Developing improved AFE materials for real-world applications requires a fundamental comprehension of the crystallography and microstructure at different length scales. A better understanding of the structural origin of antiferroelectricity offers opportunities for comprehending structure-property correlation and exploring new AFE materials [
178]. Unfortunately, a complete physical picture of antiferroelectrics has not been obtained to date owing to the complex lattice dynamics of AFE materials, thus heavily restricting the possibility of engineering the properties towards applications. The theoretical definition of AFE configuration was proposed by Kittel in 1951 as neighboring dipoles pointing in antiparallel directions [
9]. In parallel, Shirane
et al. [
14] experimentally demonstrated AFE behavior in PbZrO
3 ceramic based on its characteristic dielectric anomaly and double hysteresis loop. Following comprehensive structural investigations, the basic dipole ordering arrangement of prototypical AFE PbZrO
3 was established in which the lead ions are aligned alternately in this way (↑↑↓↓↑↑↓↓), as shown in
Fig. 4a. This defines the AFE character as per Kittel's definition [
9], and it results in a commensurately modulated structure (CMS) and the appearance of

[
110] superlattice spots in the [001] selected area electron diffraction (SAED) pattern [
179], as shown in the inset of
Fig. 4a. In principle, there are three characteristic AFE features of orthorhombic PbZrO
3, which can be associated with the instabilities of the phonon modes in the first Brillouin zone of the cubic PbZrO
3 crystal lattice [
178,
180]. First, the antiparallel Pb ion displacements along the [
110] direction are associated with instability at the Brillouin zone-boundary phonon mode [
181]. Second, the dielectric anomaly during the high-temperature cubic to orthorhombic AFE phase transition is related to instability in the Brillouin zone center mode [
182]. Third, the opposite oxygen octahedral rotations in the pseudocubic unit cell along the (001)-axis originate from instability at the R corner of the Brillouin zone [
183]. The softening of opposite oxygen octahedral rotational modes plays a significant role in structural phase transformations for FEs and AFEs [
184,
185]. The recent polarized infrared ray and Raman spectroscopic results indicated that PbZrO
3 has multiple soft modes, resulting in a flat soft polarization branch near the AFE wave vector [
180]. In PbZrO
3-based AFE materials, chemical modifications generally drive the AFE phase from CMS to an incommensurately modulated structure (IMS) [
179,
186]. In 1990s, transmission electron microscopy (TEM) studies of IMS were performed in PbZrO
3-based ceramics [
187,
188]. Later, the rather complex IMS with a sinusoidal modulation of A-site cations was first studied in (Pb
0.96La
0.04)(Zr
0.9Ti
0.1)
0.99O
3 ceramics using atomic-resolved scanning transmission electron microscopy (STEM) [
189]. Recently, Ma
et al. further revealed that the A-site cations were antiparallel but in a nearly orthogonal arrangement in (Pb
0.99Nb
0.02[(Zr
0.57Sn
0.43)
1−yTi
y]
0.98O
3 ceramics [
179]. The incommensurate modulations manifest themselves as fine fringes in bright-field TEM images and satellite spots in reciprocal space (
Fig. 4b). Most recently, the ferrielectric (FiE) configuration, containing FE segments with either angle or magnitude modulation of electrical dipoles [
188,
190], was experimentally found in Pb
0.97La
0.02(Zr
0.50Sn
xTi
0.50-x)O
3 ceramics using atomic-resolved STEM [
191], as shown in
Fig. 4c. This FiE phase mainly stems from the coupling between A-site cations and O-anions [
191]. Other functionalities may be accessed based on FiE segments, in which the sublattice polarizations do not completely cancel, resulting in a net spontaneous polarization. These effects would show the characteristic electric-field-induced transition of antiferroelectrics, but could also have a switchable polarization at lower fields. By combining advanced synchrotron X-ray, neutron and three-dimensional electron diffraction techniques, the incommensurately modulated intermediate state was revealed recently in Pb
0.99{Nb
0.02[(Zr
0.57Sn
0.43)
0.94Ti
0.06]
0.98}O
3 ceramics, which display imbalanced polarization and flexible configuration of a sinusoidal wave functional [
41], as shown in
Fig. 4d. Incommensuration in PbZrO
3-based AFE materials is related to the competing interactions between a zone-boundary ∑ phonon mode favouring AFE ordering in an orthorhombic structure and a zone-center Γ phonon mode favouring FE ordering in a rhombohedral structure [
186]. In addition, incommensurate modulations were observed in (Pb, La)(Zr, Sn, Ti)O
3 AFE tetragonal single crystals [
156], and field-induced transformation between incommensurate AFE and commensurate FE modulations enabled the emergence of complex domain structures and domain memory effects (
Fig. 4e). Past investigations revealed that the generation of incommensurate superlattice reflections is related to the addition of La
3+ in the (Pb, La)(Zr, Ti)O
3 and (Pb, La)(Zr, Sn, Ti)O
3 ceramics [
192]. These studies have been considered an important step forward to the understanding of cation displacement and dipole arrangement in IMS.