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 (∼10 -3 µ B /f.u.) caused by Ir 5 d electrons is observed below T MI . The entropy associated with MITs for Ln = Nd, Sm, and Eu is estimated to be 0.47, 2.0, and 1.4 J/(K·mole), respectively. The change in entropy is much smaller than 2 R ln 2 [11.5 J/(K·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.