Temperature-dependent dielectric functions and interband critical points of sulfur-rich TlIn(S1−xSex)2 layered solid solution crystals

Real and imaginary parts of the dielectric function of TlIn(S1-xSex)2 (x = 0.05, 0.08, 0.25) single crystals were determined in the spectral range from 1 to 5 eV within a temperature interval 140-293 K from spectroscopic ellipsometry measurements. The energies of interband transitions (critical points) of the TlIn(S1−xSex)2 crystals were obtained from the second derivative of the real and imaginary parts of dielectric function. Structural phase transitions are behind the observed change of electronic band structure.