PHASE-STABILITY AND LOW-TEMPERATURE SPECIFIC-HEAT UP TO 14 T OF BACUOX AS A FUNCTION OF OXYGEN STOICHIOMETRY

The stability of the BaCuOx (x greater than or equal to 2) phase has b een mapped over a wide range of temperature (300-1100 degrees C) and o xygen pressure(10(-5)-10(3) bar). At ambient pressure and temperature, BaCuO, is found to be in a metastable state: long annealing at 450 de grees C tends to decompose the phase into Ba2Cu3O5 and BaO2. Having ob tained the phase stability domain in the [T,p(O)(2)] plane we were abl e to prepare single-phase samples of BaCuOx with different oxygen cont ents suitable for precise intrinsic thermodynamical measurements. We s how that the behavior of the low-temperature specific heat (1.1 less t han or equal to T less than or equal to 32 K) and its dependence on th e magnetic field (0 less than or equal to B less than or equal to 14 T ) may be understood by taking into account a many-level magnetic syste m directly related to the Cu6O12 and Cu18O24 structural blocks of BaCu Ox. Depending on the oxygen concentration, competition between antifer romagnetic (AF) ordering and the many-level system is observed. With i ncreasing oxygen content, the Neel temperature decreases whereas ampli tude of the many-level system increases. The zero-held AF transition b elongs to the three-dimensional isotropic Heisenberg universality clas s.