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The orthorhombic-tetragonal YBa2Cu3O7-δ phase boundary in the δ-T diagram was determined by a detailed isobar TG measurement under the oxygen partial pressures of 1 to 0.01 atm. The phase boundary was found not to be located at δ = const., but to have a slope of dδ/dT∼-4.7×10-4 K-1. The experimental results are discussed with a simple order-disorder transformation model of oxygen atoms on the Cu plane sandwiched by Ba planes.
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Kubo et al. (1987) studied this question.
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