Ion Dynamics in Superionic Conductors Studied by a Spectral Hole-Burning Effect

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(1) The optical absorption and luminescence spectra of ^7F_0 -^5 D_0 transition in Eu^<3+> ions are studied for Eu^<3+> -doped CeO_2 -Y_2 O_3 and ZrO_2-Y_2O_3 (YSZ) systems. From a detailed study of the luminescence bands under site selective excitation, following results are obtained. In Eu^<3+> /CeO^2-Y_2O_3, three types of trivalent ion sites are found ; the site without vacancy, with one vacancy on the nearest neighbor site, and with one vacancy on the 2nd nearest site. In YSZ, trivalent ion site without vacancy and one and two vacancies on the nearest nieghbor site are found.(2) The FLN(fluorescence line narrowing) spectra are studied in Eu^<3+> -doped YSZ(ZrO_2-Y_2O_3) crystals with various compositions. The homogeneous line width is found to be proportional to T^<2.6> between 80K and 300K. Since the homogeneous and inhomogeneous line widths have different composition dependence, R(=GAMMA_<inhomo>/GAMMA_<homo>) has a maximum value at certain composition. It is found that the 19mol% YSZ has a maximum value, R=5000, at 20K. This result suggests usefulness of the Eu^<3+>/YSZ system for a frequency domain optical memory.(3) In 10mol%YSZ doped with 2%Pr^<3+>, we found a persistent hole-burning effect for ^3H_4-^1D_2 transition at 6115A. The hole can be burned below 25K and its width is 0.35GHz at 4.2K. The hole width, which is assumed to be equal to 2GAMMA_<homo>, has a T^<1.3> temperature dependence. This temperature dependence is very similar to that of the dye molecules embedded in polymers, and strongly suggests that the dephasing mechanism in this system involves a coupling of the rare-earth ions with TLS(two level system). The persistent hole can be used as a new tool to investigate the microscopic structures and the interaction of the mobile ions with the defect centers in YSZ. A demonstration as a frequency domain memory will be also a fruitful subject as a next step.