Hierarchical Chemical Bonds Contributing to the Intrinsically Low Thermal Conductivity in α‐MgAgSb Thermoelectric Materials

Abstract

Understanding the lattice dynamics and phonon transport from the perspective of chemical bonds is essential for improving and finding high‐efficiency thermoelectric materials and for many applications. Here, the coexistence of global and local weak chemical bonds is elucidated as the origin of the intrinsically low lattice thermal conductivity of non‐caged structure Nowotny–Juza compound, α‐MgAgSb, which is identified as a new type of promising thermoelectric material in the temperature range of 300–550 K. The global weak bonds of the compound lead to a low sound velocity. The unique three‐centered MgAgSb bonds in α‐MgAgSb vibrate locally and induce low‐frequency optical phonons, resulting in “rattling‐like” thermal damping to further reduce the lattice thermal conductivity. The hierarchical chemical bonds originate from the low valence electron count of α‐MgAgSb, with the feature shared by Nowotny–Juza compounds. Low lattice thermal conductivities are therefore highly possible in this series of compounds, which is verified by phonon and bulk modulus calculations on some of the compositions.

Unique Role of Refractory Ta Alloying in Enhancing the Figure of Merit of NbFeSb Thermoelectric Materials

“…Strategies for increasing zT have focused on the reduction of the lattice thermal conductivity by hierarchical microstructure, [3,4] nanostructuring [5,6] point defects, [7,8] and the enhancement of the power factor (PF = α 2 σ) by optimal doping and band engineering [9][10][11] as well as the employment of complex crystal structures that possess intrinsically low lattice thermal conductivity. [12][13][14][15][16] Half-Heusler (HH) alloys, with a valence electron count of 8 or 18, have been extensively studied as potential hightemperature TE materials due to their excellent electrical properties, mechanical properties, and high temperature stability. [17][18][19] MNiSn-and MCoSb-based (M = Ti, Zr, Hf) HH alloys are the first two most studied families as n-type materials and p-type materials, respectively.…”

“…More recently, hierarchical phonon scattering was also demonstrated to be an effective way to improve the TE properties of the NbFeSb heavy band system with relatively low mobility. [40] Despite this progress, the lattice thermal conductivity of NbFeSb-based alloys still remains relatively high (>2 W m −1 K −1 ) compared with other new TE materials (<1 W m −1 K −1 ) such as Cu 2 (S, Te), [41] SnSe crystals, [42] α-MgAgSb, [13] filled skutterudites, [43] and so on.…”

Improved thermoelectric performance of solid solution Cu4Sn7.5S16 through isoelectronic substitution of Se for S

“…The total uncertainty for ZT value was ∼18%. In addition, the heat capacities ( C p ) for the Se-incorporated sample Cu 4 Sn 7.5 S 16− x Se x ( x  = 1.0) were measured with the measurement system (PPMS, Quantum Design) in the temperature range of 2.0–153 K, and the Debye temperature (Θ D ) was then determined using the equation 19 :Here, β was obtained from a simple Debye model 13,20 in the low temperatures below 10 K,and n is the number of atoms per chemical formula. The lattice contributions ( κ L ) were obtained by subtracting the electronic contribution ( κ e ) from the total κ , i.e., κ L  =  κ  −  κ e .…”

“…This is because Grüneisen parameter increases as an anharmonicity of the lattice vibrational spectrum increases 13–16 . Although such investigations with respect to the hierarchical chemical bonds contributing to the low thermal conductivity are so far limited and are mostly focused on the cubic I-V-VI 2 semiconductors 15 and Nowotny–Juza compounds like α-MgAgSb with a non-caged structure 13 , we believe that this strategy is important for engineering the phonon transport of Cu-Sn-S compounds.…”

“…This is because Grüneisen parameter increases as an anharmonicity of the lattice vibrational spectrum increases 13–16 . Although such investigations with respect to the hierarchical chemical bonds contributing to the low thermal conductivity are so far limited and are mostly focused on the cubic I-V-VI 2 semiconductors 15 and Nowotny–Juza compounds like α-MgAgSb with a non-caged structure 13 , we believe that this strategy is important for engineering the phonon transport of Cu-Sn-S compounds. In this regard, we specially substitute Se for S in the compound Cu 4 Sn 7.5 S 16 , aiming to engineer the band structure, phonon transport, and introduce lattice disorder, similar to the cases of Cu substitution for Cd 12 in Cu 2 CdSnSe 4 and Se for Te in n-type PbTe based solid solutions 17,18 .…”