Anomalous Pr ordering in HgSr2PrCu2O6+delta and the systematic variation of T-N(Pr) for the Hg-1212-type systems

Structural, magnetic, and calorimetric studies have been made to reveal an anomalous Pr antiferromagnetic ordering at T-N(Pr) of 6 K in HgSr2PrCu2O6+delta. This compound has the Hg-1212-type tetragonal structure with lattice parameters a = 3.8532 Angstrom and c = 12.1199 Angstrom. X-ray Rietveld refinement yields a Pr-O bond length d(Pr-O) of 2.492 Angstrom between Pr and oxygen in the adjacent CuO2 layers and an O-Pr-O bond angle of 101.3 degrees. The low-temperature magnetic entropy associated with the Pr ordering is only about 10% of Rln3 expected for Pr3+ With a quasitriplet ground state, suggesting a more two-dimensional-like ordering process. This result supports the earlier observation of a systematic dependence of T-N(Pr) on Pr-O bond length in various M(Ba,Sr)(2)PrCu2Oy (M = Cu, Hg, Tl, Pb) 1212-type Pr compounds. This in turn points to the importance of a superexchange magnetic coupling mechanism.