Formation of α- and β-phase Mn-doped zinc silicate in supercritical water and its luminescence properties at Si/(Zn+Mn) ratios from 0.25 to 1.25

Mn-doped Zn₂SiO₄ (ZSM) was synthesized in supercritical water under batch conditions to study the effect of Si molar ratio on ZSM phase formation and its luminescence properties. Precursors were prepared with zinc oxalate dihydrate, manganese oxalate, and amorphous SiO₂, where Mn/(Zn+Mn) molar ratio was a constant of 0.02 and Si/(Zn+Mn) molar ratios were varied from 0.25 to 1.25. Reaction conditions were chosen to be 400 °C, a pressure of 29 MPa, and for a reaction time of 90 min. At a stoichiometric Si ratio of 0.5, green-emitting α-Zn₂SiO₄ was only phase formed. At Si ratios from 0.55 to 1.25, an α-Zn₂SiO₄ phase and yellow-emitting β-Zn₂SiO₄ phase were formed along with unreacted amorphous SiO₂. As Si ratio increased, the emission color of the materials changed from the green region to the yellow-green region. The main phases present in the system, ZnO-SiO₂-H₂O, at 400 °C, 29 MPa, and 90 min were α-Zn₂SiO₄ and β-Zn₂SiO₄, while other phases such as ZnSiO₃ and Zn₄Si₂O₇(OH)₂·H₂O did not appear after reaction in supercritical water under these experimental conditions.