Kyushu University Faculty of Science

In some rare-earth compounds, 4f electrons can be delocalized due to strong mixing with conduction electrons. When the configuration 4fⁿ of a rare earth ion is nearly degenerate with a 4f^n-1 state and a surplus conduction electron, the 4f state will fluctuate between two valence states. This behavior is called valence fluctuation. The valence fluctuating state of Eu is realized between Eu³⁺(4f⁶) and Eu²⁺(4f⁷) configurations. In some cases, valence changes abruptly in its temperature dependence, which is the valence transition. Recently, we have observed that EuRh₂Si₂ exhibits a valence transition under pressure [1]. The temperature-pressure diagram is depicted in Figure 1. The antiferromagnetic Eu²⁺ state is replaced by a nonmagnetic nearly Eu³⁺ state at low temperatures above 1 GPa.

In other cases, the two valence states are ordered periodically below the characteristic temperature due to strong Coulomb repulsion between electrons. This is the valence ordering. The ternary Eu pnictide, EuPtP had been believed to be a valence ordering system. The structure contains layers of Eu and Pt-P atoms alternating in the c-direction. This compound undergoes two first-order phase transitions at T₁=235 K and T₂=190 K. Early reports suggested that the valence ordering takes place at T₁ and another valence order state is realized below T₂. We prepared single crystals and performed resonant X-ray diffraction measurements as collaboration work with JASRI [2]. The results are shown in Figure 2. We have observed (1 1 1) reflection below T₂ and superlattice reflections (1 1 2/3) and (1 1 4/3) between T1 and T2. These results have revealed that the valence order patterns are Eu²⁺Eu³⁺Eu²⁺Eu³⁺ at T < T₂ and Eu²⁺Eu²⁺Eu³⁺Eu²⁺Eu²⁺Eu³⁺ at T₂ < T < T₁.

Finally, YbPd, which crystallizes in the cubic CsCl-type structure at room temperature and has four phase transitions, had also been anticipated to exhibit valence ordering. We have clarified by single crystal X-ray diffraction that YbPd exhibits a structural phase transition from cubic to tetragonal symmetry at T1=130 K and valence ordering transition at T2=110 K [3]. The valence ordering is exotic because integral valence (Yb³⁺) and valence fluctuation (Yb^2.6+) are arranged alternately along the c-axis. In addition, Using pure powdered samples and BL20 (iMATERIA) at J-PARC, the magnetic structure below T3=1.9 K was examined by powder neutron diffraction. It was manifested that the only Yb³⁺ ions exhibit an incommensurate sinusoidal magnetic structure with k = (0.080 0 0.32) and an amplitude of 0.3μ_B [4]. The amplitude is smaller than the theoretical magnetic moment of Yb³⁺, which suggests that Kondo effect affects the valence ordering and destabilizes the magnetic ordering.