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|Title:||Crystal chemistry of Ca-doped and Ca-free Pb₂Sr₂RCu₃O₈₊δ (R = rare earth): Synthesis, structure and physical properties|
|Authors:||Xue, Simon Jiayu|
|Abstract:||<p>A novel PbO/NaCl flux growth method has been developed to grow single crystals of Ca-doped Pb<sub>2</sub>Sr<sub>2</sub>(R/Ca)Cu<sub>3</sub>O<sub>8+δ</sub> 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 Pb<sub>2</sub>Sr<sub>2</sub>(Y/Ca)Cu<sub>2</sub>O<sub>8+δ</sub> crystals are of high quality, reasonable size and superconducting with T<sub>c</sub> 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 Pb<sub>2</sub>Sr<sub>2</sub>RCu<sub>3</sub>O<sub>8</sub> (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 Pb<sup>2+</sup> 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 Pb<sub>2</sub>Sr<sub>2</sub>RCu<sub>3</sub>O<sub>8</sub> crystal series.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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