Valence and magnetism in $\mathrm{EuP}{\mathrm{d}}_3{\mathrm{S}}_4$ and $(\mathrm{Y},\mathrm{La}{)}_{\mathrm{x}}\mathrm{E}{\mathrm{u}}_{1-\mathrm{x}}\mathrm{P}{\mathrm{d}}_3{\mathrm{S}}_4$

Abstract

${}^{151}$Eu Mössbauer spectroscopy shows that yttrium substitution in mixed-valent $\mathrm{EuP}{\mathrm{d}}_3{\mathrm{S}}_4$ 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 $\mathrm{EuP}{\mathrm{d}}_3{\mathrm{S}}_4$ 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.

16 pages, 26 figures

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