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.
Cr+3 distribution in Al1 and Al2 Sites of alexandrite (BeAl2O4: Cr3+) induced by annealing, investigated by optical spectroscopy
In order to investigate optical properties of alexandrite, the present work deals with the influence of thermal annealing on optical absorption and luminescence spectra of natural samples. The exposure time to heat treatment at 1000oC is taken into account. Possible migration of Cr3+ ions from Al1 (inversion site) to Al2 (reflection site) is detected. Sample composition is obtained through Scanning Electron Microscopy (SEM) measurements and points to a rearrangement of Cr+3 and Fe3+ ions in the alexandrite crystalline structure, un-der thermal annealing influence. This feature may be used to control the optical properties of natural alexan-drite, which can be associated to the observed laser emission effect.
EPR of Cr3+ Ions in Alexandrite
Results on the EPR of Cr3+ ions occupying mirror symmetry sites in alexandrite are described. The Spin Hamiltonian parameters are: D = ( −7.31 ± 0.02) GHz, E = = (-1.71 ± 0.02) GHz with gx = 1.978 ± 0.003, gy = 1.980 ± 0.004, gz = 1.969 ± ± 0.004 with a corresponding zero-field splitting between the Kramer's doublets of (15.7 ± 0.05) GHz. The relationships between the crystallographic axes and the axes of the magnetic complexes show slight departures from the crystallographic symmetry of pure chrysoberyl. 
 
 
 
Es werden die Ergebnisse von EPR-Untersuchungen an Cr3+-Ionen auf spiegelsymmetrischen Platzen in Alexandrit beschrieben. Die Parameter des Spin-Hamiltonian sind: D = ( −7,31 ± 0,02) GHz, E = = (-1,71 ± 0,02) GHz mit gx = 1,978 ± 0,003, gy = 1,980 ± 0,004, gz = 1,969 ± 0,004 und einer entsprechenden Null-Feldaufspaltung zwischen den Kramersdubletts von (15,7 ± 0,05) GHz. Die Beziehungen zwischen den kristallographischen Achsen und den Achsen des magnetischen Komplexes zeigen leichte Abweichungen von der Kristallsymmetrie des reinen Chrysoberylls.