Valence and magnetism in EuPd3S4 and (Y, La)xEu1-xPd3S4

151Eu Mossbauer spectroscopy shows that yttrium substitution in mixed-valent EuPd3S4 drives the initial 50:50 mix of Eu3+ and Eu2+ towards pure Eu2+, 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 behavior; however, conserving the cell volume provides a consistent description of the changes in the Eu2+ : Eu3+ 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 similar to 340 K for the pure EuPd3S4 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 Eu2+ ions.