Large Thermal Conductivity Drops in the Diamondoid Lattice of CuFeS₂ by Discordant Atom Doping

Abstract

Doping in a lattice refers to the introduction of very small quantities of foreign atoms and has a generally small effect on decreasing the lattice thermal conductivity, unlike alloying which involves large fractions of other elements and strongly enhances point defect phonon scattering. Here, we report that, by alloying only 3% of In on the Cu sites of the diamond-like lattice of CuFeS2 chalcopyrite compound (Cu1–x In x FeS2, x = 0.03) has a disproportionally large effect in reducing the lattice thermal conductivity of the compound from 2.32 to 1.36 Wm–1K–1 at 630 K. We find that In is not fully ionized to +3 when on the Cu sublattice and exists mainly in the +1 oxidation state. The 5s2 lone pair of electrons of In+ makes this atom incompatible (referred to as discordant) with the tetrahedral geometry of the crystallographic site. This causes strong local bond distortions thereby softening the In–S and Cu–S chemical bonds and introducing localized low frequency vibrations. The latter couple with the base phonon frequencies of the CuFeS2 matrix enhancing the anharmonicity and decreasing the phonon velocity, and consequently the lattice thermal conductivity. The control material in which the In doping is on the Fe3+ site of the structure at the same doping level (and found in the site-compatible In3+ state), has a far smaller effect on the phonon scattering.

Slowing down the heat in thermoelectrics

“…With an increase in temperature, the highly delocalized Cu ions lead to a liquid‐like behavior in Cu 2 Se, which significantly reduces the contribution of transverse acoustic (TA) phonons to the heat capacity and produces a lower C V of ~2 Nk B 124 . Based on this discovery, several liquid‐like thermoelectrics have been continuously developed 127–135 …”

A scalable synthesis route for multiscale defect engineering in the sustainable thermoelectric quaternary sulfide Cu26V2Sn6S32

“…[3] Fortunately, a second wind was given to the research on earth-abundant sulfides following the reports of outstanding properties in a variety of ternary and quaternary n-and ptype copper sulfides, paving a new way for cheap, light and non-toxic thermoelectric materials. Members of this particularly promising group of materials, often derivatives of natural minerals, include but are not limited to: bornite Cu 5 FeS 4 , [9][10][11] germanite derivative Cu 22 Fe 8 Ge 4 S 32 , [12,13] stannoidite Cu 8.5 Fe 2.5 Sn 2 S 12 , [14] colusites Cu 26 T 2 M 6 S 32 (T = V, Nb, Ta, Cr, Mo, W; M = Sn, Ge), [15][16][17][18][19][20][21][22][23][24] Cu 2 SnS 3 , [25] kesterite Cu 2 ZnSnS 4 , [26,27] Cu 4 Sn 7 S 16 , [28] CuFeS 2 [29] and tetrahedrites Cu 12-x T x Sb 4 S 13 (T = Mn, Fe, Ni, Zn). [30][31][32][33][34][35][36] Some of these materials exhibit bulk performances close to those of the commercially available bismuth telluride derivatives with a figure of merit (ZT = S 2 T/ρκ where T is the absolute temperature, S the Seebeck coefficient, ρ the electrical resistivity, and κ the thermal conductivity) around unity at intermediate temperatures.…”

Approximate Models for the Lattice Thermal Conductivity of Alloy Thermoelectrics

“…𝜅 latt is independent of 𝑛, and most high-performance TEs are materials with large intrinsic phonon anharmonicity and naturally low thermal conductivity. As for the electrical properties, the 𝜅 latt can also be tuned to some extent by alloying, [9][10][11] chemical doping 12 and hierarchical nanostructuring. 13 Commercial TEGs are currently based on doped Bi2Te3, which shows a 𝑍𝑇 of ~1 at room temperature corresponding to a 2-3 % heat recovery.…”

“…12 and 1.184 x 10 -12 eV 2 respectively (see Supporting Information). These are 16 × larger than the values obtained by analysing the eight-atom primitive cells as they are computed over (3 ) 216 instead of 576 pairwise interactions.…”