Observation of Pr and Gd ordering in the 2212-type compounds (Pb0.5Cu0.5)2(Ba0.5Sr0.5)2RCu2O8 (R=Pr,Gd)

Magnetic and calorimetric measurements have been made to reveal a Pr ordering in the (Pb0.5Cu0.5)2(Ba0.5Sr0.5)2PrCu2O8 compound which has the (Pb,Cu)-2212-type tetragonal structure with lattice parameters a=3.878(3) and c=27.594(9). The transition occurs at a relatively high TN(Pr) of 9 K. In comparison, isostructural (Pb0.5Cu0.5)2(Ba0.5Sr0.5)2GdCu2O8 exhibits a TN(Gd) of only 2.2 K. The magnetic entropy associated with the Pr ordering has a lower limit value equivalent to about 20% of Rln3 expected for Pr3+ with a quasitriplet ground state, suggesting a more three-dimensional-like process. These results represent the observation of anomalous Pr ordering in the 2212-type cuprate and a complete demonstration of such effects in all two-CuO2-layer m212 (m=1,2,3) systems.