Structural, optical and ferroelectric properties of KNixNb1-xO3 single crystals

Ferroelectric perovskite oxides have attracted considerable attention as photovoltaic materials owing to the bulk photovoltaic effect (BPVE). However, single crystals are necessary to investigate the mechanism underlying the BPVE and bandgap engineering methods are furthermore required in order to increase their optical absorption in the visible region of the solar spectrum. In this study, a high-quality, large-sized (the largest diameter and height were 32 and 29 mm, respectively) KNixNb1-xO3 (x = 0.006, 0.010, and 0.012) single crystal was grown using a top-seeded solution growth method. The effect of nickel doping on the optical and ferroelectric properties was investigated. It was observed that while the introduction of nickel induced bandgap narrowing and thus an increase in the visible-light absorption, the concomitant introduction of oxygen vacancies led to slight weakening of the ferroelectric properties. On the whole, however, our results indicate the high potential of KNixNb1-xO3 crystals for investigating the BPVE and as candidate photovoltaic materials.