Structural and luminescent properties of water-quenched Eu3+-doped HfW2O8 with negative thermal expansion

This study represents the first attempt to correlate the photoluminescence properties of Eu³⁺-doped HfW₂O₈ (Hf_1-xEu_xW₂O₈) with their novel thermal structural changes, i.e., negative thermal expansion (NTE). We synthesized Hf_1-xEu_xW₂O₈ (x = 0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.12, and 0.15) via a fast solid state reaction method with water quenching. Temperature dependent X-ray diffraction patterns exhibited the phase transition from α-phase (P2₁3, cubic) to β-phase (Pa-3, cubic) as the temperature increased from room temperature to 200 °C, along with highly linear contraction of lattice constants. The thermal expansion coefficient and the volume thermal expansion coefficient were determined as approximately −1.1 × 10⁻⁵ and −3.4 × 10⁻⁵, respectively. In the temperature dependent photoluminescence measurement, together with the thermal quenching behavior, we found that the asymmetric ratios decreased in accord with the structural change to higher symmetry. These findings reveal that the luminescent properties of Eu³⁺ are closely related to the structural properties in NTE HfW₂O₈.