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压电材料是一类能够实现机械能和电能相互转换的功能材料。目前发现的大多数压电材料表现出正的纵向压电效应，即极化强度随着外部拉伸应变而增加。相比之下，对负的纵向压电效应的研究显得尤为稀缺。最近，HfO2这一材料因其被预测具有负纵向压电性而受到关注，然而，对其固有负压电效应起源的理解以如何通过有效手段调控其压电系数仍缺乏系统的理论研究。
Fig. 1| Negative
piezoelectric response in HfO
2
.
来自上海交通大学的朱虹教授团队，通过引入
c
方向阳离子周围加权投影键合强度（WPB
c
）的概念，来定量表征沿应变方向的不对称键合刚度，从而揭示了铁电氧化铪中负纵向压电响应的起源。他们通过分析阳离子周围的键合环境，他们发现当WPB
c
与体自发极化方向反平行时，极化会随着拉伸应变而减小，从而导致负压电性。为了验证这一结论的普适性，同时探究WPB
c
对压电系数的影响，他们进一步在掺杂HfO
2
中进行了系统性的探究。在计算掺杂体系压电系数之前，该研究首先采用两步筛选法在49个掺杂元素中获得了26个有利于稳定铁电氧化铪的元素。
Fig.2 | Phase
stability screening of hafnia in the presence of various dopants.
接着对这26个掺杂HfO
2
体系的压电系数和键合环境进行了分析，其中Pb, Rh和Sn对氧化铪的压电系数起到增强作用。不同于本征HfO
2
中单一的键合环境，掺杂会造成阳离子周围键合环境的不同。该研究发现WPB
c
波动较大的体系所对应的压电系数普遍较小。同时平均WPB
c
的正负也与压电系数的正负显示出相关性。
Fig. 3| The calculated
e
33
and
the distribution of WPB
c
of cations in the doped hafnia.
为了建立WPB
c
和压电系数间直接关系，他们采用WPB
c
的标准差（STD-WPB
c
）来定量描述阳离子周围的局部键合波动，用平均WPB
c
来描述阳离子整体的成键方向和强度。结果表明，平均WPB
c
越负，STD-WPB
c
越小，压电响应越强。该研究为理解压电响应机制提供了新思路，并且可以用来筛选新的有前景的压电材料。
该文近期发表于
npj Computationa
l
Materials
10
: 160 (2024)
，
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。
Fig. 4| Impact of bonding
environment on
e
33
and promising candidates.
Understanding and tuning negative longitudinal piezoelectricity in hafnia
Huirong Jing, Chaohong Guan & Hong Zhu
Most piezoelectric materials exhibit a positive longitudinal piezoelectric effect (PLPE), while a negative longitudinal piezoelectric effect (NLPE) is rarely reported or paid much attention. Here, utilizing first-principles calculations, we unveil the origin of negative longitudinal piezoelectricity in ferroelectric hafnia by introducing the concept of weighted projected bond strength around cation in the c direction (WPBc), which is proposed to quantitatively characterize the asymmetric bonding stiffness along the strain direction. When the WPBc is anti-parallel to the direction of bulk spontaneous polarization, the polarization decreases with respect to tensile strain and leads to a negative piezoelectricity. Furthermore, to confirm the influence of WPBc on the piezoelectric effect and understand how the value of WPBc influences the piezoelectric coefficient e33, we acquire both the piezoelectric coefficient of doped hafnia and the corresponding bonding environment around each cation. The finding reveals that the more negative piezoelectric coefficient can be achieved through a concurrent achievement of the more negative average WPBc and the lower standard deviation (STD) of WPBc. In addition, the Sn-doped hafnia with the lowest average WPBc and smaller STD-WPBc is identified to have the highest piezoelectric coefficient (−2.04 C/m2) compared to other dopants, showing great potential in next-generation electromechanical devices.
