Dysprosium(III) Sulfate Dy2(SO4)3: Crystal and Electronic Structure, Thermal Expansion, Thermochemistry, Magnetic and Optical Properties.

Dy2(SO4)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 αV = -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 SO4 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 (Eg = 6.1 eV), narrow photoluminescence at 577 nm, suitable for Dy3+-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 Dy2(SO4)3 as a multifunctional material combining NTE with favorable optical and magnetic properties.