Fine structural analysis and phase transition behavior for Li-modified Na0.5K0.5NbO3 lead-free piezoelectric ceramics

The fine crystal structure of Lix(Na0.5K0.5)1−xNbO3 ceramics has been studied by means of Nb-K edge extended X-ray absorption fine structure (EXAFS) and X-ray internal strain measurement technique in the vicinity of the compositions showing a polymorphic phase boundary (PPB) between orthorhombic and tetragonal structures. The anisotropic distortion of the NbO6 octahedral initially occurred when x was increased from 0.050 to 0.053, prior to the completion of the phase transition from orthorhombic to tetragonal symmetry. EXAFS clearly revealed that the bond distance of Nb–O1 with [0 0 1] configuration was increased, and that of Nb–O2 with [1 1 0] configuration was oppositely decreased in the NbO6 octahedral. In the vicinity of the PPB compositions, the internal strain η(0 1 1) also increased from 4.5 × 10−3 to the maximum value of 12.0 × 10−3 in the narrow x range from 0.040 to 0.055, then decreased to 3.2 × 10−3 at x = 0.06. On the other hand, the η(1 0 0) increases from 1.5 × 10−3 to the maximum value of 2.9 × 10−3 in the next narrow x range from 0.055 to 0.060. The variation of η(1 0 0) differed in Li dependence from that of η(0 1 1), which indicates that a large anisotropic strain remains in the crystal lattice in the PPB compositions.