Isothermal susceptibility in tetragonal phase of YBa2Cu3O6+2c

Fluctuations in tetragonal phase of YBa2Cu3O6+2c compound are analyzed in t erms of the two-dimensional asymmetric next-nearest-neighbor Ising (ASYNNNI ) model. For several sets of interaction constants between oxygen atoms, VI > 0. V-2 < 0 and V-3 > 0, we calculated equidistant mu (c, T) = constant c urves to show that largest fluctuations of oxygen concentration in tetragon al phase occur for chemical potential mu approximate to mu (g1), where mu ( g1) = -8V(1) - 4V(3). We argue that this is a general property of the ASYNN NI model: it holds for all temperatures and different values of interaction constants. Analyzing the experimental results on isothermal susceptibility , chi = sigmac/sigma mu for six different temperatures ranging from 673 to 923 K, we found that the top of chi occurs for approximately the same value of oxygen chemical potential in gas phase which corresponds to mu (gi) of the ASYNNNI model. By comparison with experiment, we obtained the relation, 4 \V-2\ - epsilon = 82 mRy, where epsilon is the on-site energy of the bas al plane oxygen atoms. (C) 2001 Elsevier Science B.V. All rights reserved.