Luminescent properties of x-irradiated rare-earth-doped barium copper oxides
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M. S. Jahan Department of Physics, Memphis State University, Memphis, Tennessee 38152 (Received 29 June 1987; accepted 31 July 1987) Thermally stimulated luminescence (TSL) of x-ray-induced defects ini?-Ba-Cu-O (R = Gd, Ho, and Eu) has been measured. In the interval 25-400 °C, Ho- and Eu-doped samples exhibit similar TSL with peaks at 65, 135, and 185 °C. The GdBa 2 Cu 3 O x has, in addition to the three aforementioned peaks, a maximum at 310 °C. All TSL peaks are attributed to recombination o f f / - and Fs- center electrons with Fs-type holes, requiring typical thermal activation energies 0.5-1.0 eV. Spectral emission data support this conclusion. Irreversible quenching of TSL is found to occur in each sample. It is postulated that adsorbed oxygen molecules interact with radiation-induced Fs+ and Fs centers to produce O2 and O2 molecular ions, respectively, thereby reducing the surface oxygen vacancy concentration and quenching the luminescence. In addition to activation energies, other TSL parameters are extracted from the data. It is suggested that surface TSL in these oxide superconductors is independent of rare-earth dopant.
I. INTRODUCTION Superconductivity above 90 K in the multiphase Y Ba-Cu-O system has been well established,1-3 and identification of the phase4'5 responsible for the superconducting behavior in these oxygen-deficient perovskites has been determined. Additionally, superconductivity (with transition temperatures near 90 K) has also been observed6"8 in the rare-earth-doped barium copper oxides. These exciting discoveries have led to numerous studies of these systems in which many different experimental techniques 2 ^' 9 ' 10 have been applied. Recently, we measured the thermally stimulated luminescence (TSL) and emission spectra" of i?-BaCu-O with R = Gd, Ho, and Eu, in the temperature interval 80-300 K. The Gd-doped sample exhibited a single TSL peak at 200 K with concomitant emission at 500 nm, whereas the Ho-doped material was characterized by two closely spaced TSL peaks (160 and 172 K) with complex temperature-dependent emission. No low-temperature TSL was observed from the Eu-doped sample. Possible defect mechanisms involving F- and Vtype centers were assumed to be responsible for the observed luminescence. In the present work we extend those studies to include TSL measurements from 25 to 400 °C. II. EXPERIMENTAL ASPECTS The Ho- and Eu-doped samples used in this investigation were multiphase, i.e., they contained both the green semiconducting and black superconducting J. Mater. Res. 2 (6), Nov/Dec 1987
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phases and were prepared by the methods described in Ref. 6. Their compositions were HO[ 5 Ba! 5 Cu 2 O x and Eu; 5 Ba! 5 Cu 2 O x . Two single-phase Gd-doped samples prepared by methods of Ref. 7 were used; the composition was GdBa2Cu3O;c for each one. From Meissner and shielding effect measurements it was estimated12 that one sample was 90%-95% pure, whereas the other was 99% pure superconducting, each one exhibiting Tc near 93 K. High-temperature (25-40
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