D. R. Figueroa

Superstructure observations in NaCl: Cd2+ by ITC experiments

Abstract A new ITC peak in highly doped NaCl crystals was detected at high temperature (318 K). By studying the behaviour of this peak after adequate thermal treatments, we are able to show that it is due to the precipitates of the Suzuki phase present in the crystal.

TSDC and DR studies on PEO complexed with inorganic salts

Abstract TSDC studies have been carried out on PEO complexed with inorganic salts. The results clarify the structure of the DR spectra observed previously for these materials. In the case of the γ relaxation region, which is associated with the amorphous phase, several new relaxations are observed in PEO complexed with the various salts. Changes in the strength of these peaks give rise to the differences observed previously in the DR spectra for these materials, not only between different salts but between different concentrations of the same salt. The results show that in the amorphous phase below the glass transition, at least, the ions occupy very well defined positions vis a vis the polymer chains. Models for the relaxation are discussed along with the influence of the ions.

Surface-charge accumulation and clustering in fluorites

Abstract Comparative Thermally Stimulated Polarization and Depolarization studies have been carried out on SrF 2 crystals doped with lanthanum up to molar concentrations of 5x10 −2 and to temperatures of 500K. The first three peaks, attributed to nearest-neighbour dipoles and to clusters built with this basic unit (L-shaped La 3+ -F i2 − and the gettered 2:2:2 dimer) and the first high temperature peak are accurately reproduced in both spectra proving the dipolar origin of these relaxations. A fifth band at high temperature is present in the ITC spectrum but not in the TSP run where it is replaced by a steep increase in the current. The behaviour of this peak is studied and it is concluded that it is related to the relaxation of space charges accumulated near the electrodes.

Polarization and depolarization processes in SrF2:La from 77 TO 500K

Abstract Lanthanum doped SrF2 single crystals with concentrations ranging from 10−4 to 5 × 10−2 (in molar fraction) were studied by ionic thermal currents and thermally stimulated polarization techniques from 77 to 500 K. Current peaks identified with a La3+-F i -nearest-neighbour dipole (A), an L-shaped La3+-F i2- Complex (B), and higher clusters (C) were found by both techniques. A higher temperature peak (D), which presented an anomalous behaviour, was found at temperatures which depended on the doping level of the crystal and the thermal treatment. Additional experiments, done with different polarizing conditions, various sample thicknesses and mechanical treatments, were performed in order to elucidate the origin of this higher temperature peak. The new evidence reported here agrees better with the dislocation polarization model rather than the idea of surface accumulation of charges near the electrodes. A fifth peak is found when the sample is polarized at higher temperatures which is possibly due t...

Thermally stimulated depolarization currents and ionic conductivity of cubic lead fluoride containing small rare earth ions

Abstract This work is concerned with thermally stimulated depolarization current (TSDC) and ionic conductivity studies in lead fluoride containing the small rare earths Dy, Ho, Er and Yb. The TSDC scans from 80 to 300 K show two peaks. For Pb1−xErxF2+x one is located at 106 K, and another, which is much stronger, occurs at about 160 K. The former is associated with a dipolar defect containing at least two rare earths and the latter is attributed to the development of F− space charge during polarization of the sample. The activation energies obtained from both the high temperature TSDC peak and the ionic conductivity are the same, which corroborates the latter assignment. In addition, the ionic conductivity is shown to be independent of concentration. Those results can be understood of rare earth clustering, which is either absent or is unobservable dielectrically for large rare earths such as lanthanum, occurs extensively even at very low concentrations of the small rare earths. The explanation is that th...