Temperature-dependent materials properties: thermochromism and the Debye temperatureWednesday (19.06.2019) 12:00 - 12:30
The capability of materials to change the color with changing temperatures is well known. As early as 1909 H. Meyer  reported about the color change of Bianthron from lemon-yellow to green on heating. Whereas many examples are available for organic thermochromic compounds driven by the change of their molecular geometry [2,3] the corresponding examples of inorganic materials are rare. Minium or lead-II-orthoplumbate  is a very early example of an industrially produced red pigment (16th century in Venice, Italy) which was later found to reversibly change its color to yellow by cooling it to ~170 K. This effect originates in a structural displacive phase transition (Figure) which changes the band structure of the compounds accompanied by a band gap change leading to this color change. But also, the temperature change itself influences the band-gap width  leading to continuous color changes of the absorption energy if in the energy range of visible light, the effect is strong and the temperature interval big enough. The Debye temperature evaluated from the temperature-dependent band-gap change using e.g. the Vina  model could be used (and vice versa) for the corresponding Debye-Einstein-Anharmonicity modeling of the thermal expansion (and the associated phase-transitions) as well as heat capacity or atomic displacement Debye-Waller factors  calculations. If color changes should be directly observed (naked eye) a distinct difference of the temperature-dependent color variation is as well necessary as the associated error calculation while using cameras for an easy and continuous observation.
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