E. Giulotto

Local structure, microstructure and glass properties

X-ray small angle scattering measurements on silicate and borate glasses are described and the contrasting effects of inhomogeneities such as density fluctuations and alkali microsegregation are discussed. The consequences of microstructure for ionic diffusion, the mixed alkali effect (MAE), thermal expansion, glass fracture and polishing are considered, making use of some recent results of molecular dynamic simulations and oxygen X-ray absorption fine structure measurements.

Light-induced defects in KTaO3

Photoconductivity (PC), thermally stimulated conductivity (TSC), photoluminescence (PL), thermoluminescence (TL), and electron spin resonance (ESR) measurements have been made on single crystals of potassium tantalate over the temperature range 4.2–290 K. We revealed two sorts of O− shallow hole centers which are responsible for the two temperature regions of PL and PC enhancement: T<70 K and 100–150 K. Both O− centers were identified by their ESR spectra. While at low temperatures PL and PC have a rather intrinsic origin, i.e., they do not depend essentially on the sort or quality of crystals, at 100–150 K both quantities strongly depend on the defect content and vanish in well-oxidized crystals. We show that O− centers serve as radiative electron–hole recombination centers. Their energy levels are situated at 0.08 and 0.16 eV above the top of the valence band. Measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18–30 K and 65–70 K. There is a good correlation betw...

Origin of the blue fluorescence in Dominican amber

We report on optical absorption, fluorescence, and time-resolved fluorescence measurements in Dominican ambers. The “blue” variety reveals an intense fluorescence emission in the visible wavelength region, between 430 and 530nm, with spectral features typical of aromatic hydrocarbons. On the contrary, the “red” and “yellow” varieties have a much weaker and featureless emission. The data for blue amber, including fluorescence lifetime, allow the identification of perylene as the compound responsible for its distinctive showy fluorescence.

New remarks on the high-polarizability perovskite-related material CaCu3Ti4O12

We present results from Raman scattering, EPR and static magnetization measurements in CaCu3Ti4O12, a perovskite-related material characterized by a colossal dielectric permittivity ϵ0. The investigated samples were pure and partially substituted in the octahedral Ti sites by other transition metals. The results from the pure material are interpreted in terms of the presence of delocalized Copper-holes. Substitutions in the Ti sites drastically change the magnetic properties and, most evidently in the case of Cr, also affect the Raman spectra. These effects correlate with the known suppression of the colossal ϵ0 after the same kind of substitutions.

Cooperative phenomena induced by Nb admixture in K1-xLixTaO3

Raman scattering observations were performed on samples of K1-xLixTa1-yNbyO3, with x in the range 0.6-1.0 mol% and y in the range 0.2-0.3 mol%. Concentrations of Li and Nb, determined by analytical methods, were below the known thresholds at which these substituents can separately induce a ferroelectric transition in the quantum paraelectric KTaO3. The temperature evolution of TO1 and TO4 spectra was monitored. The results show a critical behaviour below 50 K, characterized by an abrupt increase of first-order scattering strengths and by spontaneous splitting of the non-softening TO1 mode. From our data, we infer the occurrence of a ferroelectric phase transition of order-disorder character. This proves that small additions of Nb enhance the tendency of the Li subsystem towards cooperative ordering in the KTaO3 matrix.

Shallow levels and photoconductivity in K1-xLixTaO3

We present the results of experiments performed on K1-xLixTaO3 as well as on Nb-doped and nominally pure KTaO3 single crystals, in which we compared observations of thermally stimulated currents and photoconductivity. In the Li-doped compounds, the large enhancement of conductivity caused by ultraviolet excitation at low temperature was found to correlate with the filling of shallow trapping centres, giving intense charge release below 30-40 K. No sign of a corresponding release was shown by pure or Nb-doped KTaO3, consistently with a very low yield of photocurrent which one observes in these cases. The depth of the trapping levels in K1-xLixTaO3 crystals was found to be between 50 and 70 meV. On the basis of past models and recent calculations, these levels can be identified with hole traps, originating from the perturbation of O2- states at the top of the valence band. They are a plausible source of enhanced photoconductivity, via the quenching of electron-hole recombination.

Optical study of the transition of doped potassium tantalate

Abstract We report on recent findings obtained through Raman scattering, second harmonic generation and linear birefringence on the polar phase of doped KTaO3. At low temperature, the polar coherence length ranges from a few nanometers to 20 nm for 1.6% and 6% Li doping, respectively and shows no criticality. Comparable figures are found for the case of 12% Na doping. The evolution of low energy Raman spectra and the behaviour under a poling field provides further evidence of phenomena which limit the growth and cooperative ordering of polar microregions.

Phase transitions, related properties and possible applications of (K, Li)(Ta, Nb)O3 crystals

Abstract It is shown that doping the highly polarizable KTaO3 perovskite simultaneously by Li and Nb (K1−xLixTa1−yNbyO3, KLTN) gives rise to new impressive dipole ordering effects, very unusual physical properties and strong responses with respect to doping parameters. Besides, KLTN has outstanding perspectives of application in electrically controlled holographic and compositionally graded pyroelectric devices.

Photoluminescence processes in k−1x Li x TaO3

Abstract A study of blue-green photoluminescence of K1-x Li x TaO3 single crystals, under UV irradiation, was performed in the range 15–20 K. It was shown that the emission is strongly enhanced and thermally more stable with respect to pure KTaO3, up to ∼40 K. Above this point the yield of emission decreases abruptly with an Arrhenius-like behaviour controlled by an activation energy of 70–100 meV. After low temperature excitation, a thermoluminescence spectrally very similar to PL and giving rise to a glow peak around 30K is also observed. Correlations with recent results on photoconductivity and thermally stimulated currents are illustrated. An argument in favour of close-pair recombinations between Ta4+ and O− centers is proposed.

An all optical mapping of the strain field in GaAsN/GaAsN:H wires

GaAsN/GaAsN:H heterostructures were made by an in-plane selective hydrogen incorporation controlled by H-opaque metallic masks. The strain field and hydrogen distributions in GaAsN micro-sized wires thus obtained have been mapped by an all optical procedure that combines micro-Raman scattering and photoreflectance spectroscopy. The strain field is related to the formation of N-H complexes along the hydrogen diffusion profile with an ensuing expansion of the GaAsN lattice whose patterning generates an anisotropic stress in the sample growth plane. These results highlight a powerful non-invasive tool to simultaneously determine both the H diffusion profile and the related strain field distribution.