Phase-transition induced thermochromic behavior of schafarzikites

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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.