Phase diagrams of YBa 2Cu 4O 8 and YBa 2Cu 3.5O 7.5 in the pressure range 1 bar≤P O2≤3000 bar

Abstract

The P-T-x phase diagram of the pseudobinary system (Y-Ba-Cu-O)-O 2 has been further investigated in the oxygen pressure range between 1 and 3000 bar. The stability ranges of the phases YBa 2Cu 4O 8 (124), YBa 2Cu 3.5O 7.5− x (123.5) and YBa 2Cu 3O 7− x (123) have been determined. Long duration experiments showed that the 123 phase is not stable at least down to 7 bar≤ P≤20 bar oxygen and 900°C. It is not clear whether at lower pressures and temperatures the 123 phase is thermodynamically stable or metastable due to low reaction rates. In the presence of excess CuO, the 124 is the stable phase. The melting of 124 pellets at P O2 =2800 bar shows that even at this pressure the 124 compound melts incongruently. Using the phase diagram data we could change the T c of 123.5 from 16 to 70 K by varying systematically the nonstoichiometry. Due to a narrow homogeneity range the T c of 124 remained constant but is different for powder pellets (81 K) and for crystals (70 K), probably due to the influence of the flux. Single crystals of both 124 and 123.5 with dimensions up to 4 mm were grown from the flux under high oxygen pressure.