Hybrid improper ferroelectricity in a Multiferroic and Magnetoelectric hybrid organic-inorganic perosvkite

Abstract: There is great interest in hybrid organic-inorganic materials such as metal-organic frameworks (MOFs). The compounds [C(NH${2}$)${3}$]M(HCOO)${3}$, where M=Cu$^{2+}$ or Cr$^{2+}$ are Jahn-Teller (JT) active ions, are MOF with perovskite topology which crystallizes in polar space group Pna2${1}$. In inorganic compounds, octahedral tilting and Jahn-Teller structural distortions are usually non-polar distortions. However, in this MOF cooperative interactions between the antiferro-distortive distortions of the framework and the C(NH2)${3}$ organic cation via hydrogen bonding breaks the inversion symmetry and induces a ferroelectric polarization.[Angew. Chem. Int. Ed. 50, 5847, 2011] Our ab-initio study supports the picture of an orbital-order-induced ferroelectricity, a rare example of dipolar ordering caused by electronic degrees of freedom. The switching of polarization direction implies the reversal of the weak ferromagnetic component. The microscopic mechanism in this JT-based MOF with ABX${3}$ perovskite structure displays a Hybrid Improper Ferroelectric (HIF) state, arising from a trilinear coupling between different structural deformations that comprise tilting, rotations and Jahn-Teller distortions of both the BX$_{3}$ framework and the organic cation at the A sites. Since these distortion modes in perovskite-inorganic compounds usually freeze-in at elevated temperatures, the trilinear coupling in MOF compounds may provide an interesting route towards high-temperature multiferroicity. These results offer an important starting point for tailoring multiferroic properties in this emerging class of materials for various technological applications. In particular, the high tunability of the ferroelectric polarization by means of the modification of the organic A cation has been recently shown[J. Am. Chem. Soc. 135 18126 (2013)]

• The paper demonstrates that trilinear coupling among tilting, rotation, and Jahn-Teller distortions drives hybrid improper ferroelectricity in MOF perovskites.
• The paper employs ab-initio calculations, symmetry analysis, and Monte Carlo simulations to predict polarization switching alongside weak ferromagnetic reversals.
• The paper highlights the potential for designing lead-free, high-temperature multiferroic materials by tuning organic-inorganic interactions in perovskite frameworks.

Hybrid Improper Ferroelectricity in Hybrid Organic-Inorganic Perovskites: A Detailed Analysis

The study presented investigates the emergence of hybrid improper ferroelectricity and magnetoelectric coupling in a specific metal-organic framework (MOF) of the perovskite type, combining hybrid organic-inorganic materials. The research focuses on the compounds [C(NH$_{2}$)$_{3}$]M(HCOO)$_{3}$, with M = Cu$^{2+}$ or Cr$^{2+}$, examining how structural distortions in these materials can lead to multiferroicity, a property that could hold significant implications for advanced materials design.

Key Findings

The paper delineates the presence of a hybrid improper ferroelectric (HIF) state characterized by trilinear coupling among non-polar structural deformations, including tilting, rotations, and Jahn-Teller (JT) distortions. These lead to a polarization usually absent in standard inorganic compounds where these distortion modes often manifest at elevated temperatures. This coupling mechanism suggests potential pathways for realizing high-temperature multiferroicity in MOFs, an area predominantly explored within inorganic materials.

Significant theoretical and computational methods, such as ab-initio calculations, symmetry analysis, and Monte Carlo simulations, support the proposed mechanisms and predictions. The study provides evidence that polarization switching in these MOFs is accompanied by the reversal of a weak ferromagnetic component, highlighting a strong interdependence between ferroelectric and ferromagnetic orders. This interaction suggests an enhanced magnetoelectric coupling mechanism, which has far-reaching implications for technology—particularly in designing devices that leverage both electric and magnetic properties for advanced functionalities.

Implications and Future Prospects

1. Material Design: The findings underscore the significant role of organic-inorganic duality in MOF perovskites. By adjusting the organic components, it may be possible to tailor ferroelectric properties and achieve customizable multiferroic materials. This adaptability offers a versatile approach to material synthesis for specific applications.
2. Technological Relevance: Short-term applications may focus on integrating these findings into electronic devices, sensors, or data storage systems, where the coupling between electric and magnetic states can be exploited.
3. Environmental Impact: The potential for developing lead-free ferroelectric materials presents a crucial advantage for environmental sustainability, addressing a significant concern in current material sciences.
4. Fundamental Understanding: The identification of the trilinear coupling in these hybrid systems contributes to the broader understanding of multiferroic mechanisms in condensed matter physics, particularly in systems where traditional mechanisms do not apply.

Conclusion

The paper effectively introduces a novel perspective on the mechanisms enabling hybrid improper ferroelectricity within MOFs and highlights the potential of leveraging these unique structural properties for technological innovation. Future research may continue to explore optimizing these frameworks for practical applications and increasing the operational temperatures of multiferroic behavior in such materials. Additionally, refining and expanding computational methods will be vital to accurately predict and validate new material functionalities in this ever-evolving field.