Formation of a KNbO3 single crystal using solvothermally synthesized K2-mNb2O6-m/2 pyrochlore phase

Abstract

A K2-mNb2O6-m/2 single crystal with a pyrochlore phase formed when the Nb2O5+x mol% KOH specimens with 0.6x1.2 were solvothermally heated at 230 degrees C for 24h. They have an octahedral shape with a size of 100m, and the composition of this single crystal is close to K1.3Nb2O5.65. The single-crystal KNbO3 formed when the single-crystal K2-mNb2O6-m/2 was annealed at a temperature between 600 degrees C and 800 degrees C with K2CO3 powders. When annealing was conducted at 600 degrees C (or with a small amount of K2CO3), the KNbO3 single crystal has a rhombohedral structure that is stable at low temperatures (< - 10 degrees C). The formation of the rhombohedral KNbO3 structure can be explained by the presence of the K+ vacancies in the specimen. The KNbO3 single crystal with an orthorhombic structure formed when the K2-mNb2O6-m/2 single crystal was annealed at 800 degrees C with 20wt% of K2CO3.