The primary order parameter of hybrid improper ferroelectricity (HIF) is not spontaneous polarisation but nonpolar modes such as oxygen octahedral rotations (OORs), antipolar, or even Jahn–Teller distortions; therefore, the HIF mechanism may be applied as an effective pathway to tune electronic bandgaps, control orbitals, and create multiferroicity. Most of the current experimental research on HIF is focused on layered perovskite oxides; therefore, this review focuses on the recent progress of experimental studies on HIF materials with the Ruddlesden–Popper (R–P) and Dion–Jacobson (D–J) structures. Experimental research on double-layered R–P oxides is included, and the linear relationship between the Curie temperature and tolerance factor has been established. Moreover, the coexistence of polar and weak ferromagnetic phases has been observed in iron-based double-layered R–P oxides at room temperature. The recent discovery of ferroelectricity in A-site cation-ordered triple-layered R–P oxides has significantly expanded the HIF field. HIF has also been confirmed in caesium- and rubidium-based double-layered D–J oxides, and complex OOR modes have been observed in D–J oxides. Although significant progress has been achieved for HIF materials in layered perovskite oxides, extensive research is required to reveal the mysteries of HIF and to create single-phase multiferroics in HIF materials.

Hybrid improper ferroelectricity has been extensively studied in double-layer Ruddlesden–Popper oxides in recent years. Although the hybrid improper ferroelectricity could be created among triple-layer Ruddlesden–Popper oxides with an ordered A-site cation predicted by the first-principles calculations, no experimental result has been reported yet. In the present work, the room-temperature ferroelectricity has been observed in Li2La2Ti3O10 ceramics with an A-site cation ordered triple-layer Ruddlesden–Popper structure. The polar phase P21ab has been determined by combining the first-principles calculation and the powder diffraction analysis at room temperature. The hybrid improper ferroelectricity was induced by the triple-coupled irreps including the A-site cation ordering. The variable temperature differential scanning calorimetry measurements and dielectric responses indicate no evidence of phase transition over the temperature range of 200–1080 K. The present work sheds light on designing the hybrid improper ferroelectrics in A-site ordered triple-layer Ruddlesden–Popper compounds.

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Here, we report the growth of (Sr, Ca) Nd2Fe2O7 single crystals with the Ruddlesden–Popper structure using an optical floating-zone method. A significantly anisotropic magneto-dielectric effect (MD), ab-plane and c-axial MD coefficients reaching −12.3% and −8.4% measured at 103 Hz in a 1 T magnetic field, can be obtained in a SrNd2Fe2O7 crystal at room temperature. The corresponding anisotropic MD ratio can be reached as high as 1.46. With an increase in the Ca concentration, the MD effect decreases dramatically and is eventually completely suppressed in both directions. Analysis of magnetic properties and 57Fe Mössbauer spectra suggests that the anisotropic MD effects in SrNd2Fe2O7 can be attributed to polaronic hopping between two neighboring Fe3+ ions through oxygen vacancies in an anisotropically antiferromagnetic matrix; the disappearance of the MD effect in Ca-doped SrNd2Fe2O7 is a consequence of the suppression of the antiferromagnetism. Our work suggests that the significantly anisotropic MD effect in SrNd2Fe2O7 crystals at room temperature can be used in magneto-dielectric controlled devices.