Cr3+(I) and Cr3+(II) centers in alexandrite single crystal

Nuclear magnetic resonance spectra of the Be-9 and Al-27 nuclei as well as electron paramagnetic resonance spectra of Cr3+ ion in an alexandrite single crystal (BeAl2O4:Cr) have been investigated by employing a pulse nuclear magnetic resonance spectrometer and an X-band electron paramagnetic resonance spectrometer, respectively. Be has two magnetically inequivalent sites. While Al(I) at inversion symmetry has four magnetically inequivalent Al sites, Al(II) at mirror symmetry has two. The Cr3+(I) center has four magnetically inequivalent sites, whereas the Cr3+(II) center has two. All resonance sets of nuclear magnetic resonance for Be-9 and Al-27 nuclei and electron paramagnetic resonance for Cr3+ ion are analyzed in terms of crystal symmetry, ionic size, and charge state. It is concluded that the Cr3+(I) and Cr3+(II) ions substitute for Al3+(I) at inversion symmetry and Al3+(II) at mirror symmetry, respectively, intact of Be2+ sites. (C) 2001 American Institute of Physics.