Structure Prediction And Electronic Structures Of ?-? Sulfide Alloys

ZnS-CaS alloys have great potential for applications in ultraviolet optoelectronic devices due to their large and tunable band gaps.However,the in-depth understanding of these sulfide compounds is hindered by the uncertainty of their phase relations and complex crystal structures.Here,we explored the new phases of ZnS-CaS system systematically by the simulations of an ab initio evolutionary variable-composition search and the special quasirandom structures approach for the ordered and random structures,respectively.The main results are as follows:?I?All of the experimentally synthesized phases of binary compounds were found during the structure search,together with three new stable phases of alloys I4/mcm-CaZnS₂,R3?-Ca₂ZnS₃ and R3?-Ca₇Zn₂S₉ under pressures,which can remain dynamically and mechanically stable at ambient pressure.We presented the thermodynamic convex hulls and pressure-composition diagrams of the ZnS-CaS alloys under pressures.Under the loading of pressure,it is easy for the calcium-rich phases to become thermodynamically stable,relative to the zinc-rich phases,due to the different bulk modulus of their binary analogs.?II?Although ZnS and CaS are wide-band-gap semiconductors,the band gaps could be tuned into the range below 2.0 eV by alloying,which is desirable for applications in solid-state lighting and high-efficient solar cells.Despite the band gap of CaS is smaller than that of ZnS,the band gaps of alloys are enhanced anomalously by increasing Ca concentration due to the weak p-d repulsion between S and Zn atoms.Another way to enhance the band gaps by weakening this level repulsion is the increase of the 3d-orbital binding energy induced by pressure,which leads the band gaps to first increase and then decrease with pressure for the compounds containing the d-orbital electrons.?III?Finally,we presented the mixing enthalpies curves and temperature-composition phase diagrams of ZnS-CaS alloys system under temperatures,and discussed the nature of the wide and asymmetric miscibility gap,which indicates that the heterostructural alloys can be synthesized experimentally by a nonequilibrium growth technique,and give a solid basis for experimental synthesis and further investigations.