Investigation of the Order Parameter of Pr in the Filled Skutterudite PrRu4P12 by Resonant Soft X-ray Diffraction

The filled skutterudite PrRu4P12 shows a metal-insulator (MI) transition at 62.3 K which is yet to be understood. It is believed that the Pr 4 f electrons play an integral role in the MI transition however the order parameter associated with the Pr 4 f shells remains elusive. In this study, we investigate PrRu4P12 using temperature dependent resonant soft x-ray diffraction in combination with x-ray absorption spectroscopy at the Pr M-4,M-5 edges. A resonant enhancement of the (100) reflection at the Pr M-4,M-5 edges signaling the order parameter of the Pr 4 f shells was observed below T-MI, with a steady increase with decreasing temperature. The experimental spectra and subsequent analysis rule out the existence of magnetic, orbital and significant charge order, as well as any Pr lattice displacement. The ordered state below T-MI is likely the periodic electronic energy-level modulation of Pr 4 f shells. Our results indicate that a synergistic coupling of Pr 4 f states and lattice distortion plays an important role in the metal-insulator transition associated with charge density wave state formation.