Deformation and Failure Mechanisms of Thermoelectric Type-I Clathrate Ba₈Au₆Ge₄₀

Abstract

Type-I clathrate Ba 8 Au 6 Ge 40 , possessing an interesting structure stacked by polyhedrons, is a potential "phonon-glass, electron-crystal" thermoelectric material. However, the mechanical properties of Ba 8 Au 6 Ge 40 vital for industrial applications have not been clarified. Here, we report the first density functional theory calculations of the intrinsic mechanical properties of thermoelectric clathrate Ba 8 Au 6 Ge 40 . Among the different loading directions, the {110}/⟨001⟩ shearing and ⟨110⟩ tension are the weakest, with strengths of 4.51 and 6.64 GPa, respectively. Under {110}/⟨001⟩ shearing, the Ge−Ge bonds undergo significant stretching and twisting, leading to a severe distortion of the tetrakaidecahedral cage, giving rise to the fast softening of the flank Au−Ge bonds. At a strain of 0.2655, the Au−Ge bonds suddenly break, resulting in the collapse of the cage and the failure of the material. Under a ⟨110⟩ tension, the stretching of the Ge−Ge bonds keeps accelerating the softening of the Au−Ge bonds in the top/bottom hexagons, which releases the stress and disables the structure. The Au−Ge bonds are more rigid, contributing two-thirds of the structural deformation resistance. This work provides a new insight to understand the failure mechanisms of type-I clathrates with varied framework constitutions, which should help inform the design of robust thermoelectric clathrate materials.

Deformation and Failure Mechanisms of Element-Substituted Thermoelectric Type-I and Type-VIII Clathrates

“…By comparing the failure mechanism of Ba 8 Au 6 Si 40 and Ba 8 Au 6 Ge 40 , it is found that the Ge pentagonal ring in Ba 8 Au 6 Ge 40 is a relatively flexible structure, with the Ge–Ge bond elongating and softening during the shear deformation. Moreover, the brittle failure of CoSb 3 is mainly attributed to the deconstruction of the Sb-ring and the collapse of the Co–Sb frameworks .…”

“…Here, we compare the intrinsic mechanical properties of type-I and type-VIII clathrates with those of other important TE materials, − as shown in Figure . The red column represents the ideal shear strength of the TE clathrates, which shows the disperse distribution of clathrate ideal strength among all important TE materials.…”

“…Analysis of the key bond lengths during shear deformation shows that most bond lengths in the tetrakaidekahedral cage remain stable when the strain is increased to the ultimate strain, except for the Au−Si2 (Figure 4e), which is different from the Ge1−Ge1 bond softening and breaking in Ba 8 Au 6 Ge 40 . 19 The Au−Si2 (bottom) bond length increases approximately linearly with an increasing strain. Until the strain By comparing the failure mechanism of Ba 8 Au 6 Si 40 and Ba 8 Au 6 Ge 40 , 19 it is found that the Ge pentagonal ring in Ba 8 Au 6 Ge 40 is a relatively flexible structure, with the Ge−Ge bond elongating and softening during the shear deformation.…”

“…Thermoelectric (TE) semiconductors realize direct conversion between heat and electricity based on carrier and phonon transport, which is considered to have potential applications in waste heat utilization and microrefrigeration. , The conversion efficiency of TE semiconductors is typically expressed as a dimensionless figure of merit, ZT = α 2 σ T /κ, where α is the Seebeck coefficient, σ is the electric conductivity, and κ is the thermal conductivity . Under Slack’s guidance “phonon-glass, electron crystal”, diversified TE semiconductor systems have been developed, including covalent compounds (TiNiSn and InSb), ionic compounds (PbTe, PbSe, Mg 2 Si, PbS, α-MgAgSb, CuInTe 2 , La 3 Te 4 , CaZn 2 Sb 2 , CaMg 2 Sb 2 , BiCuSeO, and Mg 3 Sb 2 ), van der Waals compounds (Bi 2 Te 3 , SnSe, α-Ag 2 S, and In 4 Se 3 ), and clathrates (skutterudite CoSb 3 , type-I Ba 8 Au 6 Ge 40 , type-VIII Ba 8 Ga 16 Sn 30 , and tI -Na 2 ZnSn 5 ). With the improvement of the TE conversion efficiency and the progress of device miniaturization technology, the importance of TE semiconductors has become increasingly prominent.…”

Deformation Mechanisms of Clathrate tI-Na2ZnSn5

“…We have reported the intrinsic mechanical properties of typical TE materials including type-I clathrate Ba 8 Au 6 Ge 40 in previous work. 21 Compared with the type-I clathrates with complete cage frameworks, tI-Na 2 ZnSn 5 has an incomplete open cage framework. Pores and tunnels may result in the change of intrinsic mechanical properties, such as degradation of ideal strength and mechanical anisotropy.…”