Luminescence from Defects and Impurity Centers in Y 2 O 3
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LUMINESCENCE FROM DEFECTS AND IMPURITY CENTERS INY2 0 3 M.S. Jahan,* D.W. Cooke, B. L. Bennett, W. L. Hults, M.A. Maez, K.C. Ott and J. L. Smith, Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT High-purity (99.999%) Y203 powder is used as a starting material for fabricating high-temperature superconductors (HTS), and is frequently found as an unreacted second phase in the final product. We have found that as-received Y20 3 contains Tb 3 + paramagnetic impurity ions as determined by TSL and emissionspectra measurements. Deep luminescence traps (presumably Tb 4 + ions) are formed in Y20 3 when it is exposed to y-rays, x rays, or fluorescent lights. These deep traps can only be removed by annealing the material to near 1600 0C. Given the short coherence length of HTS it is possible that order-of-magnitude variations in the Tb impurity concentration of starting materials may affect the ultimate value of surface resistance (Rs). INTRODUCTION Yttrium oxide Y203 is a trivalent metal oxide which has a cubic structure with space group Th 7 . The unit cell contains 32 cations and 48 anions; each anion is surrounded by four cations in a distorted tetrahedron. It has a large number of inequivalent sites for defects and substitutional impurities and, therefore, accepts rare-earth ions in the trivalent state without charge-compensating problems and ionsize limitations. For example, when activated with rare-earth ions yttrium oxide exhibits selective photoluminescence in the visible and near ultraviolet spectral region 1 and represents an interesting host material for many phosphors. In yittrium-based HTS, Y203 is used as one of the starting compounds and is frequently found in sintered YBa2Cu3Ox as an unwanted second phase. Removal of this insulating impurity, especially at the surface, is important for acheiving low values of surface resistance, which is important for practical applications of HTS. A convenient method for detecting the presence of insulating materials in HTS, especially at the surface, is that of thermally stimulated luminescence (TSL). 2 Following y, x, or ultraviolet irradiation at room temperature and subsequent heating to 3500C, Y203 emits characteristic luminescence, which can be used to identify its presence in HTS. Our objective is to investigate impurities in pure (99.999%) Y20 3 by luminescence methods and to utiize this information in fabricating improved HTS materials.
Mat. Res. Soc. Symp. Proc. Vol. 169. 91990 Materials Research Society
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MATERIALS AND METHODS High-purity (99.999%) powders of Y203 were obtained from Research Chemicals , Aldrich, and Johnson Matthey, Inc., and were used without further purification. Pressed pellets of these powders were prepared and identified as sample A (Research Chemicals), sample B (Aldrich), and sample C (Johnson Matthey, Inc.). Another group of samples was prepared by intentionally doping virgin Johnson Matthey Y203 powder with Tb. The resulting concentrations, as determined by chemical analyses, were 1, 2, 11, and 150 ppm by weight. Gamma i
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