Influence of annealing on X-ray diffraction of natural alexandrite
In this work we present results of X-ray diffraction using powder method, on natural alexandrite samples from Minas Gerais State (Brazil), as a function of a sequence of annealing. From these measurements we determine lattice parameters before (a=9.405 Å, b=5.471 Å, c=4.409 Å) and after annealing, and its structure is confirmed as orthorhombic. Measurements done after an annealing of 15 minutes at 700 °C and for 5 hours at 1000 °C indicate the migration of atoms present in the sample through different phases, which were also identified by Microprobe Analysis (WDS). However we have verified that such migration does not modify the structure. X-ray diffraction measurements have been carried out in conjunction with optical absorption in the UV–Vis as a function of annealing.
Thermal annealing-induced electric dipole relaxation in natural alexandrite
Electrical properties of natural alexandrite (BeAl2O4:Cr3+) are investigated by the thermally stimulated depolarization current (TSDC) technique. Samples are submitted to consecutive annealing processes and TSDC is carried out after each annealing, yielding bands with different parameters. These bands are fitted by a continuous distribution of relaxation parameters: activation energy and pre-exponential factor of the Arrhenius equation. It has been observed that annealing influences the dipole relaxation behavior, since it promotes a modification of Fe3+ and Cr3+ impurity distributions on sites of distinct symmetry: Al1 and Al2. In order to have a reference for comparison, TSDC is also carried out on a synthetic alexandrite sample, where the only impurity present is Cr3+ ion.
An Intensity Standard for Electron Paramagnetic Resonance Using Chromium-Doped Corundum (Al2O3: Cr3+).
We report on the preparation of a standard reference material (SRM), made of single-crystal chromium-doped corundum (synthetic ruby), for use as an intensity standard in electron paramagnetic resonance (EPR) experiments. The SRM can be used to measure, by comparison, the number of spins in an unknown test sample. We selected the above material for the standard because its magnetic resonance properties are well understood and because it is physically and chemically stable under common laboratory conditions. To prepare samples which gave satisfactory EPR signals, it was necessary to perform annealing and chemical etching after cutting. This removed strains and surface damage. After treatment, the Cr3+ resonance lines were sharper and the intensities were in good agreement with the theoretically predicted values. To aid in application, the theoretical resonance fields and line intensities were calculated and tabulated for arbitrary orientations of the sample, at several commonly used microwave frequencies. The concentration of Cr3+ in the samples was determined by measurement of the static susceptibility. A quantitative EPR intensity experiment, based on measuring the microwave power absorbed during resonance, gave the same concentration. This proves, that within experimental error the EPR experiment detects all the Cr3+ ions in the sample, which makes the ruby a useful SRM.