Pressure-induced formation of intermediate-valence quasicrystalline system in a Cd-Mg-Yb alloy

Abstract

A quasiperiodic intermediate-valence system was prepared by applying pressure to an icosahedral Cd-Mg-Yb quasicrystal. X-ray absorption spectroscopy near the Yb ${L}_{3}$ edge demonstrates that the Yb valence increases continuously upon compression from the divalent state ($4{f}^{14}$) at ambient pressure and reaches a value of 2.71 at 57.6 GPa, which is close to the trivalent state ($4{f}^{13}$). By following the trend of Yb-based intermediate-valence crystalline compounds, this large valence increase suggests the change of $4f$ character in the Cd-Mg-Yb quasicrystal from an itinerant state with weak electron correlations to an intermediate region between an itinerant and a localized state with strong correlations. The valence increases sensitively with pressure below $\ensuremath{\sim}$30 GPa; however, the increase is significantly suppressed above $\ensuremath{\sim}$30 GPa. The rate of valence increase with respect to pressure in the lower-pressure region is twice larger than that of a Cd-Yb quasicrystal in our previous study [Phys. Rev. B 81, 220202(R) (2010)]. This is mainly explained by the smaller bulk modulus of the Cd-Mg-Yb quasicrystal compared to the Cd-Yb system. The suppression of valence increase with respect to pressure in the higher-pressure region is most likely due to an increase of conduction-$4f$ electron hybridization that counteracts $4f$ localization and reduces the Yb valence increase.