Pressure-induced delocalization in charge-ordering rare-earth compounds studied bynuclear resonant scattering

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The pressure versus temperature phase diagram of the mixed-valent Eu4As3 compound was determined by means of x-ray diffraction measurements using synchrotron radiation. The structural phase transition occurs at 343 K under ambient pressure with volume reduction and the transition temperature increases linearly with pressure up to about 5 GPa. The entropy change at this transition is estimated to be 41 J/molK from the Clausius-Clapeyron relation. The value is twice lager than the configuration entropy which is expected from the random arrangement of Eu^<3+> and Eu^<2+> ions in the thermally derived valence fluctuating state of Eu_4As_3.^<151>Eu nuclear resonant inelastic x-ray scattering (NRIXS) experiments have been performed on Eu_4As_3 from 301 to 406 K. We derived the thermodynamic parameters from inelastic components in the observed NRIXS spectra using the sum rules. The mean recoilless fraction shows the softening in the lattice vibration of the Eu ion around the boundary of the B … More rillouin zone in the valence fluctuating state. We present by the temperature dependence of mean force constant that the hybridization between the 4fstates and the conduction or valence bands in Eu_4As_3 plays a role around the charge ordering temperature and in the valence fluctuating state.We have carried out high-pressure x-ray diffraction experiments on Eu_4As_3 at mom 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 pressure dependence of the distortion angle changes at 10 GPa although the volume shows no anomaly as a function of pressure. The results show that the volume of the EuAs_6 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 dear 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 10 GPa and Eu_4As_3 is in a mixed-valence state above 10 GPa. Less