 Gadolinium sulfate octahydrate

Gadolinium sulfate octahydrate is the hydrated salt $\mathrm{G}{\mathrm{d}}_2(\mathrm{S}{\mathrm{O}}_4{)}_3\cdot 8{\mathrm{H}}_2\mathrm{O}$. Gadolinium(III) ions provide localized magnetic moments, while water of crystallization is part of the crystal composition.

The material was used in the Giauque-MacDougall magnetic cooling experiment. Changing the magnetic field under suitable thermal isolation changes the spin entropy and the temperature, connecting the salt's microscopic magnetic states with refrigeration.