The valency of Cu ions in the orthorhombic and tetragonal phases of high-T c superconductor Ba2YCu3O y (6.22<y<6.96) has been studied by near-edge X-ray absorption spectroscopy. The results indicate that the Cu valency at Cu1 sites on the z=1/2 plane is strongly correlated with the high-T c (90 K) superconductivity, while the Cu valency of Cu2 sites on the basal planes affects the low-T c (50-60 K) superconductivity of the orthorhombic phase.
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Ōyanagi et al. (1987) studied this question.
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