It is known that the 123 phase is not a thermodynamically stable phase at room temperature. The 123 decomposes during low temperature annealing to form the thermodynamically stable 124 phase, as well as the 211 phase and BaCuO2 phase as follows[1]:
YBa2Cu3O6.76(s) 1/2YBa2Cu4O8(s) + 1/4Y2BaCuO5(s) + 3/4BaCuO2.01(s)
The extremely slow kinetics of this reaction cause the 123 phase to be metastable at low temperatures[1].
HRTEM micrographs of fiber samples show the results of this decomposition:
It can be seen that large numbers of stacking faults exist near the BaCuO2.01 second phase layer. The density of stacking faults is a strong function of the distance from the second phase layers and 211 particles. It is believed that the 211 particles act as a nucleation site for the above reaction. The densely populated stacking faults near the second phase layers correspond to extra CuO layers incorporated into the 123 phase, forming the 124 phase.
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[1] Z. Ahou and A. Navrosky: J. Mater. Res., 7, 2920 (1992).
©CPRL, 1995