The energetics of Schottky defects in the 123 cuprate superconductor series, RBa2Cu3O7 (where R = lanthanides) and YA2Cu3O7 (A = alkali earth), were found to have unusual relationships if only the volumetric strain was considered. Calculations revealed the effect of non-uniform changes in interatomic distances within the R-123 structures, introduced by doping with homovalent elements, upon Schottky defect formation energy. The formation energy of Frenkel pair defects, an elementary disordering event in 123 compounds, could be substantially altered by stress and chemical doping. Upon scaling the O-O short-range repulsive parameter, by using the calculated formation energy of Frenkel pair defects, the transition temperature between orthorhombic and tetragonal phases was computed by using quasi-chemical approximations. The theoretical results exhibited the same trends as the experimental measurements, in that the larger the ionic radius of R the lower was the orthorhombic/tetragonal phase transition temperature. This provided strong evidence for strain effects upon order-disorder transition due to O atoms in the CuO chain sites.

Strain Effects on Point Defects and Chain-Oxygen Order-Disorder Transition in 123 Cuprate Compounds. H.Su, D.O.Welch, W.Wong-Ng: Physical Review B, 2004, 70[5], 054517 (7pp)

Table 3

Grain-Boundary Diffusion of Fe in Polycrystalline Cr2O3