Opacification of dielectrics oxides investigated by infrared emittance spectroscopy

With increasing temperature, some compounds that are transparent in the near infrared range at room temperature become progressively opaque towards the liquid phase. Such a behavior deeply impacts their thermal radiative properties which knowledge is of main importance in crystal growth processes for example. To understand this phenomena, infrared emittance spectra were acquired from room temperature up to the liquid state on crystalline LaAlO$_{3}$ and LiAlO$_{2}$. The samples are heated with a CO$_{2}$ laser that allows a direct measurement of emittance up to their melting point and in a wide spectral range, i.e. between 50 and 10000 cm$^{-1}$. In the transparency region, we observed an increase of emittance up to 1000 K for LaAlO$_{3}$ (more than 1000 K before the solid to liquid transition) and no significant increase of emittance for LiAlO$_{2}$ before the solid to liquid phase transition. The experimental data have been fitted with a dielectric function model including a Debye relaxation term. The absorption mechanism at the origin of the opacification is thermally activated and has a relaxation time compatible with a motion due to electrons. Electrical conductivity measurements of these compounds were also made up to 800K.