The NbO 6 octahedral distortion and phase structural transition of Eu 3+ ‐doped K 0.5 Na 0.5 NbO 3 – x LiNbO 3 ferroelectric ceramics

A small quantity of Eu 3+ ions were doped in the lead‐free ferroelectric K 0.5 Na 0.5 NbO 3 – x LiNbO 3 ( KNN – x LN , 0 ≤ x ≤ 0.08) ceramics to investigate the NbO 6 octahedral distortion induced by the increasing LN content. In addition, the phase structure, ferroelectric, and photoluminescence properties of K 0.5 Na 0.5 NbO 3 – x LiNbO 3 :0.006Eu 3+ ( KNN – x LN :0.006Eu 3+ ) lead‐free piezoelectric ceramics were characterized. All the X‐ray diffraction, Raman spectra, dielectric constant vs temperature measurements and the photoluminescence of Eu 3+ ions demonstrated that the prepared ceramics undergo a polymorphic phase transition ( PPT , from orthorhombic to tetragonal phase transformation) with the rising LN content, and the PPT region locates at 0.05 ≤ x ≤ 0.06. The ferroelectric properties, Raman intensity ratios and photoluminescence intensity ratios show similar variations with the increasing LN content, all with a maximum value achieved at the PPT region. We believe that the close relationship among the ferroelectric properties, Raman intensity ratios, and photoluminescence intensity ratios is caused by the NbO 6 octahedral distortion. The photoluminescence of Eu 3+ ion was discussed basing on the crystal‐symmetry principle and Judd‐Ofelt theory.