HIGH-TEMPERATURE THERMODYNAMIC PROPERTIES, ORTHORHOMBIC TETRAGONAL TRANSITION AND PHASE-STABILITY OF GDBA2CU3OY AND RELATED R123 COMPOUNDS/

Thermogravimetric measurements of the equilibrium oxygen partial press ure (p(O-2)) as a function of temperature and oxygen content have been made on GdBa2Cu3Oy. (Gd123) within the range 3 x 10(-4) atm less than or equal to p(O-2) less than or equal to 1 atm and 450 degrees C less than or equal to T less than or equal to 800 degrees C, The partial m olar entropy Delta (S) over bar(O-2) and enthalpy Delta (H) over bar(O -2) of solution of oxygen in Gd123 are evaluated and discussed, The p( O-2) data on Gd123 are compared with measurements performed on NdBa2Cu 3Oy. (Nd123) at 800 degrees C and literature data for YBa2Cu3Oy, (Y123 ) at 600 degrees C and 800 degrees C. The orthorhombic/tetragonal tran sition temperature (T-O/T) for Gd123 and Nd123 as a function of the ox ygen partial pressure (1 x 10(-3) atm less than or equal to p(O-2) les s than or equal to 1 atm) has been determined by high temperature X-ra y diffraction (HTXRD). The oxygen content values for the O/T transitio n (y(O/T)) at several p(O-2) values for Gd123 and Nd123 have been dete rmined by means of thermogravimetric measurements. The obtained data i n the y-T diagram supports the applicability of the quasichemical appr oximation in describing the O/T transition of Gd123 and Nd123 compound s. The effect of the R atom (R = Y, Gd, Nd) ionic size on the oxygen s toichiometry and phase stability of R123 compounds at high temperature is discussed, and a possible explanation is proposed which accounts f or the latticemismatch between Ba-O and CuO2 layers. (C) 1998 Elsevier Science B.V.