Date of Award
Doctor of Philosophy (PhD)
A novel PbO/NaCl flux growth method has been developed to grow single crystals of Ca-doped Pb2Sr2(R/Ca)Cu3O8+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)Cu2O8+δ crystals are of high quality, reasonable size and superconducting with Tc 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.
Xue, Jiayu Simon, "Crystal chemistry of Ca-doped and Ca-free Pb₂Sr₂RCu₃O₈₊δ (R = rare earth): Synthesis, structure and physical properties" (1992). Open Access Dissertations and Theses. Paper 3977.