Size Effect on Crystal Structure and Phase Transition of Potassium Niobate

Powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were performed for potassium niobate (KNbO3) fine particle (down to 113 nm) prepared by a soft-chemical route. It is found that KNbO3 fine particles smaller than 150 nm change in the lattice constants as their crystal structure approaches tetragonal at room temperature. With decreasing the grain size, both phase transition temperatures from the orthorhombic to tetragonal and the tetragonal to cubic phases were shifted toward low temperature on DSC curves. The present results suggest the presence of a critical size below 100 nm at which KNbO3 transforms from orthorhombic to tetragonal phase at room temperature. The critical size empirically estimated from the particle size dependence of the phase transition temperature was about 60 nm.