An Unlikely Route to Low Lattice Thermal Conductivity: Small Atoms in a Simple Layered Structure

Abstract

In the design of materials with low lattice thermal conductivity, compounds with high density, low speed of sound, and complexity at either the atomic, nano-or microstructural level are preferred. The layered compound Mg 3 Sb 2 defies these prevailing paradigms, exhibiting lattice thermal conductivity comparable to PbTe and Bi 2 Te 3 , despite its low density and simple structure. The excellent thermoelectric performance (zT ∼ 1.5) in n-type Mg 3 Sb 2 has thus far been attributed to its multi-valley conduction band, while its anomalous thermal properties have been largely overlooked. To explain the origin of the low lattice thermal conductivity of Mg 3 Sb 2 , we have used both experimental methods and ab initio phonon calculations to investigate trends in the elasticity, thermal expansion and anharmonicity of AMg 2 Pn 2 Zintl compounds with A = Mg, Ca, Yb, and Pn = Sb and Bi. Phonon calculations within the quasiharmonic approximation reveal large mode Grüneisen parameters in Mg 3 Sb 2 compared with isostructural compounds, in particular in transverse acoustic modes involving shearing of adjacent anionic layers. Measurements of the elastic moduli and sound velocity as a function of temperature using resonant ultrasound spectroscopy provide a window into the softening of the acoustic branches at high temperature, confirming their exceptionally high anharmonicity. We attribute the anomalous thermal behavior of Mg 3 Sb 2 to the diminutive size of Mg, which may be too small for the octahedrally-coordinated site, leading to weak, unstable interlayer Mg-Sb bonding. This suggests more broadly that soft shear modes resulting from undersized cations provide a potential route to achieving low lattice thermal conductivity low-density, earth-abundant materials.

Growth and transport properties of Mg3X2 (X = Sb, Bi) single crystals

“…[53] As seen in Fig. 5c and 5d [28] showing a low-lying transverse phonon mode with similar frequency and very large Grüneisen parameter at A point of the Brillouin zone. Thus, we could conclude that the existence of low-lying vibration mode in Mg3X2 system.…”

“…and CaMg2Bi2. This issue has recently been discussed by Peng et al [28] They proposed that the diminutive size of Mg 2+ leads to low-lying transverse phonon modes and large mode Grüneisen parameters, which thus contribute to the inherently low κL of Mg3X2. To check for the existence of low-lying vibration modes, [28] the low-temperature heat capacity of both Mg3Sb2 and Mg3Bi2 was further measured and is shown in Fig.…”

“…[21] The diminutive size of Mg in Mg3Sb2, leading to weak interlayer bonding, was attributed to be responsible for its low thermal conductivity. [28] More attempts have been made to further optimize the TE performance of the Mg3Sb2-based compounds, such as proper selection of the dopants on the anion site, [29] rational choice of the alloying compositions (alloyed with Mg3Bi2), [30] manipulation of the carrier scattering mechanisms [31,32] and utilization of the possible anisotropic transport characteristics (forming textures) [33].…”

Rapid One‐Step Synthesis and Compaction of High‐Performance n‐Type Mg3Sb2 Thermoelectrics

“…The intrinsic low k L in Mg 3 Sb 2 is mainly related to the large anharmonic scattering rate induced by the soft transverse acoustic phonon modes. [11,53] Comparing with the pure Mg 3 Sb 2 prepared using the same synthesis method, [41] the Te doping in Mg 3 Sb 2 slightly reduces the lattice thermal conductivity. With the proper Sc and Te co-doping, the lattice thermal conductivity at 300 K can be effectively reduced to 1.05 W m À1 K À1 in Mg 3.5 Sc 0.04 Sb 1.97 Te 0.03 , which decreases to 0.54 W m À1 K À1 at 725 K (see Figure 4 d).…”

The Limited Predictive Power of the Pauling Rules

“…a) Illustration of the radius-ratio rule (Pauling's first rule). The figure was inspired by Ref [18]…”