Observation of two charge ordering transitions in the valence-fluctuating EuPtP by resonant x-ray diffraction

Two first-order phase transitions of the valence-fluctuating compound EuPtP at T₁ =246K and T₂ =200K were examined by means of resonant x-ray diffraction at the Eu L₃ absorption edge. Although previous Mössbauer spectroscopy and preliminary x-ray diffraction experiments have suggested charge-ordered states, characteristic energy spectra of the 111 and 11 2/3 reflections observed in the present experiment unambiguously indicated that twofold and threefold superstructures of Eu valence exist below T₂ and between T₁ and T₂, respectively. We also propose a mechanism to explain the charge ordering in EuPtP based on a charge-lattice coupling that alters a Coulomb repulsion.