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.

中文翻译：

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低矫顽力纤锌矿结构铁电体的计算研究

在我们基于第一性原理计算初步预测纤锌矿化合物中的铁电性五年后，最近在纤锌矿结构的 Sc 掺杂 AlN 薄膜中观察到铁电性。薄膜表现出比传统钙钛矿结构铁电材料 PbTiO3 高得多的矫顽电压（3 MV/cm），然而，这使得薄膜极性难以转换并限制了它们的实际应用。为了识别具有低矫顽电压的四面体铁电材料，我们使用第一性原理方法对候选二元化合物进行了更广泛的探索，从卤化物到硫族化物再到 pnictogenides。总体趋势是极化转换势垒随着阴离子与阳离子半径比的降低而降低，在铜和卤化银等一价化合物中具有最低的势垒；例如，计算得出 CuCl 的开关势垒为 0.17 eV/fu，AgI 的开关势垒为 0.22 eV/fu，其数值与 PbTiO3 (0.20 eV/fu) 的数值相似。对基面施加外延拉伸应变对于进一步降低势垒也很有效，当应用 5% 的面内膨胀时，AgI 和 CuCl 中的势垒都会降低到 0.04 eV/fu。结果表明，应该可以实现具有中等矫顽电压（低于 100 kV/cm）的四面体铁电体。对基面施加外延拉伸应变对于进一步降低势垒也很有效，当应用 5% 的面内膨胀时，AgI 和 CuCl 中的势垒都会降低到 0.04 eV/fu。结果表明，应该可以实现具有中等矫顽电压（低于 100 kV/cm）的四面体铁电体。对基面施加外延拉伸应变对于进一步降低势垒也很有效，当应用 5% 的面内膨胀时，AgI 和 CuCl 中的势垒都会降低到 0.04 eV/fu。结果表明，应该可以实现具有中等矫顽电压（低于 100 kV/cm）的四面体铁电体。