Systematic variation of T_N(Pr) for the two-CuO₂-layer cuprate m212 (m=1, 2, 3) systems

The systematic variation of anomalous Pr antiferromagnetic ordering temperature T_N(Pr) for the two-CuO₂-layer cuprate system MA₂PrCu₂O_y (M=Cu, Hg, Tl, Pb; A=Ba, Sr) was verified through the observation of a new Hg-1212 HgSr₂PrCu₂O_6+δ tetragonal compound with a T_N(Pr) of 6 K and a Pr-O bond length of 2.492 Å between Pr and oxygen in the adjacent CuO₂ layers. For the tetragonal/orthorhombic 1212 compounds including the PrBa₂Cu₃O_7-y system, T_N(Pr) was found to decrease monotonically with increasing Pr-O distance. The importance of Pr-O-Pr superexchange magnetic coupling is expected for all two-CuO₂-layer M_mA₂PrCu₂O_y w212-type (m=1, 2, 3) compounds. A similar Pr anomaly in the 2212-type system was first observed on the new tetragonal (Pb_0.5Cu_0.5)₂(Ba_0.5Sr _0.5)2PrCu₂O₈ compound with a high T_N(Pr) of 9 K.