Investigation On Thermoelectric Properties And Transport Mechanism Of Cu_0.5In_0.5Cr₂Se₄ And Its Doped Samples

Thermoelectric(TE)materials can convert directly heat energy into electric energy,and vice versa.They could be used in temperature measurement,waste heat recovery semiconductor refrigeration,etc.The properties of TE materials are characterized by the non-dimensional thermoelectric merit ZT(ZT=(?)T).Electrical performance of TE materials is characterized by power factor PF=S2?.TE materials with excellent electrical performance require a large effective mass of density of states and a high carrier mobility.The effective mass of the density of states is determined by both the effective mass of the band and the degeneracy of the band.However,the effective mass of the band couples with the carrier mobility and large band effective mass leads to low carrier mobility which deteriorates TE performance.On the other hand,modification on band effective mass will not affect the band degeneracy.Therefore,it is a concerned optimization strategy to improvement of the effective mass of the density of states by increasing the band degeneracy without affecting the carrier mobility.Cu0.5In0.5Cr2Se4 with a face-centered cubic structure has high energy valley degeneracy;meanwhile,preliminary theoretical calculations show that its the maximum of valence band(VBM)has high orbital degeneracy,and therefore has high energy band degeneracy,which is a potential thermoelectricity material.At present,there are few studies on the thermoelectric properties of Cu0.5In0.5Cr2Se4,and the transport mechanism of Cu0.5In0.5Cr2Se4 remains to be further understood.In this paper,the thermoelectricproperties and transport mechanism of Cu0.5In0.5Cr2Se4 were studied both theoretically and experimentally.Firstly,the transport mechanism of Cu0.5In0.5Cr2Se4 was studied based on the first principles calculations.Cu0.5In0.5Cr2Se4 is a semiconductor with an indirect band gap.VBM is located at K point and the minimum of conduction band(CBM)is located at ?point.VBM at the K point is four orbit degeneracies;VBM at the ? point is six orbit degeneracies.Meanwhile,Cu0.5In0.sCr2Se4 is a face centered cubic crystal and has a high degeneracy of the energy valley.Therefore,Cu0.5In0.5Cr2Se4 has a high band degeneracy and is a potential TE material.However,because the effective mass of the VBM is large and the electronic states near VBM of Cu0.5In0.5Cr2Se4 mainly distribute around the Cr atom,which results in low carrier mobility,shows the electronic localization and inhibits the thermoelectric performance of the TE material.After equivalently replacing Ag on Cu sites,the multiple orbital degeneracy at the top of K and ? points in the valence band is separated,and the number of orbital degeneracy decreases.The distribution of electronic states around Cr atom is anisotropic,which makes it difficult to form effective conductive channels,inhibit carrier migration.Secondly,the thermoelectric performance and transport mechanism of pure Cu0.5In0.5Cr2Se4 were studied experimentally.Low-to-intermediate temperature zone,the resistivity decreases with the increase of temperature,showing a semiconductor conductive behavior;Seebeck coefficient increases with the increase of temperature,showing a metal-like conductive behavior.With the increase of temperature,the electrons gain energy and make a fixed range hopping to enter the localized state,and the hole concentration in the extended state below the mobility edge increases.The electrical transport of Cu0.5In0.5Cr2Se4 follows the Anderson's localization theory.The thermal conductivity of Cu0.5In0.5Cr2Se4 is mainly contributed by the lattice thermal conductivity.Due to the dominant phonon U-process scattering,the thermal conductivity decreases with the increase of temperature.The ZT value of the sample increases with the increase of temperature and reaches the maximum value of?0.31 at 773 K.Finally,electronic localization suppresses the performance of TE materials,the band structure and thermoelectric properties of Cu0.5In0.5Cr2Se4 is adjusted by doping,so the thermoelectric performance of doped Cu0.5In0.5Cr2Se4 was studied.After doping trace Zn atom at In site,the hole concentration decreases slightly,the Seebeck coefficient increases,the carrier mobility increases substantially and the resistivity decreases substantially,which means that the electron achieves delocalization.With the increase of Zn doping,the hole concentration increases,the resistivity and Seebeck coefficient decreases.The trend of resistivity with temperature is different with that of pristine Cu0.5In0.5Cr2Se4.The resistivity of the Zn-doped samples increases monotonously with the increase of temperature;the Seebeck coefficient increases monotonously with the increase of temperature.The resistivity and Seebeck coefficient exibit the degenerate semiconductor conductive behavior.The maximum ZT value of Zndoped samples is about 0.40 at 773 K in the sample of x=0.035,which is about 30%higher than that of pure Cu0.5In0.5Cr2Se4.After the substitution of Ag in Cu site,resistivity is unchanged or increased and the Seebeck coefficient increases.Moreover the trend of resistivity and Seebeck coefficient with temperature of Cu0.5-xAgxIn0.5Cr2Se4(x=0.005,0.015,0.025)samples is almost similar to that of pure Cu0.5In0.5Cr2Se4,showing electronic localization.