M. Puma

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.

Clustering of Y3+ in BaF2 using Gd3+ as paramagnetic probes

Abstract Ionic thermocurrent (ITC) and electron paramagnetic resonance (EPR) experiments have been performed on solid solutions of the type Ba1−xYxF2+x, 0.0001≤x≤0.05, additionally doped with a small fixed molar concentration, equal to 10−4 , of Gd3+ acting as paramagnetic probes. It is shown that the more abundant polarizable defect is the next nearest neighbor (nnn) dipole, Y3+−Fi−, whose molar concentration reaches a maximum for x∼0.005. Also, the EPR experiments show the coexistence of cubic and trigonal Gd3+ centers for the whole x range studied here. The variation of the population of these two paramagnetic centers peaks around x=0.0002 and the fraction of their relative abundances deduced from the observed EPR intensity is almost constant. The existence of several weaker lines evidences the presence of Gd3+ ions in additional sites to the cubic and trigonal ones. The comparison of the results obtained from the ITC and EPR techniques indicates the presence of Y3+ in larger clusters than the simple nnn dipole. To explain the variation of the abundance of the trigonal center at low Y3+ concentrations, a mechanism is proposed for the creation of the clusters formed by the trapping of an extra Fi− by either the Y3+ or the Gd3+ nnn dipoles. The existence of clustering among the trivalent ions in this system is supported by the variation of the linewidths of the EPR lines as x increases. These linewidths do not increase linearly with x but rather present a saturation effect at high x values. This indicates that a high doping levels the yttrium ions are not mainly dispersed in the lattice as nnn dipoles, but rather they are forming higher clusters whose interaction with the Gd3+ trigonal centers is small. The ITC results show that most of the clusters are either not polarizable or present a very small dipolar moment. As all the evidence presented here indicates that clustering is important in these crystals, one can conclude that the behavior of the high temperature ITC peak is best understood if it is attributed to an interfacial polarization around the dislocations of the crystal and not to a space charge effect.

On the origin of high temperature relaxation processes in rare-earth doped BaF2 crystals and its relation to dislocations

Abstract Ionic thermal current experiments have been performed on barium fluoride crystals doped with different trivalent rare-earth ions (La 3+ , Ce 3+ , Gd 3+ and Tm 3+ ) in a wide concentration range. The samples were either untreated, plastically deformed, or thermally treated, in order to study the effect of these treatments on the behavior of the high Temperature peak, and to elucidate the role of dislocations in this polarization process. The variation of the position, T M , and of the height of the HT peak, H HT , together with the dislocation density of the crystals were followed as a function of the molar concentration of impurities, of the amount of deformation, and of the thermal treatment of the samples. The results were interpreted in the scope of the Maxwell-Wagner-Sillars model for a heterogeneous dielectric by assuming that the inclusions are the zones of the crystal around the dislocation lines and their surrounding defect atmospheres which are easy diffusion paths. The role of the trivalent ions, acting as pinning points along the dislocation lines, explains the differences in the plastic deformation experiments observed for the different cations used here as dopants. The determination of the dislocation density after each of these treatments was performed in order to verify the predictions of the model by making simple assumptions on the geometrical parameters which describe the inclusions. The model accounts for all the results reported here. The role of dislocations in the relaxation process from which the HT ITC peak originates is thus favored against the idea of a space-charge polarization.

Co-existence and aggregation of Gd3+ defects in BaF2 as-grown and thermally treated crystals

Abstract To study the different populations of defects existing in Ba1-x Gd x F2+x solid solutions for x ranging from 0.0001 to 0.1, comparative Ionic Thermal Currents (ITC) and Electron Paramagnetic Resonance (EPR) experiments have been performed. In the as-grown crystals weak trigonal centers assigned to the nnn Gd3+-F i − dipoles coexist at all concentrations with Gd3+ in cubic sites. The variation of the number of these nnn dipoles as a function of x found with the two series of experiments is a strong indication of the existence of larger clusters in the crystals. The appearance of new relaxation peaks in the low temperature zone of the ITC spectra is correlated to the formation of these aggregates together with the observed variation in the number of cubic sites. An association energy of 0.41 eV of the nnn dipole is calculated from the equilibrium of associated and free defects. The less doped crystals were treated thermally at 1223 K and variable amounts of O2− ions diffused into the samples creati...

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...

Observation of defect clustering in Gd-doped calcium fluoride

Abstract Solid solutions Ca1-xGdxF2+x for 3 × 10−7≤ x ≤10−1 have been studied by electron paramagnetic resonance (EPR) and ionic thermal currents (ITC). The EPR experiments show the presence of two single-ion sites a cubic and a tetragonal Gd3+ center which co-exist with comparable abundances for intermediate impurity concentrations. The cubic center predominates at very low and high concentrations. Seven different relaxation processes have been identified from the ITC spectra and the variation of their intensity vs. x was measured. The absolute concentrations of the cubic and nn Gd3+ dipoles were calculated. The scavenging of interstitial fluorines by the neutral clusters explains both the abundance of cubic sites at high concentration and the variety of orientable clusters detected by ITC.

Defect clustering in double doped fluorite crystals with Lu and Gd

Abstract Solid solutions Ca1 x-yLuxGdy F2+x+y for 10−4 ≤ x ≤ 2 × 10−2 and y=0.0001 have been studied by electron paramagnetic resonance (EPR) and ionic thermal currents (ITC). It has been found that the ITC spectrum from 77 to 420 K is very weak and the main peak is attributed to the relaxation of both Lu3+-F− x and Gd3+F− i nn dipoles. No polarizable clusters are present in the temperature range explored here. The EPR spectra show the presence of Gd3+ tetragonal and cubic centers due to the local and non local compensation, respectively. The continuous decrease in the molar fraction of Gd3+ tetragonal centers together with the low concentration of Lu nn dipoles is an evidence of the existence at these low and intermediate concentrations of large clusters such as the cubo-octahedral hexamer which has been proposed for CaF2 crystals very highly doped with small trivalent cations.