| Recent studies indicate that ternary A?B?C2? and A?B?C2? type compounds with chalcopyrite structure have unique mechanical,electrical and optical characteristics,and they have wide applications,such as nonlinear optics,light emitting diodes,semiconductor lasers,optical detectors,infrared parametric oscillators and solar cells.Among these ternary chalcopyrites,Cu-based chalcopyrite compounds and MSnP2-type chalcopyrite compounds have been investigated experimentally.The doping of chalcopyrite structural compounds is mostly B-site or C-site doping and rarely involves A-site doping.For all we know,this is the first time that the physical properties of Ag-doped CuInTe2 compounds and MSnP2(M=Zn,Cd,Hg)chalcopyrite are studied theoretically.Particularly,HgSnP2 has not been synthesized in experiment or even theoretically studied up to now.These researches are of great significance to the fundamental understanding of these materials,as well as their preparations and applications.Based on the plane-wave pseudopotential density functional theory(DFT),we have investigated the crystal structures,elastic behavior,lattice dynamical properties,electronic structures,optical properties and thermodynamic properties of Cu1-xAgxInTe2(x=0,0.25,0.5,0.75,1)and MSnP2(M=Zn,Cd,Hg).Cu1-xAgxInTe2 compounds belong to the tetragonal system.With the increasing Ag doping concentration,both the lattice constants and volumes of these crystals increase.Cu1-xAgxInTe2 compounds are mechanically stable,because their elastic constants satisfy the Born Huang stability criteria.In addition,Cu1-xAgxInTe2 crystals are ductile materials,and their toughness gets better after being doped with Ag.The elastic properties of Cu1-xAgxInTe2 are anisotropic,and the anisotropy of the {001} plane is more obvious than those of the {100} and {010} planes.There is no negative frequency found in the phonon spectra of Cu1-xAgxInTe2,so these structures are at their lowest energy states and dynamically stable.The point group of CuInTe2 and AgInTe2 is D2d,which includes five irreducible representations,namely A1,A2,B1,B2 and E.Cu0.5Ag0.5InTe2,Cu0.75Ag0.25InTe2 and Cu0.25Ag0.75lnTe2 belong to the S4 point group,and they have the same irreducible representations,namely,A,B and E.The atomic displacements of several typical phonon modes in CuInTe2 crystal have been analyzed to deep the understanding of lattice vibrations in Cui-xAgxInTe2 compounds.With the increasing concentration of doped Ag,the Debye temperatures of Cu1-xAgxInTe2 compounds decrease,while the heat capacities increase.Cu1-xAgxInTe2 compounds are direct band gap semiconductors,and their band gaps are respectively 1.128,1.086,1.053,1.029 and 1.01 eV.Cu1-xAgxInTe2 compounds have optical anisotropies,leading to their potential applications in the field of second harmonic generation and optical parametric oscillator.In the ultraviolet region,Cu1-xAgxInTe2 compounds have high absorption and conductivity,and the conductivity gradually increase with x increasing.In the visible region,there are obvious and sharp peaks in the conductivities of CuInTe2,Cu0.25Ag0.75InTe2 and AgInTe2,and the conductivities are higher than 2.1,indicating that they have great potentials to be photoelectric materials.MSnP2(M=Zn,Cd,Hg)compounds belong to the tetragonal system and have a space group I-42d(D2d),and their lattice constants increase in the order of ZnSnP2?CdSnP2?HgSnP2.MSnP2 compounds are thermodynamically stable and might be synthesized experimentally due to their negative results of cohesive energies and formation energies.MSnP2 compounds are mechanically stable,because their elastic constants satisfy the Born Huang stability criteria and there is no negative frequency in their phonon dispersions.Therefore,we predict that the existence of HgSnP2 is reasonable.HgSnP2 and CdSnP2 have extremely similar properties for their close elastic constants.MSnP2 compounds are elastic anisotropic,and the anisotropy of CdSnP2 and HgSnP2 is more obvious.In addition,ZnSnP2 has brittle nature,while CdSnP2 and HgSnP2 have ductile character.The phonon spectrum of each MSnP2 has 24 branches,including three acoustic branches and 21 curves optical branches.The phonon dispersions of MSnP2 compounds and their phonon density of states show that the lightest atoms(P)vibrate mainly in the high-frequency region,while Sn and M vibrations occupy the lower frequency bands.For MSnP2,the group D2d has five irreducible representations,namely Ai,A2,Bi,B2 and E.Ai and A2 modes involve only displacements of P atoms,while others involve the vibrations of M,Sn and P atoms.Based on phonon spectra,the thermodynamic properties of MSnP2 were calculate,there is not much difference among these compounds.As the order of ZnSnP2?CdSnP2?HgSnP2,the Debye temperatures decrease.With the temperature increasing,the enthalpy increases slowly,the free energy decrease,and TS increases at a fast rate.MSnP2 compounds are direct gap semiconductors,and their bad gaps are respectively 2.062,1.424 and 0.543 eV.In the ultraviolet region,MSnP2 compounds have strong absorptions and high conductivities,making MSnP2 compounds to be promising solar cell materials.In addition,in the visible region,both the absorption coefficient and the conductivity of HgSnP2 are significantly greater than those of ZnSnP2 and CdSnP2,indicating that HgSnP2 has stronger visible light absorption capacity and better photovoltaic property. |
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