Crystal chemistry of Ca-doped and Ca-free Pb₂Sr₂RCu₃O₈₊δ (R = rare earth): Synthesis, structure and physical properties

Abstract

<p>A novel PbO/NaCl flux growth method has been developed to grow single crystals of Ca-doped Pb₂Sr₂(R/Ca)Cu₃O_8+δ with R = Nd, Sm, Gd, Dy, Er, Tm, Yb, Lu and Y. The absence of CuO flux enables the crystals to be extracted nondestructively from the melt using an ultrasonic bath and methanol. The as grown Pb₂Sr₂(Y/Ca)Cu₂O_8+δ crystals are of high quality, reasonable size and superconducting with T_c up to 80K. The role of NaCl in the crystal growth process has also been investigated. The same method was applied to grow a series of Ca-free Pb₂Sr₂RCu₃O₈ (R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Y) crystals. X-ray diffraction was used to determine the crystal structures of these compounds. A monoclinic symmetry was found for those crystals with R = Pr, Nd and Tb, orthorhombic for R = La and tetragonal for R = Eu and Dy. Different crystal symmetries can be understood in terms of order/disorder among oxygens in the PbO layers which is driven by an orientational ordering of the Pb²⁺ lone pairs. Bond valence sum calculations were also performed to evaluate the residual strains existing in the crystal structures. A transition from insulating to metallic and superconducting character occurs as a function of decreasing rare-earth size in this Ca-free crystal series. Structural and chemical analyses revealed that the electronic properties of these crystals are closely related to the rare-earth deficiency. Therefore, a cation vacancy carrier-doping mechanism has been proposed for the first time as the most plausible origin of the metallic and superconducting behaviour observed in the Pb₂Sr₂RCu₃O₈ crystal series.</p>