Structure domains induced nonswitchable ferroelectric polarization in polar doubly cation-ordered perovskites

Double perovskites with cation ordering allow the coexistence of ferroelectricity and magnetization in the same phase, and thus become a good candidate for room-temperature multiferroic materials. In this paper, we suc-cessfully synthesized A-and B-site ordered double perovskites NaLnBWO6 (Ln = La, Nd, and B--Mg, Mn) ce-ramics, and investigated their microstructure by combining X-ray diffraction, aberration-corrected scanning transmission electron microscopy, and optical second harmonic generation measurements. At room temperature, both NaLaMgWO6 and NaNdMgWO6 ceramics possess the space group of P21/m, while NaLaMnWO6 has a polar crystal structure (P21) without inversion symmetry. All three samples exhibit the A-site layered ordering and B -site rock-salt ordering as directly imaged by the HAADF-STEM images. The observed structural domains arising from the cation ordering well explain the nonswitchable ferroelectric polarization due to a large energy barrier. These findings suggest that it is necessary to eliminate structural domains during the sample synthesis for obtaining large ferroelectric polarization in double perovskites.