Effects of isovalent doping on the thermoelectric properties of environmentally-friendly phosphide Ag₆Ge₁₀P₁₂

Abstract

The electrical and thermal transport properties of the isovalent Cu- or Sn-doped ternary phosphide Ag6Ge10P12 have been investigated. Polycrystalline ingots of Cu- and Sn-doped Ag6Ge10P12 were grown by the melting method. In Ag6−xCuxGe10P12 (x ≤ 0.3) and Ag6Ge10−ySnyP12 (y ≤ 0.25), the lattice thermal conductivity accounted for ∼95% of the total thermal conductivity. The lattice thermal conductivity of Cu- and Sn-doped Ag6Ge10P12 decreased with increasing doping concentration. Because of the decrease of the lattice thermal conductivity, the figure of merit ZT value of Ag5.85Cu0.15Ge10P12 reached ∼0.26, which was ∼60% higher than that of non-doped Ag6Ge10P12. Although the lattice thermal conductivity of Sn-doped Ag6Ge10P12 decreased, the electrical resistivity of Sn-doped Ag6Ge10P12 increased, resulting in the ZT value of Sn-doped Ag6Ge10P12 monotonically decreasing. Therefore, Cu doping of Ag6Ge10P12 is the effective way to enhance the thermoelectric performance.

Band Engineering and Synergistic Modulation Doping for Excellent Thermoelectric Performance in Composites Ti1–xNbxCoSb–Nb0.8+δCoSb

“…This is reasonable since the band structure modification under chemical doping depends not only on the steric effect due to ionic size mismatch but also on the electronegativity and ionization energy of the dopants. Typically, doping with an atom with substantially different electronegativity and ionization energy as compared to the host atom can result in a significant band modification without any drastic change in the lattice constant . The case of Nb doping is specifically interesting, as Nb is the most electronegative of all the dopants considered here.…”

Recent progress in phosphide materials for thermoelectric conversion

“…48 Ag 6 Ge 10 P 12 is a p-type conductor, with semiconducting ρ(T) and large S 300 K ~ 250 μV.K -1 (over 350 μV.K -1 in single crystals 48 ). 48,58,88 Hole concentrations are sensitive to non-stoichiometry, controlled by nominal excess of P during synthesis. In one study 3% excess yielded the best electronic properties with largest S 2 σ ~ 1.1 mW.m -1 .K -2 above 500 K. 58 Higher P excess leads to increased impurity phases, increased κ L and degraded thermoelectric performance.…”

“…Isovalent alloying with Cu and Sn in Ag 6-x Cu x Ge 10−y Sn y P 12 has been found to lead to a reduction in k L , but the level of Cu substitution is limited due to deterioration of the electronic performance. 88 Hole doping using Ga in Ag 6 Ge 10−x Ga x P 12 is effective and can control the hole concentration, with some additional suppression of k L . 67 The m * DoS is rather large for Ag 6 Ge 10−x Ga x P 12 , between 8 and 16m e depending on the Fermi level, due to the combination of a low dispersion band in the G-F direction and the contribution of a lower energy valence band in the G-H direction.…”

“…In single crystal Ag 6 Ge 10 P 12 , k L ∼ 1.75 W m −1 K −1 is observed at 300 K, which is reduced to 1.3 W m −1 K −1 for polycrystalline samples, with lowest observed values ∼0.8 W m −1 K −1 in Ag 5.7 Cu 0.3 Ge 10 P 12 . 48,67,88 One explanation for the low k L is the weak interaction between the covalent Ge-P framework and the enclosed [Ag 6 Ge 4 ] clusters, in which large thermal displacement parameters give Ag + a liquid-like behaviour. 48 Alternatively, the [Ag 6 ] 4+ cluster has been considered to act as a low energy "rattler", leading to suppression of acoustic phonon modes and low sound velocities.…”

“…Isovalent alloying with Cu and Sn in Ag 6- x Cu x Ge 10− y Sn y P 12 has been found to lead to a reduction in κ L , but the level of Cu substitution is limited due to deterioration of the electronic performance. 88 Hole doping using Ga in Ag 6 Ge 10− x Ga x P 12 is effective and can control the hole concentration, with some additional suppression of κ L . 67…”