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

Abstract

Magnetic and calorimetric measurements have been made to reveal a Pr ordering in the (${\mathrm{Pb}}_{0.5}$${\mathrm{Cu}}_{0.5}$${)}_{2}$(${\mathrm{Ba}}_{0.5}$${\mathrm{Sr}}_{0.5}$${)}_{2}$${\mathrm{PrCu}}_{2}$${\mathrm{O}}_{8}$ compound which has the (Pb,Cu)-2212-type tetragonal structure with lattice parameters a=3.878(3) \AA{} and c=27.594(9) \AA{}. The transition occurs at a relatively high ${\mathit{T}}_{\mathit{N}}$(Pr) of 9 K. In comparison, isostructural (${\mathrm{Pb}}_{0.5}$${\mathrm{Cu}}_{0.5}$${)}_{2}$(${\mathrm{Ba}}_{0.5}$${\mathrm{Sr}}_{0.5}$${)}_{2}$${\mathrm{GdCu}}_{2}$${\mathrm{O}}_{8}$ exhibits a ${\mathit{T}}_{\mathit{N}}$(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 ${\mathrm{Pr}}^{3+}$ 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-${\mathrm{CuO}}_{2}$-layer m212 (m=1,2,3) systems.