Switching was observed in other HIF materials with coercive fields ranging from 120 to 200 kV cm −1 3, 4, 6, and very recently, the smallest coercive field of 39 kV cm −1 was observed in single crystals of Sr 3Sn 2O 7 11.
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The high experimental coercive field is not surprising, because the energy scale that needs to be overcome for switching is considered to be determined by the octahedral rotations, which often have an energy scale significantly higher than that of the ferroelectric distortions in typical perovskite oxides. The reason behind the absence of switching in these materials was initially believed to be large intrinsic coercive fields, or defects in the materials, which typically increase the coercive field 9, 10. For example, the original HIF Ca 3Ti 2O 7 was reported to have a polar structure 20 years before the idea of HIFs was introduced 8, but the direct evidence of polarization switching was not observed until 2015 3. Among those, the list of examples that are experimentally verified includes A 3B 2O 7 HIFs (Ca,Sr) 3Ti 2O 7 3, (Sr,Ca) 3Sn 2O 7, Sr 3Zr 2O 7 4, 5, 6, as well as a weak ferromagnetic (Ca 0.69Sr 0.46Tb 1.85Fe 2O 7) 7.ĭespite the prediction of ferroelectricity and observation of a polar crystal structure in many compounds, experimentally observing the switching of polarization is challenging. A major breakthrough in perovskite-related ferroelectrics is the discovery of hybrid improper ferroelectricity (HIF) as a materials design route in 2011, which led to an explosion in the predictions of novel ferroelectric oxides 2.
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Versatility of the perovskite structure allows a large number of complex oxides to be synthesized, but among those, only a small fraction are ferroelectrics 1. Since the discovery of ferroelectricity in BaTiO 3, perovskite oxides have been heavily studied and utilized in applications as ferroelectric materials.