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Temperature dependence of the indirect band gap and related optical parameters of (NH 4 ) 2 ZnCl 4 : x Sr 2+ single crystals
Author(s) -
Gaffar M. A.,
Abu ElFadl A.,
Bin Anooz S.
Publication year - 2003
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.200301872
Subject(s) - absorption edge , analytical chemistry (journal) , band gap , attenuation coefficient , doping , atmospheric temperature range , absorption (acoustics) , photon energy , chemistry , transmittance , antiferroelectricity , materials science , condensed matter physics , optics , thermodynamics , photon , ferroelectricity , physics , dielectric , optoelectronics , chromatography , composite material
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)