Prediction, synthesis, and characterization of novel ferroelectric metal fluorides

铁电体是具有自发电极化的绝缘体，这种极化可以被外电场逆转。它们的特性使它们成为理想的功能材料，具有广泛的应用潜力，从能量转换器或传感器到信息技术中的随机存取存储器。铁电性的发生与严格的结构要求有关。自发极化只可能在属于十种热释电晶体类的极空间群中发生。此外，极化方向的反转必须通过对电各向同性超群发生。这些结构要求被亚伯拉罕总结为经验规则，并且仍然是预测以前未知的铁电性的基础。历史上，研究的重点一直是氧化铁电体。目前已知的氧化物铁电结构类型很多，但具有这种性质的氟化物却很少。在本项目中，我们的目标是预测新的铁电氟化物。为此，我们将准备一个极性金属氟化物的调查，我们将利用成功的亚伯拉罕晶体学标准结合现代量子化学方法来研究它们作为铁电体的潜力。我们将从这次调查中综合和表征有希望的候选者，以证明它们的铁电性的结构先决条件。通过对皮尔逊数据库的初步筛选，我们成功地确定了金属氟化物的105种不同极性结构类型。这些结构类型中只有九种至少有一种被证明是铁电的代表。因此，这里存在着以前未知的铁电体的高潜力。到目前为止，我们已经确定了三种新的潜在铁电结构类型β-MnF4 (R3c, hR360), Mn3F8 （P21, mS22）和RbCrF5 (Pmc21, oP28)，我们可以开始我们的工作。我们的筛选进一步表明，大约30%的结构类型存在有争议的结构建议。这是因为，特别是极性化合物，在衍射实验中，空间基团的分配并非总是可以毫无疑问地实现的。这使得高通量DFT计算或机器学习所需的数据集自动编译变得复杂。这方面突出了极性化合物的一个明确的结构化学数据集的巨大需求。凭借我们工作组的晶体学，结构化学和实验经验知识，我们希望解决金属氟化物材料类别的这一任务。

Ferroelectrics are insulators with a spontaneous electric polarization that can be reversed by an external electric field. Their properties make them desirable functional materials with broad application potential, ranging from energy converters or sensors to random access memory in information technology. The occurrence of ferroelectricity is linked to strict structural requirements. Spontaneous polarization is only possible in polar space groups belonging to the ten pyroelectric crystal classes. In addition, the reversal of the polarization direction must occur via a paraelectric isotropy supergroup. These structural requirements were summarized by Abrahams into empirical rules and are still the basis for predicting previously unknown ferroelectrics. Historically, the focus of research has been on oxide ferroelectrics. Nowadays, a large number of oxide ferroelectric structure types are known, but only a few fluoride compounds with this property. In the present project, we aim to predict novel ferroelectric fluorides. For this purpose, we will prepare a survey of polar metal fluorides, which we will investigate for their potential as ferroelectrics using the successful Abrahams crystallographic criteria in combination with modern quantum chemical methods. We will synthesize and characterize promising candidates from this survey to demonstrate their structural prerequisites for ferroelectricity. Via an initial screening of the Pearson database, we have succeeded in identifying 105 different polar structure types for metal fluorides. Only nine of these structure types have at least one representative that is proven to be ferroelectric. Thus, a high potential for previously unknown ferroelectrics exists here. So far, we have already identified three promising candidates for new potentially ferroelectric structure types with β-MnF4 (R3c, hR360), Mn3F8 (P21, mS22) and RbCrF5 (Pmc21, oP28), with which we can start our work. Our screening further reveals that controversial structure proposals exist for about 30% of the structure types. This is because, especially for polar compounds, the assignment of the space group in the diffraction experiment is not always possible without doubt. This complicates automatic compilations of data sets needed for high-throughput DFT calculations or machine learning. This aspect highlights the great need for an unambiguous structural chemical data set for polar compounds. With the crystallographic, structural chemical and experimental experience knowledge of our working group we want to tackle this task for the material class of metal fluorides.