| Summary: | 碩士 === 國立清華大學 === 物理學系 === 87 === Structural and magnetic studies have been made on the
PrBa2-xRxCu3O7+y system (R=Pr and La, 0<x<1, -0.12<y<0.4).
Powder x-ray Rietveld analysis indicates tha the structure
changes from the PrBa2Cu3O7 or CuBa2PrCu3O7 1212C-type orthorhombic phase (Pmmm with a0 < b0~0.39nm) to 1212-type
tetragonal phase (P4/mmm with at~0.39nm) around x=0.4 for
R=Pr, and 0.45 for La, and then to a new type of orthorhombic
O'-phase (Cmmm with a0'<b0'~0.55nm + superstructure modulation)
for x=0.7 for R=Pr and La.
Regardless of the structure change, the bond length between Pr and oxygen in the adjacent CuO2 layers increases with increasing x. Magnetic data show that the anomalously Neel temperature Tn(Pr) decreases monotonically from 18 K for x=0 to 2.8 K for PrBa1.3La0.7Cu3O7.24 and 2.5 K for Pr1.8Ba1.2Cu3O7.27. The increasing Pr-O bond length with decreasing Tn(Pr) indicates that Pr ordering is closely correlated with the wave function overlap between Pr-4f orbital and O-2p(pi) orbital in the CuO2 bi-layers.
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