On characteristics of oxygen absorption isotherms for RBa2Cu3O6+x(R=Y, Er) ceramics and quasi-binary phase diagram

A series of experimental and theoretical studies was analyzed. It is shown that oxygen absorption by RBa2Cu3O6+x (R=Y, Er) superconducting ceramics is described by Langmuir equation with similar values of absorption enthalpy and entropy [about - 90 kJ/mol, - 120 J/(mol.k)] and sufficiently different values of absorption center concentration (from 0.5 to 1.0 mole centers per 1 mole ceramics) for all structures, existing in T temperature range from 600 to 1200 K and x parameter range from 0.2 to 1.0. Experimental isotherms of oxygen absorption by YBa2Cu3O6+x ceramics were used to determine boundaries for five segments of quasibinary state diagram T-x. They differ by structure (tetragonal and orthorhombic hyperstructure), the character of concentration dependences of temperature of transition to superconducting state and Fermi energy, as well as by the character of temperature dependences of electric resistance, thermal expansion coefficient and root-mean-square displacement of absorbed oxygen atoms. It is shown that pair interactions between absorbed oxygen atoms in 123 type ceramics are neglible. Phase transitions between tetragonal and orthorhombic structures and hyperstructures are conditioned by other interatomic interactions and corresponding changes of electron subsystem