Pressure-Induced Melting Of Charge Order In Eu4as3 Without Structural Change

We have carried out high-pressure x-ray diffraction experiments on Eu4As3 at room temperature, and individual atomic coordinates as well as lattice parameters were derived precisely from the diffraction data. We have confirmed that a trigonal structure with the R3c space group does not change up to 19 GPa. The results show that the volume of the EuAs6 octahedron and the valence state of the Eu ion are closely correlated, and pure trivalent and divalent Eu ions occupy two nonequivalent Eu sites, respectively, in the charge-ordered state at ambient pressure. We present clear evidence on the basis of the pressure dependence of these volumes at two nonequivalent Eu sites that a melting of the static charge order occurs at about 10 GPa and Eu4As3 is in a mixed-valence state above 10 GPa.