Sulfate Recrystallization as a Precursor‐Conditioning Step in the Hydrogen‐Reduction Synthesis of Gd 2 O 2 S

Gd 2 O 2 S was synthesized from Gd 2 (SO 4 ) 3 ·8H 2 O through a three‐step thermal process consisting of dehydration, intermediate air heat treatment, and hydrogen reduction. The phase evolution and role of sulfate recrystallization were systematically investigated using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric–differential scanning calorimetry (TG–DSC). Heat treatment at 300°C produced dehydrated anhydrous Gd 2 (SO 4 ) 3 , followed by progressive recrystallization and structural evolution during subsequent treatment at 400°C–700°C. Quantitative XRD peak‐profile analysis supported increased structural ordering and crystallite coherence of the anhydrous sulfate during this intermediate treatment. Hydrogen reduction of the recrystallized precursor at 800°C proceeded through a Gd 2 O 2 SO 4 intermediate before conversion to hexagonal Gd 2 O 2 S. Direct‐reduction control experiments showed that Gd 2 O 2 S could also form without prior recrystallization; however, prolonged direct reduction resulted in detectable Gd 2 O 3 formation. In contrast, the recrystallized precursor favored phase‐selective conversion toward Gd 2 O 2 S during prolonged hydrogen treatment. These results indicate that sulfate recrystallization is not an absolute prerequisite for initial Gd 2 O 2 S formation but acts as an important precursor‐conditioning step that influences the subsequent reduction pathway and phase selectivity. The findings provide a processing basis for controlling precursor evolution and phase formation in sulfate‐derived Gd 2 O 2 S powders.