Stability,Electrical And Thermal Transport Properties Of Multinary Compounds From A First-principles Study

In current society,the phenomenon of thermal transports in solids and its related applications have become an emerging topic since its importance in many fields,such as thermoelectrics.Some typical thermoelectric materials have been found to have unique vibration characteristics,such as Cu₃SbSe₃.In previous experiments,Cu₃SbSe₃has exhibited extremelly low lattice thermal conductivity,nearly temperature-independent.In this paper,we theoretically studied the stability,electrical transport and thermal transport of Cu₃SbSe₃,Ag₃SbSe₃,Cu₃BiSe₃and Cu₃SbTe₃.The new compounds are obtained by substituting elements at each atomic position for the prototype material Cu₃SbSe₃.Compared with the substituted elements in Cu₃SbSe₃,each new element is just in the next cycle of Cu,Sb,Se in the same group of the periodic table,and they have larger relative atomic mass.First-principles calculations of stability,electrical transport and thermal transport for these four compounds were carried out,the results were brought into the expression of dimensionless figure of merit:ZT=TS²?/?.We can conclude that Cu₃BiSe₃and Ag₃SbSe₃may be thermoelectric materials with good quality when the carrier concentration can be optimized appropriately.Considering that temperature has a great influence on the anharmonicity of mate-rials,for the prototype material Cu₃SbSe₃and Ag₃SbSe₃which showed the biggest ZT in these four materials,we do ab initio molecular dynamics for them at 100K,200K,300K,400K and 100K,400K,respectively.The results show that the phonon disper-sion curves of Cu₃SbSe₃have no negative frequencies at temperatures of 100K,200K,300K,but small negative frequencies occur at the temperature of 400K,at this time,the dynamical stability of the crystal structure is destroyed,the negative frequencies may originate from the related lattice vibration of Cu1z in low frequency phonons microcos-mically and the fluidity of Cu atoms at 400 K macroscopically,the Cu sublattice should be viewed as“sublattice melting”,but the atoms of Sb and Se are still in crystalline state,so the whole system is in a mixed“Part-crystalline part-liquid state”.However,Ag₃SbSe₃exhibits very exaggerated negative frequencies at 100K and 400K,which may be related to the further increase of the amplitude of Ag atoms with large atomic displacement parameters at finite temperature.Cd₂Cu₃In₃Te₈is a recently designed material with good thermoelectric properties.We use three common metal elements Mg,Mn and Zn which are usually used in ex-periments to replace its Cd element in a similar method.We study the stability and lattice thermal conductivity of Cd₂Cu₃In₃Te₈and the newly produced Mg₂Cu₃In₃Te₈,Mn₂Cu₃In₃Te₈and Zn₂Cu₃In₃Te₈.The phonon dispersion curves without negative fre-quencies show that their crystal structures are dynamically stable,and the lattice ther-mal conductivity calculated by Debye-Callaway model is also very small.In theory,Cd₂Cu₃In₃Te₈,Mg₂Cu₃In₃Te₈,Mn₂Cu₃In₃Te₈and Zn₂Cu₃In₃Te₈may be thermoelec-tric materials with good quality.Compared to Cu₃SbSe₃and Ag₃SbSe₃,Cu₃BiSe₃,Cu₃SbTe₃through heavy element substitution,the lattice thermal conductivities of Cd₂Cu₃In₃Te₈,Mg₂Cu₃In₃Te₈,Mn₂Cu₃In₃Te₈,Zn₂Cu₃In₃Te₈are much bigger,because Cu₃SbSe₃,Ag₃SbSe₃,Cu₃BiSe₃and Cu₃SbTe₃have stronger anharmonicity due to their complex crystal structure and Cu?Ag?atoms act as“ratting-like”damping in the heat conduction.