Dysprosium(III) Sulfate Dy 2 (SO 4 ) 3 : Crystal and Electronic Structure, Thermal Expansion, Thermochemistry, Magnetic and Optical Properties

Dy 2 (SO 4 ) 3 is shown to exhibit negative thermal expansion (NTE) in all three crystallographic directions, suggesting that rare earth sulfates may serve as a novel platform for 3D NTE materials. High‐temperature x‐ray diffraction (453–703 K) reveals a NTE volume coefficient of α V = ‐18.27(7)·10 −6 K −1 , attributed to anisotropic compression along the b‐ and c‐axes. Combined DFT calculations and vibrational spectroscopy identify low‐frequency phonon modes with pronounced rotational motions of SO 4 tetrahedra, suggesting a rigid unit mode (RUM) contribution to the lattice compression. In addition to the NTE, the material exhibits high thermal stability (onset of decomposition at 783°C), a wide band gap (E g = 6.1 eV), narrow photoluminescence at 577 nm, suitable for Dy 3+ ‐based laser applications, and pronounced magnetic anisotropy, indicating strong crystal field effects. These results extend the known families of NTE to sulfate‐based structures and position Dy 2 (SO 4 ) 3 as a multifunctional material combining NTE with favorable optical and magnetic properties.