The observation of phase transition near 470 K in YBa2Cu3O6.9

In heat capacity of YBa2Cu3O6.9 the anomaly has been extracted in the range 250-600 K with maximum at ∼470 K. This anomaly has been shown to correspond to the II kind phase transition from the phase with low mobility of oxygen atoms O1 to the phase with high mobility. At T>600 K the indication of another anomaly has been discovered, probably of ortho-tetra phase transition. Also, the different behavior of resistance ρ(T) at fast and slow heating has been discovered. Such behavior revealed itself in region of the II kind phase transition. Apparently, it was caused by the oxygen subsystem reorganization