ELECTRON-PARAMAGNETIC RESONANCE SPECTROSCOPY OF TETRAHEDRAL CR-4+ IN CHROMIUM-DOPED FORSTERITE AND AKERMANITE

The observation of near IR laser action in chromium activated forsterite by Petricevic et al. [Appl. Phys. Lett. 52, 1040 (1988)] has spawned considerable interest in identifying and characterizing the quadrivalent chromium centers purportedly responsible for laser action in this material. The EPR (electron paramagnetic resonance) spectroscopy reported here of chromium-doped forsterite and structurally related a(over circle)kermanite provides compelling evidence for the presence of tetrahedral Cr4+ centers in both materials. The EPR spectra reveal substitutional incorporation of chromium ions with triplet spin multiplicity, assignable to Cr4+. The fine structure and electron Zeeman interaction parameters of the centers in both materials are found to be very similar. In a(over circle)kermanite, the Cr4+ center is unequivocally accommodated at tetrahedral lattice sites. The similarity of the spectroscopic proper-ties of the Cr4+ ions in the two materials thus implicates incorporation at the analogous tetrahedral sites in forsterite. In conjunction with the findings of recent optical Zeeman studies [K. R. Hoffman et al., Phys. Rev. B 44, 12589 (1991); T. S. Rose et al., OSA Proceedings on Advanced Solid State Lasers 13, 17 ( 1992) ], these EPR results demonstrate that tetrahedral Cr4+ is responsible for the near IR laser activity in chromium-doped forsterite.