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Published online by Cambridge University Press: 28 February 2011
Elastic and structural instability have been reported in YBa2Cu3O7‐δbetween 210K and 270K. We have carried out an ultrasonic study of these instabilities as a function of both oxygen concentration and annealing temperature. The results strongly indicate that an athermal (martensitic‐type) transition starts at a temperature Ts which decreases with decreasing oxygen concentration. A linear relationship between Ts and Tc suggests that the second phase formed is essential for the high Tc.