We report the coexistence of ferroelectricity and a spin liquid state in hexagonal TbInO 3 with quasitwo-dimensional triangular spin lattice. Geometrical ferroelectricity associated with In trimerization accompanies topological ferroelectric structural defects. Magnetic susceptibility data show in-plane magnetic anisotropy of Tb spins without any long-range order above 1.8 K, and we also confirm no trace of any phase transition down to 0.15 K from a specific heat measurement, which indicates that this system is highly frustrated and may host a spin liquid ground state. By analyzing the Schottky anomaly in the specific heat results, we propose a model where crystal-field levels are different in each Tb sites, and only one of them has a magnetic ground state and forms a unique honeycomb spin lattice. These observations put forward an interesting possibility where spin liquid and ferroelectric behaviors coexist, and the atomically sharp ferroelectric domain walls may host new magnetic edge states or local spin excitations.

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Dion‐Jacobson compound CsBiNb2O7 exhibits supposedly hybrid improper ferroelectricity (HIF), resulting from two different structural distortions. The highly‐cleavable single crystal of CsBiNb2O7 has been successfully grown and found large ferroelectric polarization as large as 30 μC cm‐2 with the coercivity of 310 kV cm‐1 from polarization‐electric field measurements on the crystals. Using piezoelectric force microscopy, a systematic study is on the ferroelectric domain structure and dynamics of CsBiNb2O7, unveiling the rich nature of its domain/domain wall evolution upon poling. The record large switchable polarization in CsBiNb2O7 among HIF compounds brings HIF closer to the real Pb‐free ferroelectric applications and the high cleavability of CsBiNb2O7 crystals provides a unique opportunity to explore ultrathin ferroelectrics with in‐plane polarization.