Indirect band gap and optical parameters of pure and doped potassium ferrocyanide single crystals

The optical transmittance for potassium ferrocyanide (KFCT) single crystals were measured as a function of the wavelength at different temperatures in the range 275-320 K along the two axes (I 0 (1) over bar) and (0 1 0) and hence the absorption coefficient (a) and optical band gap E-g were deduced. The type of transition was determined and the validity of Urbach's rule was checked. The steepness parameter (sigma) and the exciton energy E-0 were calculated at different temperatures. The temperature dependence of the energy gap was estimated and the result was confirmed through the calculation of dE(g)/dT from information about sigma. The refractive index, the extinction coefficient and both the real and imaginary parts of the dielectric permittivity were also calculated as a function of photon energy and the results were discussed. The effect of doping KFCT with 2% of sodium nitroproced, cobalt sulphate or potassium dichromate on the same physical parameters was also discussed. (C) 2000 Elsevier Science B.V. All rights reserved.