Structure–Property Relationship of Piezoelectric Properties in Zeolitic Imidazolate Frameworks: A Computational Study

Metal–organic frameworks (MOFs) are a class of
nanoporous
crystalline materials with very high structural tunability. They possess
a very low dielectric permittivity εr due to their
porosity and hence are favorable for piezoelectric energy harvesting.
Even though they have huge potential as piezoelectric materials, a
detailed analysis and structure–property relationship of the
piezoelectric properties in MOFs are lacking so far. This work focuses
on a class of cubic non-centrosymmetric MOFs, namely, zeolitic imidazolate
frameworks (ZIFs) to rationalize how the variation of different building
blocks of the structure, that is, metal node and linker substituents
affect the piezoelectric constants. The piezoelectric tensor for the
ZIFs is computed from ab initio theoretical methods. From the calculations,
we analyze the different contributions to the final piezoelectric
constant d14, namely, the clamped ion
(e140) and the internal strain (e14int) contributions
and the mechanical properties. For the studied ZIFs, even though e14 (e140 + e14int) is similar for all ZIFs,
the resultant piezoelectric coefficient d14 calculated from piezoelectric constant e14 and elastic compliance constant s44 varies
significantly among the different structures. It is the largest for
CdIF-1 (Cd2+ and −CH3 linker substituent).
This is mainly due to the higher elasticity or flexibility of the
framework. Interestingly, the magnitude of d14 for CdIF-1 is higher than II–VI inorganic piezoelectrics
and of a similar magnitude as the quintessential piezoelectric polymer
polyvinylidene fluoride.

金属有机框架（Metal–organic frameworks, MOFs）是一类具有极高结构可调性的纳米多孔晶体材料。由于其多孔性，这类材料具备极低的介电常数εr，因此适用于压电能量收集领域。尽管作为压电材料拥有巨大潜力，但目前学界对金属有机框架的压电特性仍缺乏系统性的详细分析与构效关系研究。
本研究聚焦于一类立方非中心对称金属有机框架，即沸石咪唑酯骨架（zeolitic imidazolate frameworks, ZIFs），旨在阐明结构的不同基元——金属节点与配体取代基——的变化如何影响压电常数。
研究通过从头算理论方法计算了这类沸石咪唑酯骨架的压电张量。基于计算结果，我们分析了最终压电常数d₁₄的各项贡献，包括钳位离子贡献（e₁₄⁰）、内应变贡献（e₁₄^int）以及材料的力学性能。
对于本次研究的沸石咪唑酯骨架而言，尽管总压电常数e₁₄（即e₁₄⁰与e₁₄^int之和）在所有样品中差异不大，但由压电常数e₁₄与弹性柔度常数s₄₄计算得到的最终压电系数d₁₄却在不同结构间存在显著差异。其中CdIF-1（对应Cd²+与-CH₃配体取代基）的d₁₄数值最大，这主要源于其框架具有更高的弹性与柔性。
值得注意的是，CdIF-1的d₁₄数值高于II-VI族无机压电材料，且与经典压电聚合物聚偏氟乙烯的压电系数处于同一量级。