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Rare-earth topological insulator reveals its thermoelectric properties

By Eva Karatairi April 2, 2019
thermoelectric matls LaP
(a) Rocksalt (NaCl) structure of rare-earth monopnictides RX; (b) ZT for LaP as a function of temperature T and for several different carrier concentrations n. A higher ZT and thus higher efficiency can be obtained by increasing the temperature and tuning the carrier concentration. Credit: Physical Review B, Journal of Applied Physics

Harvesting waste heat from cars, aircraft, and power plants to turn into electricity could prove a valuable asset in the fight against global warming and also a good solution for preventing and controlling an energy crisis. Thermoelectric materials hold great promise for this, as they can convert heat to electric power and vice versa. Researchers are working to identify new thermoelectric materials with better efficiencies than the ones that already exist. Amongst the new candidates, lanthanum phosphide (LaP) has intrigued the interest of a team of scientists from Sichuan University, China; Chongqing Normal University, China; and the Chinese Academy of Engineering Physics in Mianyang, who dedicated their latest report in a recent issue of the Journal of Applied Physics to the thermoelectric properties of this binary compound, with the simple crystal structure of NaCl.

So far, experiments and calculations have shown that LaP is a topological insulator, like other members of the lanthanum monopnictides family (with the general formula LaX, where X is P, As, Sb or Bi). This means that the material combines an insulating bulk phase with a surface that can conduct electricity. Xiang-Rong Chen, a professor at Sichuan University and co-author of the article, says that LaP was chosen because there is no experimental or theoretical data available in the literature on its thermal transport properties, which play a crucial role in determining the efficiency and reliability of devices.

“Searching for high-performance thermoelectric materials is challenging and urgent,” says Chen; “Our [computational] study not only presents comprehensive investigation on the unexplored phonon and electrical transport properties of LaP, but also provides data on the thermoelectric performance of LaP, which would be of significance for further studies and applications of topological insulators.”

To characterize the performance of LaP, researchers used the thermoelectric figure of merit ZT, a dimensionless constant. For thermoelectric materials, the higher the figure of merit, the more stable the material, the better its performance.

ZT is defined as S2σT/κe+κl, where S is the Seebeck coefficient, σ the electrical conductivity, T the absolute working temperature, and κe and κl are the electronic thermal conductivity and the lattice thermal conductivity, respectively. To raise ZT, a big power factor (S2σ) and a low thermal conductivity κ (κe+κl) are needed.

Neither is easy to achieve. For one thing, thermoelectric materials have a high lattice thermal conductivity κl , which makes decreasing thermal conductivity κ a major challenge. Furthermore, while maximizing the power factor would apparently require an increase of the Seebeck coefficient S, the latter is unfortunately coupled to the electrical conductivity σ and they are oppositely proportional to the carrier density. As it was shown in this study, the golden ratio between the three quantities is a difficult task to achieve.  

The researchers calculated the lattice thermal conductivity of LaP at room temperature to be κl = 3.19 W/mK, which is less than the value calculated for the next member of the LaX family, lanthanum arsenide (LaAs, 5.46 W/mK). Therefore, the researchers believe that LaP may be a better thermoelectric material than LaAs. The results also suggest that κl is significantly influenced by very large phonon phase velocities relevant to phonon transport in the crystal.

The transport coefficients for the electron part—S, σ, and κe—were calculated using the Boltzmann theory. The carrier concentration strongly affected S, σ, and κe; while S increased with increasing temperature, a result consistent with experimental data. Because of the larger Seebeck coefficient, the power factor, and eventually the figure of merit ZT, also increased as temperature increased. From 300 K to 700 K, the figure of merit ZT is about 0.36 at a carrier density n = 1020 cm−3.  For thermoelectric materials, a high figure of merit (>1.5) above 600 K means stability and high performance and these values are still quite low, considering that PbTe-based thermoelectric materials have achieved ZTmax ranging up to ∼2.2 at high temperatures.

The researchers believe that thermoelectric performance may be improved by raising the temperature and tuning the carrier concentration, which is what they will explore next, according to Chen.

“The improvement in computational tools has allowed increasingly powerful and convincing predictions of thermoelectric performance, and this is a nice example highlighting the prediction of all relevant thermoelectric properties in a potentially interesting material for experimentalists to investigate further,” says Michael Gaultois, who did not participate in this study and is the Theme Lead at the Experimental Materials Design Leverhulme Research Centre for Functional Materials Design at the University of Liverpool, UK.

“It’s always interesting to see deceptively simple materials emerge in a mature field, and the simple composition and crystal structure of LaP provide a rich crystal-chemical playground for experimentalists, so I look forward to seeing the future studies that result from this report,” Gaultois says.

Read the abstract in the Journal of Applied Physics.