Metal-Insulator Transitions in Pyrochlore Oxides Ln2Ir2O7

We report the physical properties of Ln(2)Ir(2)O(7) (Ln = Nd, Sm, Eu, Gd, Tb, Dy, and Ho), which exhibit metal-insulator transitions (MITs) at different temperatures. The transition temperature T-MI increases with a reduction in the ionic radius of Ln. The ionic radius boundary for MITs in Ln(2)Ir(2)O(7) lies between Ln = Pr and Nd. MITs in Ln(2)Ir(2)O(7) have some common features. They are second-order transitions. Under the field cool condition, a weak ferromagnetic component (similar to 10(-3) mu(B)/f.u.) caused by Ir 5d electrons is observed below TMI. The entropy associated with MITs for Ln = Nd, Sm, and Eu is estimated to be 0.47, 2.0, and 1.4 J/(K center dot mole), respectively. The change in entropy is much smaller than 2R ln 2 [11.5 J/(K center dot mole)] expected in a magnetic transition due to localized moments of S = 1/2. The feature of continuous MITs in Ln(2)Ir(2)O(7) is discussed.