LAKHTIN (b), and V. A. MEZRIN (a) The YBa,Cu,O, -d-gas equilibrium is studied theoretically in terms of the approach proposed earlier by the authors. The contribution of the electron-hole system of the HTSC oxide to the equilibrium thermodynamics is studied for several models of the electronic structure. Oxygen-oxygen interactions in the basal plane are taken into account to calculate the chemical potential of oxygen ions in the oxide, distinct from the previous paper where these interactions are disregarded. It is shown that the equilibrium is well described for a sufficiently broad (much broader than k T ) valence band with a hole effective mass mp = 3 to 5 free electron masses.

The solid solution MO, + MeO, (M = Zr, Hf, Ce, T h ; Me = Ca, Sr, Sc, Y, the lanthanides) is considered. A model describing the defect structure of a crystal with consideration of short-range order in the arrangement of defects is proposed. The defect interaction is expressed in the pairwise approximation. The short-range order is characterized by the mean probability of finding a dopant cation in the site nearest to the oxygen vacancy. This probability is calculated using the minimum free energy condition. On the basis of the proposed model expressions are derived for the chemical potentials of components and for the ionic conductivity of the system as a function of composition. The calculated curves are in good agreement with the experimental ones. PaCCMOTpeH TBepnbIB MO, + MeO, (M = Zr, Hf, Ce, T h ; Me = Ca, Sr, SC, Y, JlaHTaHOHnbI). npe&TIOXeHa M O H e n b , Il03BOJrHIorrlaH OIlHCaTb He4eHTHyIO CTpYIETYPy paCTBOp KpmTanna npti yseTe 6 n~m~e r o rIopsxEm B pacnonomeHmm ne$emoB. B3aHMOp&iCTBHe rrocneaHmx ~~~E I C~I B~~T C I I B napHoM n p~6 n~w e~r n~; 6nHm~11B nopxno~ xapamepa-3YeTCR CpeHHefi BepORTHOCTbIo HaXO?KneHHFI IIpkiMeCHOrO KaTHOHa B y3m, 6JIHlrradIIIeM K BaKaHCHEl HElCJIOpO~a. 3Ta BepOHTHOCTb paCCsllTaHa 113 YCJIOBHR MHHHMYMa cBO6OHHOfi 3HeprHM. Ha OCHOBe Ilpej&TIOmeHHOB MOHenkl IlOJIy4eHbI BbIpameHElR HJIH XHM119eCIiIIX nOTeHUHaJIOB KOMIlOHeHTOB, a TaHlKe QJIH ElOHHOfi npOBOnHMOCT11 CEl CTeMbI B 3aB11CMMOCT11 OT COCTBBB. PaCueTHbIe Hp11BbIe COrJlaCyIoTCR C 3HCnepHMeIITaJlbIIbIMH.

The thermodynamies of phase transitions associated with Frenkel aniondisorder in CaF2−type superionic conductors is considered. The short‐range order in the arrangement of vacancies in the main anion sublattice and of anions in the octahedral interstices is describedin a pairwise approximation (of the Bethe type) with allowance for intractions of is described in a pairwise appromean‐field approximation and the Frenkel‐pair formation entropoy phenomenologically, with retention of linear and quadratie terms in defect concentration. Genetically related first and second order and diffuse phase transitions may be realized within the model proposed. A satisfactory quantitative agreement is obtained between the theoretical temperature dependence of heat capacity and that observed experimentally for CaF2 type compounds on diffuse transition to superionic state. The reason is discussed why the classical theory of point defects cannot be applied to these systems over theentire transition range.

The equilibrium between the high-T, oxide YBa,Cu,O,-, and the gas phase is studied for some models of the ionic and electronic (e.g., two-band model) systems of the compound. The calculations show that the contribution of the electron-hole system to this equilibrium is essential. The comparison of the theory with experiment allows to obtain such parameters as binding energy of oxygen ions, their effective Einstein frequency 52, the hole effective mass rnp, etc., the 52 and rnp values being in agreement with those reported elsewhere. A number of relations, e.g. the electron and hole concentrations versus oxygen pressure Po, and as well as the influence of doping (with donors or acceptors) on these relations and on the oxygen content are predicted using the parameters obtained. The region of high Po, is also studied.