Phase-transition induced thermochromic behavior of schafarzikites

对于给定的温度，热致变色材料可用作优良的光学指示器。通过这种方式，可以将热致变色相集成到结构部件中，这表明在材料因温度而部分或完全失效的情况下，由于颜色变化而存在一定的风险。例如，在太空探索中，它可以作为热反射和吸热材料发挥作用，这取决于太阳的相对位置，从而取决于温度。为此，计划研究一旦达到临界温度就会发生颜色变化的材料。热致变色可以通过具有可逆位移型相变的材料来实现，如在沙法沸石类型的结构中发现的，导致在临界温度TC上可逆的颜色变化。此外，不可逆的重建相变或变换会导致永久性的颜色变化。如果有必要，可以将这两者结合起来。在这方面，重要的是要了解关于热梯度和/或导热系数的颜色变化机理。除了相变温度(TC)外，如果只用一个物理参数来描述热致变色转变将是有利的。因此，我们提出了一个单一参数，它可以从一个转移函数中推导出来。这个转变函数被指定为两个相关相的热膨胀行为，通过相变的德拜-爱因斯坦-非谐(DEA)模型来描述。为了实现跨越相变的热致变色，我们提出了由立体化学活性孤电子对稳定的莫来石型O8沙法兹石相及其固溶体作为模型体系。该项目着眼于以下里程碑：(I)从实验上验证并在必要时进一步开发“相变热膨胀”模型；(Ii)表征模型体系L2(M1-xM‘x)O4(L=Pb2+，M=Pb4+，Sn4+，Pt4+)，以更好地了解相变热致变色的晶体-化学-物理基础；(Iii)建立至少相关的热致变色氧化物库，并对允许较宽温度范围的热色转变的环境友好相进行分类。

A thermochromic material can be used as an excellent optical indicator for a given temperature. In this way, a thermochromic phases can be integrated into structural components, indicating a certain risk due to color changes in case of temperature-driven partial or complete failure of materials. Alternatively, it can function, for instance, in space exploration, depending on the relative position of the sun and thus depending on the temperature, as both heat-reflecting and heat-absorbing material. For this purpose, it is planned to investigate materials that can undergo color changes as soon as the critical temperature is reached. Thermochromism can be realized by materials with reversible displacive phase transitions as found in schafarzikite-type structures, leading to reversible color changes over a critical temperature TC. In addition, irreversible reconstructive phase transitions or transformations lead to permanent color changes. If necessary, these two could be combined. In this respect, it is important to understand the color change mechanism with respect to thermal gradient and/or thermal conductivity. Beside the phase-transition temperature (TC), it would be advantageous if the thermochromic transition could be described with only one physical parameter. As such, we propose a single parameter, which can be derived from a transition function. This transition function is assigned to the thermal expansion behavior of the two involved phases, described by the Debye-Einstein-Anharmonicity (DEA) model across the phase transition. To realize thermochromism across phase transitions we propose mullite-type O8 Schafarzikite phases stabilized by stereochemically active lone electron pairs, and their solid solutions as model systems. The project aims at the following milestones: (I) to experimentally validate and, if necessary, to further development the "thermal expansion across phase transition" (TEAPT) model, (II) to characterize the model system L2(M1-xM'x)O4 (L = Pb2+, M = Pb4+, Sn4+, Pt4+) to better understand the crystal-chemico-physical basis of thermochromism across the phase transitions, (III) to create at least the relevant thermochromic oxide library and categorize the environmental-friendly phases that allow a wide temperature range for thermochromic transitions.