Abstract
The spectroscopic properties of Cr impurities in four Bi 4Ge 3O 12 single crystals of different origins have been investigated in detail with three complementary techniques. Optical absorption, electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) demonstrate that Cr 4+ at the tetrahedral germanium site is by far the dominant species in these crystals. Low temperature MCD is shown to be appropriate for a nondestructive evaluation of the amount of dopant. Lattice defects appear to be present in as-grown crystals. Diamagnetic centers, contributing to the near-u.v. absorption, are also formed by u.v. irradiation at 310 nm. Finally X-ray irradiation at 15 K produces similar but more drastic effects on the absorption spectra.
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