Temperature dependence of the fundamental absorption edge in Cu2GeSe3

The temperature dependence of the energy gap EG in Cu2GeSe3 has been obtained from optical absorption measurements in the temperature range from 10 to 300 K. X-ray powder diffraction analysis indicates that the compound crystallizes in an orthorhombic structure with the unit cell parameters a=11.878(8), b=3.941(3), and c=5.485(3) Å. The data were analyzed by considering several models reported in the literature for the variation of EG vs T. The best fit was obtained by using an expression proposed by Pässler [Phys. Status Solidi B 200, 155 (1997)]. From the analysis, the Debye temperature was estimated to be around 170 K. Also obtained was the fact that the effective energy of phonons that participate in the shift of the band gap with temperature is about 15 meV and corresponds to the acoustic phonon branch.