Two new fluorooxoborates Rb3B5O8F2 and K0.6Rb2.4B5O8F2 were successfully obtained by cation regulation. Both of them crystallize in non-centrosymmetric space group P21. The structural evolution from Li2Na0.9K0.1B5O8F2 to Rb3B5O8F2, as well...

Similar to the canonical antiferroelectric (AFE) compound PbZrO3 in Pb(Zr,Ti)O3 solid‐solutions, the presence of double hysteresis loops and that of electric field–induced phase transitions are important characteristics of NaNbO3 AFE materials; yet the phase transition behavior in the latter system is typically irreversible with the related mechanisms not fully understood. Here, we explore the phase transition mechanism of ferroelectric and AFE phases in NaNbO3 based on measurements of single crystals with different directions in conjunction with density‐functional theory (DFT) calculations. The tilting and distortion behaviors of the [NbO6] octahedra are explained by DFT, and the ion displacement in the lattice is traced. The tilting and distortion behaviors of the [NbO6] octahedra with different orientations are compared. We confirm that the tilt and distortion of the [NbO6] octahedra along the [1 1 1] direction is the main reason to improve the stability of AFE phase. This conclusion is verified by the experimental characterizations of large‐size NaNbO3 single crystals successfully obtained in this study.