| Research Abstract |
The XRD analysis determined the time-temperature-transformation(T-T-T) diagram for sol-gel derived GeO_2 gels heated under various conditions of temperature and time. Sol-gel derived GeO_2 glasses were obtained by heating the gels according to the T-T-T diagram. For comparative purposes, GeO_2 glass was prepared by melt-quenching method. The sol-gel derived glasses showed strong, white luminescence (400-700 nm). White afterglow was also observed under irradiation at 254 and 365 nm. On the other hand, the melt-quenched glass gave a luminescence band at 410 nm, and the afterglow was decayed quickly. The sol-gel derived glass included defects and impurities such as Ge^<2+> center, Ge-OH,H2_O,R-OH (R : alkyl group), CH_3-,-CH_2-,CH_3CH_2-,CO_2,Ge-O-C,GeC,Ge-O-R, and carbon-related radicals, while the melt-quenched glass contained Ge^<2+> and GeE' centers, and Ge-OH. The white luminescence and afterglow observed in the sol-gel derived GeO_2 glass are considered to be caused by the Ge^<2+> c
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enter and Ge-O-C.
In Tb^<3+> ions doped GeO_2 glasses, under UV excitation at 254 and 365 nm, fluorescence bands due to the transitions of ^5D_4→^7F_j(j=3,4,5, and 6) in the Tb^<3+> ions appeared at 489,544,586, and 620 nm, respectively. In addition, the Tb^<3+> afterglow was observed and lasted longer with increasing the UV irradiation time. However, when the UV irradiation time was too long, the afterglow disappeared quickly. It is found that the intensity of an absorption band due to the tradition 4f^7→4f^65d^1 of the Tb^<4+> ions increased with an increase in the UV irradiation time. Therefore, it is considered that the afterglow may be affected by the Tb^<4+> ions.
Mn ion doped ZnO-GeO_2 glasses and glass ceramics were prepared from solutions of (CH_3COO)_2Zn ・2H_20,Ge(OC_2H_5)_4, and Mn(NO_3)_2 ・6H_2O by a sol-gel method and the subsequent heat treatment up to 1000 ℃. Under UV irradiation of 254 or 365 nm, the glass ceramics showed green luminescence at 535 am, which may be due to the ^4T_1→^6A_1 transition of Mn^<2+> ions incorporated in Zn_2GeO_4 polycrystals. The intensity of this green luminescence strengthened and the crystallinity of Zn_2Ge0_4 became high with increasing the heat-treatment temperature up to 1000O℃. In materials with low crystallinity and large amount of MnO, we observed a broad, featureless ESR signal of g=2.01. On the other hand, in a high-crystallinity and low MnO content sample heat treated at 1000℃, 0.1MnO-40ZnO-60GeO_2, many ESR signals were found in the range of about 20 to 500 mT; typical signals having six hyperfine lines were at g=2.01(A=8.3-8.7 mT) and g=4.27 (A=8.2-8.3 mT). All of these ESR signals are probably due to tetrahedrally coordinated Mn^<2+> ions incorporated in Zn_2GeO_4 polycrystals.
Local structure around Er^<3+> and Ge^<3+> ions in sol-gel derived GeO_2 glasses were studied by vibrational and XAFS spectroscopy. Less
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