Optimising Ferroelectric Hybrid Frameworks through Tuning Electronegativity

在过去的二十年里，将联合收割机金属离子和有机组分作为结构单元结合在一起的杂化骨架材料因其所表现出的结构和功能特性的多样性而引起了全世界的关注。它们的结构中具有多孔通道，可以分离和储存各种分子气体，因此长期以来一直受到人们的关注。然而，最近，它们已经开始显示出其电子和磁性的巨大前景，这对支撑许多现代技术至关重要，并且长期以来一直与金属氧化物陶瓷有关。这些属性包括：1.铁电性，即材料中电荷的可切换极化，使其具有带正电和带负电的一端。铁电体在许多行业都有应用，包括基于压电响应的医疗和国防超声技术，其中材料在应用的电场中膨胀和收缩。2.多铁性是指材料同时具有铁电有序性和磁有序性，在信息技术中具有超高密度存储器存储的巨大潜力。一些功能氧化物结构的类似物对于具有电子功能的混合框架仍然很重要。这包括钙钛矿相关的结构，其中小的八面体金属阳离子在其角落处被阴离子桥接，而较大的阳离子，混合物中的有机分子，位于该扩展框架之间的空间中。然而，混合框架的电子性质可以从根本上不同于氧化物中发现的来源产生;例如，混合钙钛矿中的带电分子阳离子可以表现出铁电有序，因为分子和阴离子框架之间的相对弱的超分子相互作用，例如氢键。这使得它们能够以独特的方式进行修改，例如，混合物中对所施加电场的铁电响应可以被优化为在宽温度范围内以通常与弛豫氧化物材料相关的方式保持一致，而不引入这种氧化物所需的阳离子的复杂混合物。这种弛豫性质如何发生在混合框架的细节仍然不清楚，这将是重要的是要发展的理解，这发生在为了能够修改混合优化其物理properties.As的相互作用负责铁电秩序在混合框架是弱得多的氧化物，这是至关重要的，以增加这些接触的强度。这将使分子阳离子能够在更高温度下采用铁电性所需的有序模式，从而开发出在室温以上的有用条件下工作的材料。实现这一点的一种途径是使较小的金属阳离子和阴离子的电负性（离子和电子之间的吸引力强度）之间的差异最大化，这间接地增强了负责分子阳离子的铁电有序的超分子相互作用。该提案将通过制造混合钙钛矿框架来探索实现这一目标的两种途径，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，这些材料将被筛选，以确定其在室温以上具有铁电有序模式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质，以建立优化的铁电性能。实验和计算技术的结合，然后将被用来了解这些材料中的原子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响应的独特方式。

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.