The mechanism for the formation of ZrSiO
4 from a system of Si(OC
2H
5)
4-ZrOCl
2·8H
2O-C
2H
5OH solution was investigated in relation to the T-→M-transformation of ZrO
2. The heat-treated powders were mixtures of metastable fine T-ZrO
2 and amorphous SiO
2. The T-ZrO
2 aggregates were gradually transformed into M-ZrO
2 with an increase in temperature. This is due to the decrease of surface energy. The solid state reaction between ZrO
2 and SiO
2 is crystallographically assumed to have a large activation energy, and, therefore, the T-→M-transformation is preferentially proceeded below ca. 1300°C without forming ZrSiO
4. Beyond the temperature, the interface reaction between T-ZrO
2 and SiO
2 took place and resulted in the formation of ZrSiO
4. With an increase in temperature, the formation reaction was accelerated due to the Hedvall effect, whence T-ZrO
2 was responsible only for the reaction not for the transformation. With decreasing amount of T-ZrO
2, the formation reaction terminated, but the addition of transition metal ions such as Ni
2+ promoted the interface reaction between M-ZrO
2 and SiO
2. In order to increase the formation rate of ZrSiO
4, it was important to maintain the T-ZrO
2 powders metastable and fine by adding transition metal ions and by grinding the dried powders.
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