Variation of antiferromagnetic Pr ordering in the Tl(Ba_1-xSr_x)₂PrCu₂O_7-δ system

Structural, magnetic, calorimetric, and transport studies have been made on a synthesized Tl(1212) system of Tl(Ba_1-xSr_x)₂PrCu₂O_7-δ (0≤x≤1). Based on x-ray Rietveld refinement data, the bond length between Pr and oxygen in the adjacent CuO₂ layers increases with increasing x, even though the tetragonal lattice parameters decrease with progressive Sr substitution. This may result in a lower degree of hybridization between extended Pr 4f and O 2pπ orbitals and a corresponding reduction of superexchange magnetic coupling strength among Pr moments. Indeed, the anomalously high Néel temperature T_N(Pr) decreases from 8.0–8.2 K for x=0 to 3.6–4.2 K for x=1. The magnetic entropy associated with this antiferromagnetic Pr ordering is estimated from specific-heat data, in which a heavy-fermion-like contribution also prevails.