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KTaO 3 with small amount ( x < 0.01) of Li and simultaneously diluted by Nb (K 1 m x Li x Ta y Nb 1 m y O 3 , KLTN) reveals original dipole ordering structural effects. Small Nb co-dopings ( y < 0.007) enhance the trend of the Li + off-centers related subsystem towards cooperative ordering and order-disorder phase transitions. At 0.01 < y < 0.05 the intrigue features of displacive and order-disorder transitions reveal. The most interesting features are the appearance of soft-mode driven ferroelectric transitions accompanied by a Li + off-centers system freezing followed by a reentrant dipole glass-like phase formation, and the emergence of a narrow transition with giant magnitude of l '(T). Evidences of reentrant dipole glass formation were observed in KTa 0.981 Nb 0.019 O 3 too. A possible model developed is based on the Li-Nb polar clusters, local configuration instabilities and superparaelectricity conceptions.

We studied the soft phonon Raman spectra in samples of K 1 m x Li x Ta 1 m y Nb y O 3 , with x = 0.0015, y = 0.046 and x = 0.0024, y = 0.01 mol% in order to explore the complex structural effects arising in KTaO 3 matrix, weakly doped by Li ( x < 0.01) and simultaneously moderately doped by Nb (0.01 h y h 0.05). The results provide new evidence that the character of the phase transitions in KTaO 3 moderately doped with Nb, can be markedly modified by the addition of small amounts of Li. In particular K 0.9976 Li 0.0015 Ta 0.954 Nb 0.046 O 3 exhibits a complex scenario of structural transformations, that differs from the known behaviour in KTN close to the multicritical point.

AbstractRaman investigations on single crystals of K1−xLixTa1−yNbyO3were 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.

AbstractExperiments on photoconductivity and thermally stimulated currents were performed on Li-doped KTaO3single 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.

AbstractA study of blue-green photoluminescence of K1-xLixTaO3single 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.

Zirconia-based electrolytes, stabilized either with yttrium oxide or calcium oxide, were prepared in 100 to 200 µm thick layers by plasma spraying and densified by high-temperature vacuum sintering. The structure and the microstructure were investigated by optical microscopy, scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The results are compared with the data we obtained on the powders used for plasma spraying and on single crystals. In the zirconia-yttria system, dense and fully stabilized zirconia films with structural properties similar to the corresponding single crystals were obtained. On the other hand, cracks and deformations were observed on calcia-stabilized films. This phenomenon is explained by the dramatic increase of monoclinic phase content due to the preferential evaporation of calcium oxide which occurs during high-temperature vacuum sintering.

In this paper we report some new measurements of field-effect modulated absorption for real surfaces of germanium. The spectra, taken for various values of the surface potential, show a broad band (α) at 0.39 eV and a narrow band (β) at 0.44 eV superimposed to a large background due to free-hole or free-electron absorption. The experimental data suggest the hypothesis that the «β»-band could arise from the change of the optical absorption of the OH vibrational line at 0.44 eV under the effect of the applied electric field. On the contrary the «α»-band seems due to electronic transitions from surface states to the conduction band. In questo lavoro si riportano nuove misure, sulle superfici reali del germanio, di assorbimento ottico modulato da effetto di campo. Gli spettri, presi per vari valori del potenziale di superficieus0, presentano una larga banda (α) a 0.39 eV e una stretta banda (β) a 0.44 eV sovrapposte ad un continuo dovuto ad assorbimento di cariche libere. I dati sperimentali suggeriscono l’ipotesi che la banda (β) sia dovuta alla variazione di assorbimento ottico sotto l’azione di un campo elettrico della riga vibrazionale del gruppo OH. Al contrario la banda (α) sembra dovuta a transizioni elettroniche da stati di superficie al fondo della banda di conduzione. В зтой статье мы сообшаем о некоторых новых иэмерениях влияния поля, модулируюшего поглошение для реальных поверхностей германия. Спектры для раэличных эначений поверхностного потенциала обнаруживают щирокую эону (α) при 0.39 зВ и уэкую эону (β) при 0.44 зВ, наложенные на больщой фон, обусловленный поглошением свободных дырок или свободных злектронов. Экспериментальные данные предполагают гипотеэу, что эона «β» может воэникать иэ иэменения оптического поглошения для вибрационной линии OH при 0.44 зВ, при действии приложенного злектрического поля. Напротив, эона «α», кажется, обусловлена злектронными переходами иэ поверхностных состояний в эону проводимости.

20 patients who had undergone microvascular decompression for the treatment of “idiopathic” trigeminal neuralgia (9 cases), hemifacial spasm, (7 cases), glossopharyngeal neuralgia (3 cases) and paroxysmal vertigo and tinnitus (1 case) were followed up for 25 months on average. Permanent relief of symptoms was observed in 19 (95%), with sparing of cranial nerve function. Analysis of the clinical data shows that the patients described in the present series did not differ from those considered to suffer from “idiopathic” cranial nerve dysfunction syndromes.

The nature of lattice disorder in electroreduced Zirconia is considered in the light of electrical and optical observations. Discussion about the relative role of metallic precipitates vs. point defects and electronic centres shows new points in favour of the latter.

We present second harmonic generation results which, for the first time, allow us to assess the correlation length of the order parameter exhibited by the mixed crystal K1−xNaxTaO3. Our data, including linear birefringence results, evidence a transition to a polar state characterized, for a 15% Na doped sample, by clusters of size 0.12 to over 1 μm and a spontaneous polarization of 503 mC/m2. Even if these findings can be consistent with a ferroelectric transition characterized by long-range order, precursor effects and the behaviour of hysteresis are more indicative of a glass-type transition.

DTA and NMR measurements in graphite nitrate are presented. Information is obtained concerning the dynamical situation of the intercalated layers. An interpretation of the reflectivity data is proposed, and the main differences between the electronic spectrum of graphite and the ones of its intercalated compounds are discussed.

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.

Physical properties of stoichiometric and non stoichiometric CuO are reported. The electrical conductivity has been measured on CuO pellets, sintered with the usual procedure for the preparation of Y Ba-Cu O superconductors and then deoxygenated under a nitrogen flow. It is shown that oxygen deficient CuO pellets present abrupt and large resistivity drops as the temperature decreases below 230-200 K. Possible correlations between the electrical instability of oxygen deficient CuO and the high temperature superconductivity problem are discussed

We report on the transition to the polar phase of Nadoped KTaO3. Soft mode, value of local polarization, orientation and dimensions of correlated domains have been investigated through Raman, birefringence and non-linear-optics techniques.