Temperature dependence of the indirect band gap and related optical parameters of (NH₄)₂ZnCl₄:xSr²⁺ single crystals

Abstract

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.

Electron irradiation‐induced effects on optical spectra of (NH4)2ZnCl4: x Sr2+ single crystals

“…For x=0.087 or 0.144, this band is completely disappeared and instead we observe a change in the rate by which α increases with hν. The reason for the existence and disappearance of this band was discussed elsewhere [19]. It is worthy to know that similar absorption bands for different crystals of the A 2 BX 4 family were also observed [20,21].…”

“…Fig. (5) contains also the same dependence for irradiation with γ the same samples extracted from another study in our laboratory for the sake of comparison [19]. The observed difference between the two cases may be due to the difference between the absolute value of the energy of the γ-and e-beam irradiation used in the two cases.…”