Yumi Inagaki

Dielectric and Piezoelectric Properties of Mn-Doped Na0.5K0.5NbO3 Single Crystals Grown by Flux Method

Lead-free piezoelectric Mn-doped Na0.5K0.5NbO3 (NKN) single crystals have been fabricated by self flux method using KF–NaF eutectic composition. The color of the obtained crystals was different depending on the doped Mn-chemicals. The large-sized single crystals with crystal face of orthorhombic (110) were obtained by optimized heat-treatment condition of the holding time at 1050 and 950 °C of 5 h and the cooling rate of 0.25 °C/min, and their piezoelectric properties were successfully measured by a resonance-antiresonance method. The piezoelectric strain constant (d33) of 0.5 mol % Mn-doped NKN single crystal was 161 pC/N, and the longitudinal electro-mechanical coupling factor (k33) showed 0.64.

Ferroelectric Domain Structure of Na0.5K0.5NbO3 Crystal Grown by Floating Zone Method

The ferroelectric domain structures of orthorhombic Na0.5K0.5NbO3 (NKN) crystals grown by the floating zone method are characterized. Three types of laminar domain of 30–80, 10–15, and 5–10 µm in width were found in the crystal cut perpendicular to the growth direction. The thinnest laminar domains were observed between two different 90° domains. Raman spectroscopy confirmed that the NbO6 units of the NKN perovskite-type structure in the thinnest domains were distorted compared with these in the neighboring laminar 90° domains. When an external electric field was applied vertically to the domain walls, the thinnest domain could release strain caused by the external electric field, and the strain was dispersed in several directions within the thinnest domains. The polarization response per cross section was enhanced by distorted NbO6 units in the case that the domain structures included the thinnest domains between wide laminar 90° domains.

Growth Condition and Ferroelectric Property of Mn-Doped Na0.5K0.5NbO3 Crystal

Mn-doped Na0.5K0.5NbO3 (NKN) crystals have been grown by self-flux method under several heat-treatment conditions. The cooling rate affected the quality of Mn-doped NKN crystals significantly. When the cooling rate was 0.5 oC/min at temperatures ranging from 1050 to 950 oC, the synthesized Mn-doped NKN crystal exhibited a single nucleation growth, compared with multinucleation growth when the cooling rate was 0.25 oC/min at temperatures ranging from 1050 to 950 oC. The frequency dependence on the ferroelectric P-E hysteresis loop of the Mn-doped NKN crystal was not observed at measurement frequency of 0.1 – 25 Hz. The annealed 0.5 mol% Mn-doped NKN crystal exhibited excellent P-E hysteresis loop with Pr of 45.0 μC/cm2 and Ec of 7.2 kV/cm.

Structural phase transitions in KNbO3 and Na0.5K0.5NbO3

The in-situ high-temperature single-crystal X-ray diffraction experiments on KNbO3 (KN) and Na0.5K0.5NbO3 (NKN) revealed a series of structural evolution associated with two transitions among orthorhombic, tetragonal and cubic forms. Upon heating, the pseudocubic unit cell volume collapsed discontinuously at these transition points, whereas the reverse took place upon cooling, indicating that both transitions were of the first-order. The coordination numbers of Nb showed a decreasing tendency with decreasing temperature, i.e., 6 in cubic, 5+1 in tetragonal, and 4+2 in orthorhombic. The difference in ionic size between Na and K gave the Na atom an attribute of rattling in the cavity surrounded by corner-sharing NbO6 octahedra in NKN. The small Na not only vibrated with large amplitude, but also shifted further from the centre of the cavity than K. The coordination number of Na was always small compared with K at all temperatures, and showed an decreasing tendency upon cooling, i.e., 8+4 in tetragonal and cubic, and 7+5 in orthorhombic. The structural phase transition of KN occurred in a similar way as NKN, though the transition temperatures were shifted slightly toward the high temperature side.