PRESSURE EFFECTS ON SUPERCONDUCTIVITY AND STRUCTURAL PHASE-TRANSITIONS IN LA2-XMXCUO4 (M=BA,SR)

Relations between the superconductivity and the crystal structure of La2-xMxCuO4 (M=Sr,Ba) are studied under high pressures up to 20 kbar. In the Sr-doped crystal, bulk superconductivity is observed both below and above the critical pressure P(c) where the high-temperature tetragonal (THT) phase is stabilized down to less than 20 K, the lowest temperature examined, instead of the orthorhombic phase. The superconducting transition temperature T(c) of the orthorhombic phase below P(c) is depressed in comparison with that of the THT phase above P(c). The depression of T(c) is caused by a tilt of the CuO6 octahedra in the orthorhombic phase. It is found for the Ba-doped crystal that the depression of T(c) for x congruent-to 0.11-0.14 is removed, except for x congruent-to 0.125 when a structure transition at T1 approximately 60 K disappears under high pressure. In the vicinity of x= 0. 125, there exists another origin for the depression of T(c) in addition to the structural transition at T1.