NMR studies of ordered structures and valence states in the successive valence-transition system EuPtP

Abstract

We have studied EuPtP, which undergoes two successive valence transitions at T A ∼ 240 K and T B ∼ 200 K by 31 P-nuclear magnetic resonance (NMR) measurements. From the analysis of NMR spectra, we obtained plausible ordered structures and Eu valence states in three phases divided by T A and T B . These ordered structures well explain observed inequivalent P sites and the intensity ratio of the NMR spectra arising from these P sites. The results are also in good accordance with mean Eu valence measured by the x-ray absorption spectroscopy. We also discuss Eu 4f states and the origin of the transitions from the measurements of nuclear spin lattice relaxation rate and hyperfine coupling constant.

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

“…One may think of the discrepancy between the phase transitions and the gradual change in the valence of Eu. The deviation of the valence from the corresponding value of the γ-phase is due to a small portion of residual stacking of −(2+) − (2+) − (3+)−, which was suggested by the 31 P-NMR measurements [29]. The gradual changes of the valence was also observed at around the γ-δ phase transition in EuNiP [29] and at around the first-order valence transition in Eu(Pd 0.85 Pt 0.15 ) 2 Si 2 [37] and EuRh 2 Si 2 under high pressure [38].…”

“…The deviation of the valence from the corresponding value of the γ-phase is due to a small portion of residual stacking of −(2+) − (2+) − (3+)−, which was suggested by the 31 P-NMR measurements [29]. The gradual changes of the valence was also observed at around the γ-δ phase transition in EuNiP [29] and at around the first-order valence transition in Eu(Pd 0.85 Pt 0.15 ) 2 Si 2 [37] and EuRh 2 Si 2 under high pressure [38]. We think that the valence estimated from XAS tends to show gradual change associated with even a first-order valence transition.…”

“…Thus, EuPtP shows two successive valence transitions that occur simultaneously with the Eu valence order. Very recently, our collaborators have observed the valence order by single-crystal x-ray diffraction (XRD) [28] and nuclear magnetic and quadrupole resonance (NMR/ NQR) [29,30]. In particular, Moriyoshi et al have proposed more detailed crystal structures for the βand γphases.…”