monocrystal was obtained in a vacuum sealed quartz ampoule inside a vertical furnace by
Bridgman's method. The optical transmittance measurement enables to evaluate the values
of Eg. By a fitting procedure of Eg as a function of temperature is possible to extract the
parameters that govern its behavior. The variation of Eg with temperature was determined
as: Eg (T)= Eg (0)-aT2/(a+ T), with: Eg (0)=(2.435±0.008) eV, a=(8.7±1.3) x 10-4 eV/K and …