We report the physical properties of Ln 2 Ir 2 O 7 (Ln = Nd, Sm, Eu, Gd, Tb, Dy, and Ho), which exhibit metalinsulator 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 À3 B /f.u.) caused by Ir 5d 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/(KÁmole), respectively. The change in entropy is much smaller than 2R ln 2 [11.5 J/(KÁmole)] expected in a magnetic transition due to localized moments of S ¼ 1=2. The feature of continuous MITs in Ln 2 Ir 2 O 7 is discussed.

nescence changes from blue to yellow after grinding. Like other such compounds, its original lumi-nescent state is restored upon dissolution and recrystallization, and this process could be repeated for 20 cycles without any decrease in luminescence. Structural and spectroscopic studies indicate that the long-lived blue emission in the crystal is intramolecular in origin and phos-phorescent (a localized intraligand π-π* transition), whereas the yellow emission appears to arise from an amorphous phase characterized by aurophilic interactions: intermolecular interactions between gold atoms.-PDS

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We found two distinct maxima in the ac magnetic susceptibility χac of dysprosium titanate pyrochlore Dy2Ti2O7. Their different frequency dependence of χac suggest the existence of two different types of dynamical behaviour. One of them, observed below 2 K is the relaxation becoming very slow at lower temperature T, possibly related to a highly degenerate ground state (`spin ice' state). Another slowing down phenomenon is evident above ~10 K. Surprisingly, however, there is no anomaly in the dc magnetic susceptibility at this temperature range. Analysing the frequency dependence of χac, we found that in spite of a structurally ordered system the magnetic relaxation is well expressed by a Davidson-Cole formula, which is characterized by a wide distribution of the relaxation time τ to the short-τ region and a temperature dependent cut-off, τc, in the slow relaxations. The magnetic relaxation of the spin ice compound Dy2Ti2O7 is uniquely in contrast with known slow dynamics.

AC and DC magnetic susceptibilities have been measured on frustrated pyrochlore ferromagnets Ho2Sn2O7 and Ho2Ti2O7 at temperatures down to 100 mK. In the AC magnetic susceptibility, a dramatic drop of χ' (in-phase component) and a single maximum of χ'' (quadrature component) are observed at around 1 K. The frequency dependence of these peak positions indicates the presence of a slow magnetic relaxation at low temperature, whose relaxation time obeys the Arrhenius formula with the energy barrier Eb = 19.6 K and 27.5 K for Ho2Sn2O7 and Ho2Ti2O7, respectively. Strong irreversibility is also observed in the temperature dependence of the DC magnetization of Ho2Sn2O7 at 1 kOe below Tp~ 0.75 K; the zero-field-cooling (ZFC) curve indicates a very sharp peak at Tp, whereas the field-cooling curve has no anomaly at Tp and increases monotonically with decreasing T. In the DC magnetization process of the ZFC state at 0.46 K, a steep increase of the moment occurs above 2 kOe, and the irreversibility disappears at around ~15 kOe where the moment ferromagnetically saturates. These results indicate a clustering of magnetic moment whose size is of the order of a few tetrahedra, consistent with a recently proposed `spin ice' model.