Optical transmittance measurements near the absorption edge of [Kx(NH4)1x]2ZnCl4 mixed crystals, where x ¼ 0.00, 0.232, 0.522,
0.644, 0.859 and 1.00, are reported over 276–350K range. Analysis reveals that the type of transition is the indirect allowed one. The
absorption edge shifted towards lower energy with increasing temperature. It is shown that [Kx(NH4)1x]2ZnCl4 mixed crystals with
xp0.644 reveal a phase transition at 319 K, this phase disappeared at high concentrations of K+ ions. The steepness parameter is given,
its value is used to estimate the temperature dependence of the indirect energy gap. In the region of the absorption edge, the absorption
coefficient obeys Urbach’s rule. Urbach parameters are investigated as a function of temperature.
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