Effects of Cu addition on rapid solid-state crystal growth of (K,Na)NbO3 single crystals

We investigated the effects of 0.5 mol% to 6.0 mol% CuO addition on crystal growth by the rapid solid-state crystal growth method to synthesize large unitary rectangular (K,Na)NbO3 (KNN) single crystals several square centimeters within 24 h. X-ray photoelectron spectroscopy and X-ray powder diffraction measurements revealed that CuO and Cu2O segregated in the region near the coarsened single crystal/polycrystal boundary. A positive correlation between crystal growth and the addition of CuO up to 1.5 mol% was observed; however, for CuO amounts larger than 1.5 mol%, this correlation became negative. To investigate these phenomena, a high-precision synchrotron powder diffraction measurement was performed. Significant precipitation of the CuO phase was detected in the calcined powders with a CuO addition larger than 1.5 mol%. The effects of this precipitation on the growth mechanism for coarsening the single crystals were discussed.