Crystal Structures of Ca-Free Pb2Sr2RCu3O8 (R = La, Pr, Nd, Eu, Tb, and Dy) Single Crystals

Structures of Pb2Sr2RCu3O8 have been determined by single crystal X-ray diffraction. A monoclinic symmetry (C2/m, Z = 2) was found for those crystals with R = Pr, Nd, and Tb, orthorhombic (Cmmm, Z = 2) for R = La, and tetragonal (P4/mmm, Z = 1) for R = Eu and Dy. The relationship among different crystal symmetries can be understood in terms of different oxygen orderings in the PbO layers driven by the Ph2+ lone pair orientation. There is evidence for a ∼ 9% deficiency in electron density at the R = Eu and Dy sites (and ∼ 4.5% for R = Tb) which has been confirmed by mass spectroscopy as being due to cation vacancies at these sites. Bond valence sum calculations have been performed to evaluate the strains existing in the crystal structures of Pb2Sr2RCu3O8.