Physical properties of DyB4 have been studied by magnetization, specific heat, and ultrasonic measurements. The magnetic entropy change and ultrasonic properties in intermediate phase II indicate that the degeneracy of internal degrees of freedom is not fully lifted in spite of the formation of magnetic order. The ultrasonic attenuation and huge softening of C44 in phase II suggest the existence of electric-quadrupolar (orbital) fluctuations of 4f -electrons. These unusual properties originate from a geometrical quadrupolar frustration.

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Dependence on lithium-ion batteries for automobile applications is rapidly increasing. The emerging use of anionic redox can boost the energy density of batteries, but the fundamental origin of anionic redox is still under debate. Moreover, to realize anionic redox, many reported electrode materials rely on manganese ions through π-type interactions with oxygen. Here, through a systematic experimental and theoretical study on a binary system of Li 3 NbO 4 −NiO, we demonstrate for the first time the unexpectedly large contribution of oxygen to charge compensation for electrochemical oxidation in Ni-based materials. In general, for Ni-based materials, e.g., LiNiO 2 , charge compensation is achieved mainly by Ni oxidation, with a lower contribution from oxygen. In contrast, for Li 3 NbO 4 −NiO, oxygen-based charge compensation is triggered by structural disordering and σ-type interactions with nickel ions, which are associated with a unique environment for oxygen, i.e., a linear Ni−O−Ni configuration in the disordered system. Reversible anionic redox with a small hysteretic behavior was achieved for LiNi 2/3 Nb 1/3 O 2 with a cation-disordered Li/Ni arrangement. Further Li enrichment in the structure destabilizes anionic redox and leads to irreversible oxygen loss due to the disappearance of the linear Ni−O−Ni configuration and the formation of unstable Ni ions with high oxidation states. On the basis of these results, we discuss the possibility of using σ-type interactions for anionic redox to design advanced electrode materials for highenergy lithium-ion batteries.

Magnetic field (B) variation of the electrical polarization P(c) (∥c) of the perfect triangular lattice antiferromagnet RbFe(MoO(4))(2) is examined up to the saturation point of the magnetization for B⊥c. P(c) is observed only in phases for which chirality is predicted in the in-plane magnetic structures. No strong anomaly is observed in P(c) at the field at which the spin modulation along the c axis, and hence the spin helicity, exhibits a discontinuity to the commensurate state. These results indicate that the ferroelectricity in this compound originates predominantly from the spin chirality, the explanation of which would require a new mechanism for magnetoferroelectricity. The obtained field-temperature phase diagram of ferroelectricity agree well with those theoretically predicted for the spin chirality of a Heisenberg spin triangular lattice antiferromagnet.