Experimental manifestations of the Nb^{4+}-O^{-} polaronic excitons in KTa_{0.988}Nb_{0.012}O_{3}

The formation of the photo-polaronic excitons in ABO_{3} perovskite type oxides has been detected experimentally by means of the photoinduced electron paramagnetic resonance studies of KTa_{0.998}Nb_{0.012}O_{3} crystals. The corresponding microwave X-band spectrum at T < 10 K consists of a narrow, nearly isotropic signal located at g ~ 2 and a strongly anisotropic component. The first signal, which has a rich structure due to hyperfine interactions with the lattice nuclei, is attributed to the single trapped charge carriers: the electrons and/or the holes. The anisotropic spectrum is caused by the axial centers oriented along the C_{4} pseudo-cubic principal crystalline axes. The spectrum angular dependence can be described well by an axial center with S = 1, g_{\parallel) = 0.82, g_{\perp} = 0.52 and D = 0.44 cm^{-1}. The anisotropic spectrum is attributed to the Nb^{4+}-O^{-} polaronic excitons. The temperature dependence of the anisotropic component is characterized by two activation energies: the internal dynamics activation E_{a1} = 3.7\pm0.5 meV, which makes the EPR spectrum unobservable above 10 K, and the destruction energy E_{a2} = 52\pm4 meV. By comparing the anisotropic photo-EPR spectrum and the photoinduced optical absorption temperature dependencies, we found that the Nb^{4+}-O^{-} polaronic excitons also manifested themselves via the ~0.7 eV wide absorption band arising under UV light excitation in the weakly concentrated KTaO_{3}:Nb crystals.Comment: PDF, 15 pages, 6 figures (submitted to Physical review B

2-5 pyrochlore relaxor ferroelectric Cd2Nb2O7 and its Fe2+/Fe3+ modifications

Copyright 2001 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.International audienceThe weak-field dielectric dispersion (100 Hz-1.8 GHz) studies both of pure and Fe2+/Fe3 modified Cd2Nb2O7 ceramics over the temperature range of 90-380 K are presented and discussed from the viewpoint of relaxor and glassy properties of the system. It is revealed that Cd2Nb2O7 pyrochlore is intolerant of the addition of 25 mol % Fe2+ or Fe3+ for Cd2+. From the x-ray diffraction analysis, pure Cd2Nb2O7 forms a single-phase pyrochlore, while the compositions Cd1.5Fe0.52+Nb2O7 and Cd1.5Fe0.53+Nb2O7 give CdNb2O6 columbite doped with Fe2+ or Fe3+ on the Cd sites (<8 and <2 mol %, respectively), except for minor amount of parasitic hematite. The novel CdNb2O6 type compounds are not ferroelectrics, unlike Cd2Nb2O7. In the latter, at TC=196 K the dielectric relaxation due to the motion of ferroelectric domain walls driven by an external ac electric field is observed. A polydispersive dielectric response of Cd2Nb2O7 around 188 K has characteristics of the relaxor ferroelectrics with glassy behavior (like PMN). Near the characteristic freezing temperature of the zero-field-cooled state (Tf=183 K) the dielectric absorption spectra and the relaxation-time distribution strongly broaden and tend to flatten out, while below Tf the imaginary part of the dielectric permittivity becomes nearly frequency independent. The dielectric response of Cd2Nb2O7 dominating far below TC (around 150 K) and that of Fe2+/Fe3+ doped CdNb2O6 between 90 and 380 K are typical of glass-forming systems at temperature far above Tglass. The relaxational process is characterized by (i) a significant frequency dependence of the peak permittivity position, (ii) non-Arrhenius behavior, and (iii) increasing asymmetry of the dielectric absorption spectrum at the low-frequency side with decreasing temperature, without broadening the relaxation-time distribution and freezing the peak-absorption frequency. It is proposed that although the nature of structural disorder in Cd2Nb2O7 pyrochlore and Fe2+/Fe3+ doped CdNb2O6 columbite is different, in both systems the off-center displacements of the A-site ions act as a random field and are responsible for the relaxor and dipolar glass-like behavior upon cooling. The Debye-like HF dielectric relaxation (1 MHz-1.8 GHz) observed both in Cd2Nb2O7 and its isostructural analog Cd2Ta2O7 at RT and higher (a centrosymmetric phase) is attributed to fluctuations in polarization of the dynamically reoriented O(7th)-Cd-O(7th) dipoles due to dynamical off-center location of Cd ions

Optical properties of excitons in CdS semiconductor-insulator quantum wires

The characteristic features of the luminescence spectra of CdS semiconductor nanocrystals, crystallized in hollow channels in a dielectric template, are explained in terms of excitonic transitions in semiconductor-insulator quantum wires. The excitonic transition energies agree with the values calculated taking into account the effects of size quantization and the “dielectric enhancement of excitons” — the large increase in the electron-hole attraction as a result of the difference between the permittivities of the semiconductor and insulator. The theoretically computed binding energies of excitons in CdS quantum wires with a diameter of 10 nm reach 170 meV. It is shown that the excitonic transition energy is constant for a wide range of wire diameters