npj: 无金属有机钙钛矿—大压电响应
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新近研究表明，不含金属元素的有机钙钛矿铁电材料具有较大的自发电极化强度、环境友好、易于合成等诸多优良特性。特别是将有机胺、无机铵和氢卤酸的溶液缓慢蒸发后所得到的MDABCO-NH4-X3类材料（X为卤族元素）。但他们在压电方面的物理性能尚缺乏深入的研究。
来自河南科技大学物理与工程学院的王翚副教授和上海科技大学物质科学与技术学院的韩晗副研究员共同领导的团队，采用第一性原理计算了无金属有机钙钛矿MDABCO-NH4-X3 (X=Cl,Br, I)的结构、电子态密度、压电系数和弹性性质。他们发现MDABCO和NH4基团的偶极矩非常小，因此MDABCO-NH4-I3 的自发电极化主要源自MDABCO和NH4基团相对于I离子中心的偏离，压电应变分量dz3计算值（14.4 pC/N）与实验值（~14 pC/N）非常吻合。虽然有机材料的压电响应一般都很弱，但该有机钙钛矿材料的压电应变分量dx4 和dx5却很大；计算发现当X为Cl、Br、I时，其最大的压电分量dx5值分别为119、248和178 pC / N。这一特性与顺度系数s44密切相关，说明MDABCO-NH4-X3 的大压电应变响应主要源于它们的柔性。MDABCO-NH4-X3 的这些独特性能使其成为柔软、轻便和生物友好型压电材料的理想选择，适用于各种重要方面的应用，如柔性设备、可植入系统和微型机器人。
该文近期发表于npj Computational Materials 5: 17 (2019)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
Large piezoelectric response in a family of metal-free perovskite ferroelectric compounds from first-principles calculations
Hui Wang, Huihui Liu, Zeyu Zhang, Zihan Liu, Zhenlong Lv, Tongwei Li, Weiwei Ju, Haisheng Li, Xiaowu Cai & Han Han
Abstract Metal-free organic perovskite ferroelectric materials have been shown recently to have a number of attractive properties, including high spontaneous polarization and piezoelectric coefficients. In particular, slow evaporation of solutions containing organic amines, inorganic ammoniums, and dilute hydrohalogen acid has been shown to produce several attractive materials in the MDABCO-NH4-I3 family (MDABCO is N-methyl-N’-diazabicyclo[2,2,2] octonium). In the present work, we study by first-principles calculations the origin of polarizaiton, electronic density of state, piezoelectric response, and elastic properties of MDABCO-NH4-X3 (X = Cl, Br, I). We find that the dipole moments of the MDABCO and NH4 groups are negligible, and the large spontaneous polarization of MDABCO-NH4-I3 mainly results from MDABCO and NH4 being off-center relative to I ions. Although the piezoelectric response of organic materials is usually very weak, we observe large piezoelectric strain components, dx4 and dx5; the calculated dx5 is 119 pC/N for MDABCO-NH4-Cl3, 248 pC/N for MDABCO-NH4-Br3 and 178 pC/N for MDABCO-NH4-I3. The large value of dx5 is found to be closely related with the large value of elastic compliance tensor, s44. These results show that MDABCO-NH4-X3 metal-free organic perovskites have large piezoelectric response with soft elastic properties.
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