The effect of a small amount of K
2O on the crystallization behavior of the system of Li
2O-SiO
2 glass was investigated. Lithium disilicate (Li
2O•2SiO
2; L•2S) and lithium metasilicate (Li
2O•SiO
2; L•S) crystals precipitate simultaneously, but only L•2S crystals grow upon further heat treatment in the glass without K
2O (Li glass). In the glass containing K
2O (K3 glass), only L•S crystals precipitate and grow. The stable crystalline phases are L•2S and SiO
2 crystals in both glasses. Phase separation was observed before crystallization; a Li
2O-rich phase formed an isolated droplet phase in Li glass and the continuous phase in K3 glass. The Li
+-K
+ ion exchange test confirmed these structures. [SEM-EDX] analysis confirmed that K
2O exists in the Li
2O rich phase after phase separation in K3 glass. Thus, K3 glass separates into two phases, one of which is a continuous phase rich in Li
2O and containing considerable amount of K
2O. The L•S crystal precipitates and grows in this phase upon further heat treatment. Consequently, K
2O suppresses the crystallization of L•2S and promotes the precipitation of L•S crystals.
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