Determination of the complex refractive index of thermochromic perovskite manganites with mixed-valences at cryogenic temperatures

J. Manara1, J. Hauck1, M. Arduini-Schuster1, M. Keller1 and M. Rydzek1

1Bavarian Center for Applied Energy Research (ZAE Bayern), Germany

Keywords: infrared radiation, spectroscopic techniques
property: thermocromic, magnetic order
material: perovskite manganite

Materials with defined infrared-optical properties are desirable for many thermophysical applications. In some tasks, a low thermal emittance is advantageous for a better thermal insulation whereas for other employments, a high thermal emittance is wanted for dispersing the heat from a device. Possible enhancements are smart materials which exhibit different thermal emissivities, depending on the temperature of the material. They are therefore called thermochromic materials.

Mainly the switch of the infrared-optical properties (e.g. spectral emittance) is of interest, which can be modeled by the change of the complex refractive index. Different functional ceramics are identified as thermochromic materials in literature. This works focuses on doped perovskite manganites with mixed-valences. The thermochromic switch occurs at temperatures below ambient temperature. Hence, a measurement extension was developed for an existing FTIR-spectrometer for determining the reflectance for wavelengths between 2 µm and 25 µm at cryogenic temperatures down to 77 K.

The infrared-optical properties of the presented Sm1-xCaxMnO3 perovskite manganite ceramics are correlated with the magnetic and the electronic properties of the materials which in turn depend on temperature. The transport of electrons is not only associated with the composition, but also with the charge transfer between the manganese ions (Mn4+ and Mn3+) which can be described, inter alia, by orbital-charge ordering, double-exchange and super-exchange. These exchange mechanisms are strongly associated with the crystal structure and the orientation of the electron spins which, on the other hand, are affected by the magnetic order (ferromagnetic, antiferromagnetic, paramagnetic, etc.) which depends on temperature.

In this work, monolithic samples with a defined composition are prepared by a sol-gel process and a subsequent sintering of the nanocomposite green body. From the measured reflectance of the opaque samples at cryogenic temperatures the spectral refractive index is derived as a function of temperature. Finally the measurement setup, the evaluation procedure and the results are discussed and a correlation between the infrared-optical properties and the magnetic order is given.

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