Bond-valence analysis of the charge distribution and internal stresses in the RBa 2Cu 3O 7−δ system (R = rare earth)

Abstract

The influence of rare-earth ions on the crystal chemistry, charge distribution and internal stresses in RBa 2Cu 3O 7−δ (R = rare-earth ion) system has been analyzed in terms of the bond-valence sums (BVS) using the structural data of Guillaume et al. [Z. Phys. B 90 (1993) 13]. The bond valences on both Cu(1) and Cu(2) sites increase with decreasing rare-earth ionic radius. The proportion of Cu 3+ varies from 14.6 to 38.9% at the Cu(1) site and from 11.3 to 31.1% at the Cu(2) site for R varying from La to Yb. While the bond-valence sums on La, Pr, and Nd are significantly more than the nominal valence of 3 indicative of a larger ion in the lattice cage, other rare-earth ions with a valence less than 3, show evidence for a compressional stress in the system. A valence sum of 3.39 on Pr suggests a supratrivalent Pr ion in PrBa 2Cu 3O 7−δ. The compressional strain int he barium cage increases with decreasing rare-earth ionic radius, forcing an increasing orthorhombic distortion. The bond-valence sums on all the oxygens except O(4) are around 2 and they show little variation with the rare-earth size. A relatively lower bond valence on O(4) suggests that this oxygen is more likely to share the holes on copper.