Ancient blue, modern speed: microwave-assisted synthesis and the role of structural evolution in Han blue luminescence

Han Blue (BaCuSi4O10) is a silicate pigment of significant technological interest due to its intense near-infrared (NIR) photoluminescence. Although conventional synthesis techniques require prolonged processing times and high energy consumption, microwave-assisted synthesis offers a rapid and efficient alternative. In this work, a comparative study is presented between Han Blue samples obtained solely through microwave activation and a hybrid protocol that integrates a subsequent conventional thermal annealing phase. The investigation focuses on the variability of structural properties and the formation of secondary phases, analyzing how highly accelerated reaction kinetics influence the structural ordering along the crystallographic c-axis and the local microenvironment of the Cu2+ emitting centers, influencing the luminescence properties. The results highlight that understanding the structural variability derived from different synthetic routes is paramount for optimizing the photophysical properties of these phosphors. In this light, this study provides valuable insights into the microwave-assisted solid-state synthesis of Han Blue, suggesting a practical approach to tailor and enhance the properties of NIR phosphors.