Origin of giant piezoelectric effect in lead-free K 1-x Na x Ta 1-y Nb y O 3 single crystals

A series of high-quality, large-sized (maximum size of 16 × 16 × 32 mm 3) K 1-x Na x Ta 1-y Nb y O 3 (x = 0.61, 0.64, and 0.70 and corresponding y = 0.58, 0.60, and 0.63) single crystals were grown using the top-seed solution growth method. The segregation of the crystals, which allowed for precise control of the individual components of the crystals during growth, was investigated. The obtained crystals exhibited excellent properties without being annealed, including a low dielectric loss (0.006), a saturated hysteresis loop, a giant piezoelectric coefficient d 33 (d 33 = 416 pC/N, determined by the resonance method and d 33 ∗ = 480 pC/N, measured using a piezo-d 33 meter), and a large electromechanical coupling factor, k 33 (k 33 = 83.6%), which was comparable to that of lead zirconate titanate. The reason the piezoelectric coefficient d 33 of K 0.39 Na 0.61 Ta 0.42 Nb 0.58 O 3 was larger than those of the other two crystals grown was elucidated through first-principles calculations. The obtained results indicated that K 1-x Na x Ta 1-y Nb y O 3 crystals can be used as a high-quality, lead-free piezoelectric material.