Emergence of a new valence-ordered structure and collapse of the magnetic order under high pressure in EuPtP

The layered hexagonal EuPtP is a rare substance that exhibits two successive valence transitions occurring simultaneously with valence ordering transitions and an antiferromagnetic order. Anticipating that the application of pressure to this sample would induce a new valence-ordered structure and/or a new phenomenon associated with valence fluctuation, we examined the electrical resistivity ρ, the Eu L₃-edge x-ray absorption spectroscopy, and the powder x-ray diffraction under high pressure. We found a new valence transition at around P = 2.5 GPa. After the transition, a new valence-ordered structure is realized at the lowest temperature. The valence-ordered structure is inferred to be stacking of (2+: Eu²⁺ layer, 3+: Eu³⁺ layer) along the c-axis. Upon further increases in pressure, the valence-ordered structure is suppressed and another valance-ordered phase is realized up to P = 6 GPa. The antiferromagnetic order collapses in the pressure range between 6 GPa and 8 GPa.