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

Abstract

Two first-order phase transitions of the valence-fluctuating compound EuPtP at ${T}_{1}=246\text{ }\text{K}$ and ${T}_{2}=200\text{ }\text{K}$ were examined by means of resonant x-ray diffraction at the $\text{Eu}\text{ }{L}_{3}$ absorption edge. Although previous M\"ossbauer spectroscopy and preliminary x-ray diffraction experiments have suggested charge-ordered states, characteristic energy spectra of the 111 and $11\frac{2}{3}$ reflections observed in the present experiment unambiguously indicated that twofold and threefold superstructures of Eu valence exist below ${T}_{2}$ and between ${T}_{1}$ and ${T}_{2}$, respectively. We also propose a mechanism to explain the charge ordering in EuPtP based on a charge-lattice coupling that alters a Coulomb repulsion.