PHASE EQUILIBRIA AND CHEMICAL PROCESSES IN SYSTEMS ZnO-SiO₂-NiO AND ZnO-SiO₂-MnO

Zaitseva N.A., Samigullina R.F., Dobrynenko E.S., Krasnenko T.I.

Institute of Solid State Chemistry UB RAS

620077, Ekaterinburg, Pervomaysksya st., 91

This paper examines chemical interactions and phase equilibria in the ternary systems ZnO-SiO₂-NiO and ZnO-SiO₂-MnO. This information is necessary for selecting optimal conditions for the synthesis of functional materials based on willemite Zn₂SiO₄ (green phosphor Zn₂SiO₄:Mn, cool blue pigment Zn_2-2xNi_2-2xSiO₄) and olivine Ni₂SiO₄. Solid-phase reactions between three reactants begin at the interfaces of only two at a time. In the ZnO-SiO₂-NiO and ZnO-SiO₂-MnO systems synthesized in air, the first pair of oxides to react are metal oxides, forming solid solutions of Ni_1-xZn_xO (rock salt structure) and Zn_1-xMn_xMn₂O₄ (spinel structure), respectively. With further increase in annealing time, the proportion of the dopant ion in the solid solutions increases. Upon heating to 900 °C, zinc and silicon oxides begin to react in both systems, forming willemite Zn₂SiO₄, which gradually becomes enriched with dopant metal ions to form the solid solution Zn_2-2xM_2-2xSiO₄ (M = Ni or Mn).

At higher temperatures, the reaction of Ni_1-xZn_xO and SiO₂ in the ZnO-SiO₂-NiO system results in the formation of a phase with the olivine structure Ni_2-2xZn_2-2xSiO₄. Phase equilibria in the system are determined by the quasi-binary equilibrium of the boundary compositions of the Zn_1.8Ni_0.2SiO₄ and Ni_1.5Zn_0.5SiO₄ solid solutions and the conodes connecting these compositions with the Ni_0.83Zn_0.17O solid solution, as well as the conode connecting the boundary composition of the Ni_1.6Zn_0.4O solid solution with Zn₂SiO₄.

In the ZnO-SiO₂-MnO system, no phases with an olivine structure were detected. In the temperature range of 800-1250 °C, the sequential appearance of intermediate phases ZnMnO₃, ZnMn₂O₄, and MnSiO₃ was detected. Phase equilibria were established for temperatures of 800 °C and 1250 °C. It is shown that the phase relationships in the MnOx–ZnO–SiO₂ system are determined by changes in the charge states of manganese ions with increasing temperature, leading to phase transformations of manganese oxide MnO→Mn₂O₃→Mn₃O₄→MnO. Triangulation of the Mn₂O₃–ZnO–SiO₂ system at 800°C is determined by the conode ZnMn₂O₄–Zn₂SiO₄ and decomposes the system into elementary triangles ZnO–Zn₂SiO₄–ZnMn₂O₄, Zn₂SiO₄–ZnMn₂O₄–SiO₂, and ZnMn₂O₄–SiO₂–Mn₂O₃. The triangulation of the ternary system MnO–ZnO–SiO₂ is determined by the elementary triangle Zn_1.6Mn_0.4SiO₄–ZnO–MnSiO₃.

This work was fulfilled as part of the Government Assignment to the Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences (project No. 124020600004-7).