Makoto Wakeshima

Reversible Mechanochromic Luminescence of [(C6F5Au)2(μ-1,4-Diisocyanobenzene)]

Reversible mechanochromic luminescence of [(C6F5Au)2(mu-1,4-diisocyanobenzene)] is reported. Grinding of the complex induced a photoluminescent color change, which was restored by exposure to a solvent. This cycle was repeated 20 times with no color degradation in the emissions. Their optical properties, X-ray crystallographic analysis, IR, and XRD measurements strongly suggested that the change in the molecular arrangement is responsible for this mechanochromic property. Intermolecular aurophilic bondings presumably play a key role in the altered emission.

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.

Specific Heat Study on Sm-based Filled Skutterudite Phosphides SmT4P12(T=Fe, Ru and Os)

Sm-based filled skutterudite phosphides SmFe 4 P 12 , SmRu 4 P 12 and SmOs 4 P 12 synthesized at high temperatures and high pressures have been studied by specific heat measurements in magnetic fields. SmFe 4 P 12 shows an anomaly at 1.6 K attributed to a ferromagnetic transition. SmRu 4 P 12 shows a metal–insulator transition at 16 K. This transition consists of two successive transitions, which suggests orbital and antiferromagnetic orderings. SmOs 4 P 12 shows a sharp anomaly at 4.5 K due to an antiferromagnetic ordering. We discuss the characteristic behavior of those phase transitions and the crystalline electric field state in Sm T 4 P 12 ( T =Fe, Ru and Os). Interestingly, the magnetic entropy of SmFe 4 P 12 have only 0.16 R ln 2 below the ferromagnetic ordering temperature. This suggests a strong spin fluctuation effect. On the other hand, the variation of magnetic entropy in SmRu 4 P 12 and SmOs 4 P 12 reaches to nearly R ln 4 below the ordering temperature.

Metal-insulator transition in pyrochlore iridates Ln2Ir2O7 (Ln = Nd, Sm, and Eu)

We report that Ln 2 Ir 2 O 7 for Ln = Nd, Sm, and Eu have metal–insulator transitions (MITs) at 36, 117, and 120 K, respectively. Their electrical resistivities and thermoelectric powers are not discontinuous and exhibit no thermal hysteresis at their transition temperatures T MI , indicating second-order phase transitions. In this letter, we focus on the MIT of Sm 2 Ir 2 O 7 . The specific heat and the magnetic susceptibility show a clear anomaly at T MI . These results indicate that this MIT involves a magnetic ordering produced by 5 d electrons in Ir.

Magnetic properties of the antiferromagnetic double perovskite Ba2PrRuO6

The ordered perovskite compound Ba2PrRuO6 is prepared and its magnetic properties are investigated. The Rietveld analysis of the neutron diffraction profiles measured at 150 K shows that the Pr3+ and Ru5+ ions are arranged with regularity over the six-coordinate B sites of the perovskite ABO3 and that Ba2PrRuO6 belongs to space group P21/n, with a = 6.0063(5), b = 5.9863(4), c = 8.4677(7) A and 90.04(2)°. The magnetic susceptibility and the heat capacity measurements show that this compound transforms to an antiferromagnetic state below 117 K. From the neutron diffraction patterns measured at 7 K, the magnetic structure is determined to be of Type I and the magnetic moments of Pr3+ and Ru5+ are estimated to be 2.2(1) and 2.0(2) µB, respectively. Their values are discussed on the basis of theoretical calculations for the crystal field splitting.

Crystal structures and magnetic properties of ordered perovskites A2R3+Ir5+O6 (A=Sr, Ba; R=Sc, Y, La, Lu)

Abstract The ordered perovskite-type compounds A 2 RIrO 6 (A=Sr, Ba; R=Sc, Y, Lu, La), with the exception of Sr 2 LaIrO 6 , were prepared. Using the Rietveld method, their crystal structures were assigned to be monoclinic (space group P2 1 /n ) with a fully ordered arrangement between R and Ir ions. For Sr 2 ScIrO 6 and Ba 2 ScIrO 6 , small amounts of disordered arrangements between Sc and Ir ions were found. The magnetic susceptibilities of these compounds show paramagnetic behavior down to 4.5 K. The magnetic behavior for these compounds is explained by Kotani’s theory and the spin–orbit coupling constants for the Ir(V) ion were calculated.

Emergence of Magnetic Long-range Order in Frustrated Pyrochlore Nd2Ir2O7 with Metal–Insulator Transition

In this study, we performed powder neutron diffraction and inelastic scattering measurements of frustrated pyrochlore Nd 2 Ir 2 O 7 , which exhibits a metal–insulator transition at a temperature T MI of 33 K. The diffraction measurements revealed that the pyrochlore has an antiferromagnetic long-range structure with propagation vector q 0 of (0,0,0) and that it grows with decreasing temperature below 15 K. This structure was analyzed to be of the all-in all-out type, consisting of highly anisotropic Nd 3+ magnetic moments of magnitude 2.3±0.4µ B , where µ B is the Bohr magneton. The inelastic scattering measurements revealed that the Kramers ground doublet of Nd 3+ splits below T MI . This suggests the appearance of a static internal magnetic field at the Nd sites, which probably originates from a magnetic order consisting of Ir 4+ magnetic moments. Here, we discuss a magnetic structure model for the Ir order and the relation of the order to the metal–insulator transition in terms of frustration.

Systematic Study of Lattice Specific Heat of Filled Skutterudites

The lattice specific heat C lat of La-based filled skutterudites La T 4 X 12 ( T = Fe, Ru and Os; X = P, As, and Sb) has been systematically studied, and both the Debye temperature Θ D and the Einstein temperature Θ E of La T 4 X 12 were carefully estimated. We confirmed that a correlation exists between Θ D and the reciprocal of the square root of average atomic mass for La T 4 P 12 , La T 4 As 12 , and La T 4 Sb 12 . The Θ D of filled skutterudites was found to depend mainly on the nature of the species X forming the cage. The temperature dependence of C lat / T 3 for La T 4 X 12 exhibited a large broad maximum at low temperatures (10–30 K), which suggests a nearly dispersionless low-energy optical mode characterized by Einstein specific heat. Since no such broad maximum exists for the unfilled skutterudite RhP 3 , the low-energy optical modes are associated with vibration involving La ions in the X 12 cage (the so-called “guest ion modes”). The Θ E of filled skutterudites was found to roughly correspon...

Magnetic properties of iridium pyrochlores R2Ir2O7 (R = Y, Sm, Eu and Lu)

Magnetic susceptibility, magnetic hysteresis, Mossbauer effect, and specific heat measurements were performed for R2Ir2O7 (R = Y, Sm, Eu and Lu), which show magnetic transitions at 150, 115, 120 and 120 K, respectively. Below the transition temperatures there is a large difference in the temperature dependence of the magnetization measured under zero-field-cooled conditions (ZFC) and under field-cooled conditions (FC), but no magnetic hysteresis loop has been observed. The magnetic properties of the Ir pyrochlores are similar to those of isostructural Ru pyrochlores. No sharp anomaly is observed in specific heat data for any of these compounds down to 1.8 K. These results suggest that R2Ir2O7 shows spin-glass-like behaviour below the transition temperatures.

Magnetic properties of ruthenium pyrochlores R2Ru2O7 (R = rare earth)

Magnetic susceptibility and magnetic hysteresis measurements were performed for rare earth pyrochlores R2Ru2O7 (R = Pr, Nd and Sm-Yb). The compounds of R = Pr, Nd, Sm and Eu show magnetic transitions at 165, 150, 135 and 120 K, respectively. Below the transition temperatures, there is a large difference in the temperature dependence of the magnetization measured under zero-field-cooled conditions (ZFC) and under field-cooled conditions (FC), but no magnetic hysteresis has been observed. These results show that below the transition temperatures, the magnetic state of these pyrochlores transforms to a spin-glass state. When the temperature is further decreased through ~20 K, apparent cusps have been observed for R = Nd, Sm and Eu in the susceptibility-temperature curves. Magnetic hysteresis measurements at 5.0 K indicate that there is a possibility of a contribution of weak ferromagnetism to the magnetic behaviour of Sm2Ru2O7 at very low temperatures. That is, at these very low temperatures, both the spin-glass state and a weak ferromagnetic state may coexist for Sm2Ru2O7.