Pyrochlore transparent ceramics (i.e. ceramics with the general composition A₂B₂O₇ and cubic structure) are promising candidates for use as scintillator hosts and luminophores [1]. These materials are, however, still mostly in the experimental stage and are due to the issues with the optical quality rarely applied in practice [2]. To achieve high optical quality, the light scattering at the wavelengths of interest must be reduced as much as possible. The sources of this undesirable effect are in the case of cubic ceramics most often residual porosity, secondary phases and impurities.
 
The issue of light scattering itself is, however, much more complicated than just eliminating the sources because the effect is, in fact, an interplay between the refractive index mismatch, wavelength, scatterer size (distribution), concentration and thickness of the whole system. Therefore, in order to quantify the effect of light scattering through modeling, the optical properties of the ceramics must be known, and a suitable modeling approach has to be applied [3].
 
The aim of this contribution is to examine and explain this interplay in a comprehensive way and describe the light scattering effects in pyrochlore systems. First, the refractive index-wavelength dependence for pyrochlore systems based on a thorough literature review will be presented and fitted using the so-called Sellmeier equations, which allow us to extrapolate refractive indices for different wavelengths. Based on this data, results of the light scattering predictions in the VIS and IR ranges of the electromagnetic spectrum will be presented to explain the effect of light scattering by different sources in pyrochlore-based materials. Finally, different approaches to light scattering predictions (numerical Mie theory versus its approximations with analytical solutions) will be discussed.
 
In particular, it will be demonstrated how the porosity and pore size distribution, as well as potential secondary phases and impurities affect the transparency at different wavelengths of interest.
 
References:
[1] Y. He, K. Liu, B. Xiang, C. Zhou, L. Zhang, G. Liu, X. Guo, J. Zhai, T. Li, L. B. Kong: An overview on transparent ceramics with pyrochlore and fluorite structures. Journal of Advanced Dielectrics. 10(3), 2030001 (2020). doi: 10.1142/S2010135X20300017.
 
[2] L. An, Z. Wang, A. Ito, G. Zhou, T. Goto, S. Wang: Transparent ceramics based on pyrochlores, pp. 399-432 (Chapter 12) in A. Chowdhury (ed.): Pyrochlore Ceramics – Properties, Processing, and Applications (Elsevier Series on Advanced Ceramic Materials). Elsevier, Amsterdam 2022. doi: 10.1016/B978-0-323-90483-4.00012-X.
 
[3] S. H?íbalová, W. Pabst: Modeling light scattering by spherical pores for calculating the transmittance of transparent ceramics – All you need to know. Journal of the European Ceramic Society. 41(4), 2169-2192 (2021). doi: 10.1016/j.jeurceramsoc.2020.11.046.
 
Acknowledgement:
This work is part of the project “Low-phonon energy transparent ceramic luminophores emitting in the short- and mid-infrared region“ (GA22-14200S), supported by the Czech Science Foundation (GA?R).