Anomalous Pr ordering and the systematic variation of(Pr) for the Hg-1212-type systems

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