Electronic structure and optical properties of amorphous GeO2 in comparison to amorphous SiO2

Amorphous germania (a-GeO2) is an excellent glass former of great industrial and scientific importance. However, in comparison with a-SiO2, its structure and fundamental properties were less well studied. Using a large near-perfect continuous random network (CRN) model with 1296 atoms and no over- or under-coordinated atoms, we have investigated the structural, electronic, and optical properties of a-GeO2 glass. Our results show that the bond length and bond angle distributions in a-GeO2 are larger than in a-SiO2. The gross features of the electronic density of states in a-GeO2 are similar to a-SiO2, but the Ge-O bonds are weaker than Si-O bonds as reflected in the lower calculated total bond order density. The average tetrahedral angle (theta) and bridging angle (phi) are smaller in a-GeO2 than in a-SiO2. The calculated optical absorption spectrum shows two distinctive peaks in excellent agreement with experiment. The calculated refractive index of a-GeO2 (n = 1.69) is also in close agreement with the measured value. In contrast to a-SiO2, there is no clear evidence of excitonic peak in a-GeO2. These a-GeO2 and a-SiO2 models could be used as a prototype for other investigations of these glasses or their mixtures containing defects, substitutional impurities and in the form of vitreous nano-particles. (c) 2015 Elsevier B.V. All rights reserved.