Effect of the Amount of Vacancies on the Thermoelectric Properties of Cu–Ga–Te Ternary Compounds

Abstract

The CuGaTe ternary compounds: Cu 3 Ga 5 Te 9 , Cu 2 Ga 4 Te 7 , CuGa 3 Te 5 , CuGa 5 Te 8 , and CuGaTe 2 have zinc-blende or chalcopyrite structure. The compounds except for CuGaTe 2 contain vacancies in the cation site due to the valence mismatch between cation and anion, and the vacancy concentration is different between these compounds. Here we investigated the effect of the amount of vacancies on the thermoelectric (TE) properties of the CuGaTe ternary compounds. At room temperature, the presence of vacancies reduced the Hall mobility (® H ) and the lattice thermal conductivity (¬ lat ), showing that the vacancies scattered both carriers and phonons. It was found that the decreasing rate of the ® H was larger than that of the ¬ lat . Therefore, the presence of vacancies degraded the TE performance of the CuGaTe ternary compounds. In other words, CuGaTe 2 without vacancies would show the best TE figure of merit. The room temperature ZT values for Cu 3 Ga 5 Te 9 , Cu 2 Ga 4 Te 7 , CuGa 3 Te 5 , CuGa 5 Te 8 , and CuGaTe 2 were 2.0 © 10 ¹2

Enhanced thermoelectric performance via the solid solution formation: The case of pseudobinary alloy (Cu2Te)(Ga2Te3)3 upon Sb substitution for Cu

“…At x=0.2, trace impurity phase Sb2Te3 (PDF: )is identified, indicating that the solubility of Sb in CGT is less than 0.2. . 16 It is interesting that both the a and c values show a decreasing tendency with Sb content increasing in the range of x≤0.05, and then remain relatively constant. Such a non-linear relation seems that the element Sb is not completely incorporated into the CTGT crystal lattice.…”

“…It is observed that the sample at x=0.05 gives the highest ZT value (0.58 at 723 K), which is about two times that of the intrinsic CTGT, and 0.35 higher than the reported value 0.23. 16 This result indicates that the solid solution formation via the mixed Sb occupations is an effective approach to optimize the TE performance. The abnormal variation of the Hall carrier concentration with Sb content might be related to the structural modification.…”

“…In order to identify the beneficial effect on the transport properties, we have measured the Hall coefficient at RT and 390 K. The calculated Hall carrier concentration (nH) and mobility (μ) are displayed in Fig.3, where the nH value from Plirdpring at RT for the CTGT is presented for comparison. 16 The nH values at x=0, which are nH = 2. The TE properties as a function of temperature are presented in Fig.4.…”

“…While at y=0.5, none of the cation sites are structural vacancies, the nH value at RT is only 2.1×10 24 m -3 .In addition, the κ value of the former (κ≈1.1 Wm -1 K -1 ) is lower than that of the latter (κ≈2.2 Wm -1 K -1 ) at 750 K. 19 Hence, the former should have a more prospective to be a potential TE candidate than the latter. However, the ZT value of CTGT is still low (ZT≈0.22 @ 735 K), 16 due to low electrical conductivity and relatively high thermal conductivity of lattice contribution (κL≈1.0 WK -1 m -1 @ 700K). 17 In order to reduce the κL, it is necessary to introduce extra point defects via the solid solution formation..…”

“…However, the total cations (Cu + +Ga 3+ ) in this compound is 2, less than anions Te 2-(2y+1) at 0.5≤y<1.0, hence (2y-1)/(2y+1) of cation sites are structural vacancies. 16 The structural vacancy is essential to the carrier concentration and lattice thermal conductivity. For example, at y=0.75, i.e.…”