An optical spectroscopic study was made of Er-related luminescence in Er-doped and O-doped material which was subjected to hydrostatic pressures. It was found that the application of the pressure caused new Er-related centers to become optically active under host photo-excitation. Two new sets of 4I13/2  4I15/2 transitions appeared in different pressure ranges. It was concluded that these 2 sets of crystal field-split transitions were due to the same Er center, and that this Er center underwent a sudden change in local configuration at a certain pressure.

R.A.Hogg, K.Takahei, A.Taguchi, K.Takarabe, T.Mizushima, S.Minomura: Journal of Applied Physics, 1997, 82[2], 813-6