Temperature dependence of the indirect band gap and related optical parameters of (NH 4 ) 2 ZnCl 4 : x Sr 2+ single crystals

Optical transmittance measurements were carried out near the absorption edge of (NH 4 ) 2 ZnCl 4 : x Sr 2+ single crystals, where x = 0.00, 0.020, 0.039, 0.087 or 0.144 wt%. The dependence of the absorption coefficient ( α ) on photon energy in the range 3.4–6.4 eV and on temperature in the range 276–350 K was examined. With increasing temperature the absorption edge was shifted to the lower energy side; this shift was attributed to the temperature dependence of electron–lattice interactions. Due to the presence of Sr 2+ in (NH 4 ) 2 ZnCl 4 : x Sr 2+ a change in the absorption band centred at 5.30 eV was observed. In the case of higher strontium doping, the optical absorption coefficient showed a ‘pre‐transition’ stage just before the onset of the band‐to‐band transition. Exponential dependence of α on hν according to the Urbach rule was observed and the Urbach parameters were determined. The steepness parameter was strongly affected by doping; its behaviour at 319 K evidently showed the antiferroelectric to commensurate phase transition. The temperature dependence of E g was also estimated using data for the steepness parameter, and hence E g at different temperatures and Sr 2+ concentrations was evaluated. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)