Reentrant valence transition in YbCu4.5 under pressure

The electronic structure of ${\mathrm{YbCu}}_{4.5}$ under pressure has been studied with x-ray emission spectroscopy. Pressure-induced first-order valence transition to the divalent Yb state is found around 0.6--2.7 GPa, accompanied by the structural transition at the same pressure range of 0--1.0 GPa suggested by the change in the x-ray diffraction pattern. We also measured temperature dependencies of the Yb valence and magnetic susceptibility, which are compared with the single impurity Anderson model.