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In this paper, the composition-dependent electronic-structure and thermodynamic properties for hexagonal wurtzite (WZ) and face-centered cubic zincblende (ZB) ZnS1−xSex ternary alloys have been studied utilizing first-principles pseudopotential plane-wave self-consistent field calculations. Special quasi-random structure method was applied to generate disordered structures of 16-atom supercell for x = 0.25, 0.5 and 0.75. We obtained a direct band-gap at Г point and a shrinking tendency of the band-gap with increasing Se content in both phases. The reasons for the band-gap shrinkage have been discussed based on the analysis of electronic density of states and band offsets. Δ-sol method was applied to correct the band gaps calculated by density functional theory. The Δ-sol corrected band-gaps are in good agreement with experimental band-gaps. The thermodynamic properties were calculated by the cluster expansion method. It turns out that, the stretching forces decrease considerably and linearly with the bond length, while the bending forces are dependent insignificantly on the bond length. The phase diagrams reveal that the lattice vibration effects can influence the critical temperature Tc and the solubility of S- and Se-rich ZnS1−xSex alloys, but the influence is not obvious for ZB-ZnS1−xSex. Based on the calculated phase diagrams without lattice vibration effects, the Tc for WZ- and ZB-ZnS1−xSex are 559 K and 421 K, respectively. When the vibration enthalpy is included, the Tc values of both phases are lowered to 543 K and 418 K, respectively. And if the vibration entropy is also taken into account, the Tc values of both phases are further lowered to 529 K and 415 K, respectively. With inclusion of lattice vibration effects, the ZnS1−xSex alloy starts a phase transition from ZB to WZ at a temperature of 1600 K, and the solid solubility of Se increases with the temperature rise.

Two-dimensional transition metal dichalcogenide materials are of great interest for the development of semiconductor devices because of their diverse properties. In this paper, we discovered the ground states of WxMo1−xS2 alloys and discussed their electronic structures and phonon properties in detail by the cluster expansion method and the first principle calculation. Three ground-state structures of WxMo1−xS2 alloys, W1/3Mo2/3S2, W1/2Mo1/2S2, and W2/3Mo1/3S2, have low order–disorder phase transition temperatures, T1/3 = 102.3 K, T1/2 = 87.8 K, and T2/3 = 102.2 K, estimated by mean field theory. Their bandgaps exhibit a nonlinear increase with increasing W content. All ground structures are dynamically stable without imagine frequency in phonon dispersion. W doping into single layer MoS2 leads low frequency optical modes hybriding with acoustic modes and lowering down the low frequency band due to larger mass of W atoms. All these may improve the developments of WxMo1−xS2 alloys devices.