Valence and magnetism in EuPd3S4 and (Y,La)x

The double Dirac candidate EuPd${}_{3}$S${}_{4}$ appears to exhibit a roughly 50:50 mixture of Eu${}^{2+}$ and Eu${}^{3+}$. Substituting yttrium rapidly drives the remaining europium divalent, whereas lanthanum has the opposite effect. Analysis shows that average site volumes play a dominant role in the valence distribution suggesting that local strains matter more than electron count. Remarkably, lanthanum substitution also promotes valence instability leading to the appearance of intermediate valence ``Eu${}^{2.5+}$'', the fraction of which increases with both lanthanum content and temperature. Extrapolation of the onset temperature to zero lanthanum predicts ${T}_{\text{onset}}$ \ensuremath{\sim} 340 K matching recent reports of a structural transition attributed to a loss of valence order.