A computational search for wurtzite-structured ferroelectrics with low coercive voltages
A computational search for wurtzite-structured ferroelectrics with low coercive voltages
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DOI：
10.1063/5.0023626
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发表时间：
2020-12-01
期刊：
影响因子：
6.1
通讯作者：
Itoh, Mitsuru
中科院分区：
文献类型：
--
作者：
Moriwake, Hiroki;Yokoi, Rie;Itoh, Mitsuru
最近在纤锌矿结构的Sc掺杂的AlN薄膜中观察到了铁电性，这距离我们基于第一性原理计算的纤锌矿化合物的铁电性的初步预测已经过去了五年。与传统的钙钛矿结构铁电材料PbTiO_3相比，该薄膜具有更高的矫直电压(3 mV/cm)，但其极性转换困难，限制了其实际应用。为了识别具有低矫直电压的四面体铁电材料，我们用第一性原理方法对候选的二元化合物进行了更广泛的探索，从卤化物到硫系化合物再到钒系化合物。总体趋势是，极化转换势垒随着阴阳离子半径比的减小而减小，最低的势垒出现在单价化合物中，如铜和银卤化物；例如，CuCL的转换势垒被计算为0.17 eV/fU。AgI为0.22 eV/fU，与PbTiO_3(0.20 eV/fU)相近。在基面上施加外延拉伸应变也是进一步降低势垒的有效方法，AgI和CuCL中的势垒都降低到0.04 eV/fU。当应用5%的平面内扩展时。结果表明，可以获得具有中等矫直电压(低于100kV/cm)的四面体铁电材料。
Ferroelectricity has recently been observed in wurtzite-structured Sc-doped AlN thin films, five years after our initial prediction of ferroelectricity in wurtzite compounds based on first-principles calculations. The thin films exhibited a much higher coercive voltage (3 MV/cm) than that of conventional perovskite-structured ferroelectric material PbTiO3, however, making it difficult to switch the films' polarity and limiting their practical application. To identify tetrahedral ferroelectric materials with low coercive voltages, we have carried out a wider exploration of candidate binary compounds, from halides to chalcogenides to pnictogenides, using first-principles methods. The overall trend is for polarization switching barriers to decrease with decreasing anion-to-cation radius ratio, with the lowest barriers found in monovalent compounds such as the copper and silver halides; e.g., CuCl is calculated to have a switching barrier of 0.17 eV/f.u. and that of AgI is 0.22 eV/f.u., values similar in magnitude to that of PbTiO3 (0.20 eV/f.u.). Applying an epitaxial tensile strain to the basal plane is also effective for lowering the potential barrier further, with barriers in both AgI and CuCl decreasing to 0.04 eV/f.u. when a 5% in-plane expansion is applied. The results suggest that tetrahedral ferroelectrics with moderate coercive voltages (below 100 kV/cm) should be achievable.