Optimising Ferroelectric Hybrid Frameworks through Tuning Electronegativity

杂化骨架材料将金属离子和有机成分组合成一个扩展的网络，在过去的二十年里，由于它们表现出的结构和功能特性的多样性，引起了全世界的关注。长期以来，人们一直对其结构中的多孔通道感兴趣，这种通道使各种分子气体能够被分离和存储。然而，最近，它们开始显示出巨大的电磁性能，这是支撑许多现代技术的关键，长期以来一直与金属氧化物陶瓷联系在一起。这些特性包括：1.铁电性，即材料中电荷的可切换极化，使其具有正负电荷的一端。铁电材料在许多行业中都有应用，包括基于压电响应的医疗和国防超声技术，即材料在应用的电场中膨胀和收缩。多铁性，即一种材料既具有铁电有序又具有磁性有序，在信息技术中具有巨大的超高密度存储潜力。某些功能氧化物结构的类似物对于具有电子功能的杂化骨架仍然是重要的。这包括与钙钛矿相关的结构，其中小的八面体金属阳离子在它们的角落被阴离子桥接，而较大的阳离子，杂化的有机分子，位于这个扩展的框架之间的空间。然而，杂化骨架的电子性质可能来自与氧化物中发现的根本不同的来源；例如，杂化钙钛矿中的带电分子阳离子可以表现出铁电有序，因为分子和阴离子骨架之间的超分子相互作用相对较弱，如氢键。这使得它们能够以独特的方式进行修饰，例如，可以优化杂化材料对外加电场的铁电响应，使其在较宽的温度范围内保持一致，这种方式通常与弛豫氧化物材料有关，而不会引入此类氧化物所需的复杂的阳离子混合物。杂化骨架中这种弛豫类性质如何发生的细节仍不清楚，为了能够对杂化骨架进行修饰以优化其物理性质，了解其发生的原因将是非常重要的。由于杂化骨架中导致铁电有序的相互作用比氧化物中的弱得多，因此增加这些接触的强度是至关重要的。这将使分子阳离子在更高的温度下采用铁电性所需的有序化模式，从而开发出在室温以上的有用条件下运行的材料。实现这一点的一种方法是最大化电负性之间的差异，即离子和电子之间的吸引强度，较小的金属阳离子和阴离子之间的电负性之间的差异，这间接地加强了导致分子阳离子铁电有序的超分子相互作用。这项提议将探索两条实现这一目标的途径，即通过制备杂化钙钛矿骨架来实现这一目标，其中八面体阳离子是电负性较低的镁、钙或锶阳离子，或者阴离子是电负性较强的，即氯、溴或碘。然后将对这些材料进行筛选，以确定其具有高于室温的铁电有序图案的分子阳离子的能力，然后将表征满足该标准的材料的物理性能，以建立最佳的铁电性能。然后，将结合实验和计算技术来了解这些材料中原子能级结构和物理性质之间的联系，特别关注在混合框架中出现弛豫样反应的独特方式。

Hybrid framework materials, which combine metal ions and organic components as building blocks in an extended network, have attracted worldwide attention over the last two decades for the diversity of structures and functional properties they exhibit. They have long been of interest for the porous channels in their structures, which enables a wide range of molecular gases to be separated and stored. More recently, however, they have begun to show tremendous promise for their electronic and magnetic properties, which are critical to underpinning many modern technologies and have long been associated with metal oxide ceramics. These properties include:1. Ferroelectricity which is, the switchable polarisation of charge in a material such that it has a positively and negatively charged end. Ferroelectrics have applications in many industries, including medical and defence ultrasound technologies based on piezoelectric responses, where a material expands and contracts when in an applied electronic field.2. Multiferroicity, where a material has both ferroelectric and magnetic order, which has great potential for ultra-high density memory storage in information technologies. Analogues of some functional oxide structures remain important for hybrid frameworks with electronic functions. This includes perovskite related structures, in which small octahedral metal cations are bridged by anions at their corners, with a larger cation, an organic molecule in hybrids, sitting in the spaces between this extended framework. The electronic properties of hybrid frameworks can, however, arise from fundamentally different origins than found in oxides; for example charged molecular cations in hybrid perovskites can exhibit ferroelectric order because of relatively weak supramolecular interactions, such as hydrogen-bonding, between the molecule and the anionic framework. This enables them to be modified in unique ways, for example the ferroelectric response to applied electric fields in hybrids can be optimised to be consistent over a wide temperature range in a way typically associated with relaxor oxide materials, without introducing the complex mixture of cations needed for such oxides. The details of how such relaxor-like properties occur in hybrid frameworks is still unclear and it will be important to develop an understanding of this occurs in-order to be able to modify hybrids to optimise their physical properties.As the interactions responsible for ferroelectric order in hybrid frameworks are much weaker than in oxides it is critical to increase the strength of these contacts. This will enable the molecular cations to adopt the ordering patterns required for ferroelectricity at higher temperatures, leading to the development of materials that operate at useful conditions, above room temperature. One route to achieving this is to maximise the difference between the electronegativity, the strength of attraction between an ion and an electron, of the smaller metal cation and the anions, which indirectly strengthens the supramolecular interactions responsible for ferroelectric order of the molecular cations. This proposal will probe two routes for achieving this goal by making hybrid perovskite frameworks where either the octahedral cation is a less electronegative magnesium, calcium or strontium cation, or the anion is more electronegative i.e. chlorine, bromine or iodine. These materials will then be screened for their ability to feature molecular cations with ferroelectric ordering patterns above room temperature and the physical properties of materials meeting this criteria will then be characterised to establish optimised ferroelectric performance. A combination of experimental and computational techniques will then be used to understand the link between atomic level structure and physical properties in these materials, with a particular focus on the unique way in which relaxor-like responses arise in hybrid frameworks.