Evidence of a first order phase transition in YBa2Cu3O6·9 at T = 230 K

In order to bring more information on the behaviour of high Tc superconductors, in the normal phase, we have undertaken detailed low frequency acoustic, specific heat and resistivity experiments between Tc and room temperature. The obtained results presented in this paper, allow us to conclude that YBa 2 Cu 3 O 6·9 undergoes a first order phase transition in the 200–240 K temperature range. The three above mentioned properties display clear anomalies which are not observed on a non-superconducting sample YBa 2 Cu 3 O 6·0 .