atoms are also bridged by one of the two benzoato ligands and a significant shortening of the transannular Cu…Cu distance (2.994(2) A in B versus 3.359(2) A in A) is observed. The eight-membered Cu2P4C2 ring adopts an approximate chair conformation in A and a saddle conformation in B. The structures of A and B have been determined by single crystal X-ray diffraction method. The crystals of A are triclinic, space group P, with Z = 1 in a unit cell of dimensions a = 11.841(2), b = 11.366(2), c = 10.769(2) A, α = 100.68(2), β = 101.45(2) and γ = 93-37(2)°. The crystals of B are monoclinic, space group P21/a, with Z = 4 in a unit cell of dimensions a = 21.295(3), b = 13.295(2), c = 22.068(3) A and β = 114.72(2)°. Final R was 0.032 (2553 reflections) for A and 0.058 (3909 reflections) for B. Bulky anions react with A and B metathetically removing one of the benzoato groups or, when coordinating, both of them. The characteristics of the new anions modulate the conformation of the Cu2(dppm)2 unit. Thermally stimulated depolarization current (TSDC) measurements have also been performed on B to study the electric polarization phenomena induced by reorientation of dipole moments of the crystallization water molecules in the temperature range 100 ⩽ T ⩽ 350 K. The reorientation ability of the dipole water molecules is probably favoured by the dynamic disorder of the atoms, which surround the water molecules in the crystal lattice.

Thermally Stimulated Depolarization Studies of Ionic Solids

Relevant physical properties of crystalline solids (for instance electric resistivity, color, mechanical strength, etc.) and applications (integrated electronics, laser etc.) are determined by the presence of lattice defects (vacancies, interstitials, impurities and complexes built by them). According to the specific nature of defects, suitable techniques have been developed. In ionic solids, defects often bear an electric charge (cation and anion vacancies, interstitial ions, aliovalent impurities). As a consequence of the Coulomb interaction between defects of opposite charge, complexes may be formed, which exhibit an electric dipole moment. Other defects, even in non ionic solids, may he endowed by their own dipole moment. Hence electrical methods such as dielectric losses (tgδ), isothermal depolarisation currents and more recently ionic thermocurrents (ITC) are suitable to monitor such defects. This last method, introduced by Bucci and Fieschi in 1964, deals with the detection and analysis of thermostimulated depolarization currents arising from ion redisplacements in solids whose electronic conductivity is neglegible (1,2). In a sense ITC enters in the wide class of methods based on the thermal release of stored energy, as for instance thermoluminescence (TL), thermally stimulated currents (TSC) and thermally stimu lated depolarization currents (TSDC). ITC deals with TSDC as well, but is restricted to polarization mechanisms, in which only ion redisplacements are involved ruling out a wide class of phenomena which occur in electrets such as carrier injection, electron and/or hole trapping. In this way ITC is more descriptive and specific term than the more general TSDC, which is used by some authors. Due to this restriction and its successful application to simple model systems, such as ionic crystals, it has opened the way for a quantitative study of dipolar processes also in more complex systems of technological interest.

Rare-Earth Doped Sol-Gel Silicate Glasses for Scintillator Applications

Radio-, photo- and thermally stimulated—luminescence (RL, PL, TSL) measurements have been performed on SiO2 sol-gel glasses doped by 0.1 mol% Ce and 3 mol% Gd, and on (0.1 mol% Ce, 3 mol% Gd) co-doped samples. Ce3☎ 5d-4f emission peaking at about 2.7 eV has been observed in the RL of SiO2: 0.1 mol% Ce, while the typical 6P-8S emission of Gd3☎ centred at 3.97 eV has been detected in SiO2: 3 mol% Gd. The co-doped sample displays both 5d-4f Ce3☎ and 6P-8S Gd3☎ emissions with reduced intensities with respect to those observed in the singly doped glasses. Moreover, in co-doped glasses the PL time decay patterns of both rare earth ions show a non exponential dependence and are significantly shortened. To explain such an effect non radiative de-excitation of both RE ions excited states involving energy transfers to defect levels is suggested. Bidirectional Gd3☎ ↔ Ce3☎ energy transfers could also occur. Complementary TSL measurements put in evidence the existence of broad glow peaks at about 100 K and 220 K. The TSL spectra feature the RE ions emissions.