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We present a theoretical study of the optical absorption coefficient Urbach tail broadening parameter Γ behavior in the Hg1−xCdxTe alloy semiconductor in the 0.21 ≤ x ≤ 0.6 alloy composition interval. This x interval corresponds to the very attractive 0.10 ≤  EG ≤ 0.75 eV medium infrared (MIR) and far infrared (FIR) optical ranges. We compare two absorption coefficient nonparabolic models based on Kane 4-band formalism, one including the Burstein–Moss shift called the NPBM-model and the other one without and called the NP-model. By comparing the results of both models with existing experimental and theoretical data, we show the strong nonparabolic behavior of the absorption coefficient in Hg1−xCdxTe in agreement with previous studies. The best fitting is obtained with the NPBM-model, where Γ is used as an adjustable parameter varying with x, temperature (T), and photon energy ( ħω) in the E ≤  EG sub-bandgap energy range. With decreasing x, Γ is found to increase first slightly with x in the 0.443 ≤ x ≤ 0.6 MIR range and then strongly and nonlinearly in the 0.21 ≤ x < 0.443 FIR range. These unusual Γ(x, ħω) dependences suggest a strong influence of nonparabolicity and band state mixing effects, which become strongly enhanced in the FIR range between strongly interacting and almost overlapping bands as x tends to 0.16 of the critical value, making Hg1−xCdxTe experience a semiconductor–semimetal transition.

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