P. Camagni

Fluorescence line narrowing spectroscopy of a lead germanate glass doped with Eu3

Abstract Site selective luminescence spectra of Eu 3+ in a lead germanate glass were measured at 150 K as a function of the excitation wavelength by using laser-induced fluorescence line narrowing (FLN) techniques. The energies of the Stark components of the 7 F 1 and 7 F 2 states were obtained by Gaussian fitting of the spectra. Assuming a C 2 v site symmetry, a crystal field (CF) analysis was carried out. The behaviour of the major CF ratios B 44 /B 40 versus −B 22 /B 20 seems to agree with the trend predicted by the model proposed by Brecher and Riseberg for the co-ordination of Eu 3+ in oxide glasses. No effect due to the occurrence of an incipient amorphous phase separation is observed.

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.

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.

Characterization of ferroelectric micro-regions in KTaO3:Na

Abstract Low-frequency Raman scattering in the mixed perovskite KTaO3 (Na 12%) is shown to give information on the correlation length of the local order parameter which is in agreement with that obtained by combined measurements of linear birefringence and second harmonic generation. Accordingly, the behavior of this system is interpreted in terms of a frustrated structural transition, characterized by freezing of the polar domain dimension (300A) in the low temperature region.

Raman study of Cu-doped K1−xLixTa1−yNbyO3

Abstract The temperature evolution of TO1 Raman spectra was investigated in single crystals of K1−xLixTa1−yNbyO3:Cu with different Li and Nb amounts. Auxiliary observations of TO4 spectra were also performed. In a sample with x = 0·002 and y = 0·025 and containing 190 ppm of Cu, the results indicate a clear-cut transition to an ordered phase around 55 K, in good agreement with previous findings from dielectric measurements in the same crystal. However, no evidence was found of a role of Cu in this structural modification. Samples with x = 0·01 and y = 0·003 revealed features of an order-disorder phase transition around 45 K, irrespective of the presence of Cu.

Photocurrents in the polar phase of K1−xLixTaO3

Abstract Experiments on photoconductivity and thermally stimulated currents were performed on Li-doped KTaO3 single crystals. The large enhancement of conductivity, caused by UV excitation at low temperature was found to correlate with a subsequent release of trapped charge between 30 and 40 K. No corresponding release was shown by pure KTaO3, consistently with a very low yield of photoconduction. It is concluded that the presence of hole traps, with the consequent quenching of electron-hole recombination, is at the origin of the photoconductive response of K1−xLixTaO3.

The polar transition of Li-doped and Nb-doped KTaO3: Comparative analysis from hard-phonon raman spectra

Abstract Raman investigations have been performed on K0.984Li0.016TaO3 and KTa0.976Nb0.024O3 single crystals, in order to follow the peculiarities of non-ferroelectric phonons, which may be indicative of a different evolution of microscopic order during the polar transition. Specific features in the Nb case confirm a ferroelectric behaviour, whereas in the Li-doped sample the results are in agreement with a dipole glass transition.

Structural effects of Li and Nb codoping in a KTaO3 matrix: A Raman investigation

Abstract Raman investigations on single crystals of K1−xLixTa1−yNbyO3 were performed to identify structural effects arising from the simultaneous presence of off-centre Li and Nb ions in the incipient ferroelectric KTaO3. Two soft-mode transitions were evidenced at 45K and 30K in a sample with x = 0.0014 and y = 0.024 after both zero-field and field cooling. These temperatures are close to the maximum and to the extrapolated Curie temperature for nonrelaxing permittivity reported in the same system. A sample with x = 0.0028 and y = 0.013 displayed a ferroelectric transition just below 30K only after field cooling, also being consistent with permittivity data.