High temperature optical absorption investigation into the electronic transitions in sol–gel derived C12A7 thin films

Abstract

Optical absorption into 6 mm thick sol–gel derived films, annealed at 1300 °C of 12CaO·7Al<inf>2</inf>O<inf>3</inf> calcium aluminate binary compound on MgO〈100〉 single crystal substrates was studied at temperatures ranging from room temperature to 300 °C. Experimental data were analysed in both Tauc and Urbach regions. The optical band gap decreased from 4.088 eV at 25 °C to 4.051 eV at 300 °C, while Urbach energy increased from 0.191 eV at 25 °C to 0.257 eV at 300 °C. The relationship between the optical band gap and the Urbach energy at different temperatures showed an almost linear relationship from which the theoretical values of 4.156 and 0.065 eV were evaluated for the band gap energy and Urbach energy of a 12CaO·7Al<inf>2</inf>O<inf>3</inf> crystal with zero structural disorder at 0 K.