Valence and magnetism in $\rm EuPd_3S_4$ and $\rm (Y,La)_xEu_{1-x}Pd_3S_4$

TL;DR

This study investigates how yttrium and lanthanum substitutions affect valence states and magnetic properties in EuPd3S4, revealing volume conservation as key to valence changes and a temperature-dependent transition in mixed-valent behavior.

Contribution

It demonstrates that cell volume conservation explains valence shifts in EuPd3S4 with Y and La substitution, and uncovers a temperature-driven transition from static to dynamic mixed valence.

Findings

Y substitution drives Eu towards Eu2+ state

La substitution causes a transition to dynamic mixed valence at higher temperatures

Magnetic order persists despite reduced Eu2+ content

Abstract

$^{151}$Eu M\"ossbauer spectroscopy shows that yttrium substitution in mixed-valent $\rm EuPd_3S_4$ drives the initial 50:50 mix of Eu$^{3+}$ and Eu$^{2+}$ towards pure Eu$^{2+}$, whereas lanthanum substitution has the opposite effect, but only for substitution levels above 50\%. We find that total valence electron count and chemical pressure effects cannot account for the observed behaviour, however conserving the cell volume provides a consistent description of the changes in the Eu$^{2+}$:Eu$^{3+}$ ratio. Remarkably, lanthanum substitution also leads to a clear transition from static mixed-valent behavior at lower temperatures to dynamic mixed valent behavior at higher temperatures, with the onset temperature monotonically increasing with Eu content and extrapolating to a value of $\sim$340~K for the pure $\rm EuPd_3S_4$ compound. Magnetic order persists at least as far as x=0.875 in both series, despite the drastic reduction in the amount of moment-carrying Eu$^{2+}$ ions.