Excitation of different chromium centres by synchrotron radiation in MgO:Cr single crystals

The excitation spectra for the emissions of chromium-containing centres have been measured at 10 K using synchrotron radiation of 4-32 eV in MgO single crystals with different content of Cr³⁺ (5-850 ppm) and Ca²⁺ impurity ions. Both virgin crystals and the samples preliminarily irradiated with x-rays at 295 K have been studied. The role of complex chromium centres containing two Cr³⁺ and a cation vacancy (sometimes nearby a Ca²⁺ ion) on the luminescence processes and the transformation/creation of structural defects has been analysed. Such anharmonic complex centres could serve as the seeds for the creation of 3D defects that facilitate the cracking and brittle destruction of MgO crystals under their irradiation with ∼GeV heavy ions providing extremely high excitation density within cylindrical ion tracks.