Charge ordering in Eu₄as₃ under pressure

The pressure versus temperature phase diagram of the mixed-valence Eu ₄As₃ 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 mol^-1 K ^-1 from the Clausius-Clapeyron relation. The value is twice larger than the configuration entropy which is expected from a random arrangement of Eu³⁺ and Eu²⁺ ions in the thermally driven valence fluctuating state of Eu₄As₃. Moreover, a new structural phase was observed above 20 GPa at room temperature.