Investigation on the valence of Cr ions in LiAlO2

The angular dependence of the electron paramagnetic resonance (EPR) spectra of Cr:LiAlO2 does not fit Cr4+ with S=1, but rather Cr3+ with S=32. The energy level (740 cm−1) of the first excited state estimated from the temperature dependence of the EPR line width is consistent with the energy separation between two groups of zero-phonon lines in the near-infrared luminescence observed at 10 K. The paramagnetic center observed by the EPR technique is the same as the optically active center. These results suggest the possibility that trivalent chromium ions substitute for trivalent aluminum ions in LiAlO2 with tetrahedral symmetry, without local charge compensators.