Pressure-induced melting of charge order in Eu₄As₃ without structural change

We have carried out high-pressure x-ray diffraction experiments on Eu ₄As₃ 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.