Percolation nature of the 60-K to 90-K phase transition in Y[Ba.sub.2][Cu.sub.3][O.sub.6+[delta]]

The temperature dependence of the heat capacity of the HTSC Y[Ba.sub.2][Cu.sub.3][O.sub.6+[delta]] is measured in the temperature range 1.8-300 K and the doping range 0.70 < [delta] < 0.86. The results obtained suggest that the transition from the 60-K to the 90-K phase in Y[Ba.sub.2][Cu.sub.3][O.sub.6+[delta]] has a percolation nature and that the underdoped 60-K phase represents a heterophase structure in which numerous superconducting clusters are embedded into an insulating matrix.