High Thermoelectric Performance in the Wide Band‐Gap AgGa_1‐xTe₂ Compounds: Directional Negative Thermal Expansion and Intrinsically Low Thermal Conductivity

Abstract

A deficiency of Ga in the wide bandgap AgGa 1-x Te 2 semiconductor (1.2 eV) can be used to optimize the electrical transport properties and reduce the thermal conductivity to achieve ZT >1 at 873 K. First-principles DFT calculations and a Boson peak observed in the low temperature heat capacity data indicate the presence of strong coupling between optical phonons with low frequency and heat carrying acoustical phonons, resulting in a depressed maximum of Debye frequency in the first Brillouin zone and low phonon velocities. Moreover, the Ag-Te bond lengths and Te-Ag-Te bond angles increase with rising temperature, leading to a significant distortion of the [AgTe 4 ] 7tetrahedra, but an almost unmodified [GaTe 4 ] 5tetrahedra. This behavior results in lattice expansion in the ab plane and contraction along the c axis, corresponding to the positive and negative Gruneisen parameters in the phonon spectral calculations. This effect gives rise to the large anharmonic behavior of the lattice. These factors together with the low frequency vibrations of Ag and Te atoms in the structure lead to an ultralow thermal conductivity of 0.18 Wm -1 K -1 at 873 K.

High ZT 2D Thermoelectrics by Design: Strong Interlayer Vibration and Complete Band‐Extrema Alignment

“…However, because of the lower κ L, it shows better TE performance than KAgSe monolayer. The p‐type ZT at 700 K reaches an ultrahigh value of 2.08, which is significantly higher than those of recently reported promising nanoscale TE materials (0.025, 0.45, and 0.63 for SnSe nanosheets, single‐layer Bi 2 Se 3 , and Tellurium nanofilms) [ 36,37,39 ] and Ag‐based bulk TE materials (0.7, 0.85, and 0.9 for CsAg 5 Te 3 , Ag 8 SnSe 6 , and AgBi 3 S 5 , respectively) [ 40–44 ] (Figure 5g). More importantly, such a high ZT, only realized in bulk SnSe (2.6 ± 0.3) [ 45 ] and microscale Ge‐alloyed SnSe (≈2.1), [ 46 ] will be of important practical significance for nanoscale TE applications.…”

Silver vacancy concentration engineering leading to the ultralow lattice thermal conductivity and improved thermoelectric performance of Ag1-xInTe2

“…In this estimation, however, the average of the speed of sound ( ν m ) is used. Therefore, the κ min of AgInTe 2 (~0.11 WK −1 m −1 ) can be easily deduced from the κ min value (0.18~0.23 WK −1 m −1 ) of AgGaTe 2 7 , by using the ν m (1240 m/s) and Debye temperature Θ D (156 K) of AgInTe 2 and ν m (2402 m/s) and Θ D (192 K) of AgGaTe 2 7 . The ultralow lattice part in Ag 1- x InTe 2 is largely attributed to the enhanced phonon scattering on the point defects due to the increased concentration of the silver vacancy (V Ag ).…”

“…On the other hand, the Debye temperature ( Θ D ) and average sound velocity of AgInTe 2 are 113–202 K and ν age = 1240 ms −1 respectively 11,14,15 , which are very low compared to those of CuInTe 2 ( Θ D = 197.5 K; ν age = 3420 ms −1 ) 16 and CuGaTe 2 ( Θ D = 229.0 K; ν s = 2072.0 ms −1 , ν l = 3817.0 ms −1 ) 1 . Besides that, the average sound velocity is only about half that of AgGaTe 2 ( ν s = 1812.0 ms −1 , ν l = 2990.0 ms −1 ) 7 . It implies that the compound AgInTe 2 , naturally, has a low lattice thermal conductivity 17 , hence it is potentially a good TE candidate.…”

“…Therefore, it is quite necessary to explore novel TE candidates. Ternary I-III-VI 2 (I = Cu, Ag; III = Al, Ga, In; VI = S, Se, Te) chalcogenides, such as CuGaTe 2 1 , CuInTe 2 2 , AgInSe 2 3 and AgGaTe 2 4,5 , have recently attracted much attention in thermoelectrics because of their unique crystal and band structures 6,7 , among which the cation vacancy and crystal structure are two important factors in engineering both the carrier concentration and phonon transport 8–10 . However, to our best knowledge, AgInTe 2 has not received due recognition.…”

Co-regulation of the copper vacancy concentration and point defects leading to the enhanced thermoelectric performance of Cu3In5Te9-based chalcogenides

“…4b is the temperature-dependent electrical conductivities (s), which increase with temperature until at $730 K. Above $730 K, the materials exhibit metallic-like behavior, as was observed in AgGa 1Àx Te 2 system. 46 Besides, it is noted that the electrical conductivity at $822 K increases from 5.7 Â 10 3 U À1 m À1 (x ¼ 0, V c ¼ 0.11) to 11.4 Â 10 3 U À1 m À1 (x ¼ 0.3, V c ¼ 0.078) before it starts to fall to the 8.7 Â 10 3 U À1 m À1 (x ¼ 0.4, V c ¼ 0.067), which corresponds to the variations in carrier concentration and mobility (Fig. 3).…”