Materials with low thermal conductivity, which can minimize heat dissipation, play a central role in many practical applications, such as thermoelectric devices and thermal barrier coatings. We report the synthesis and low thermal conductivities of narrow band gap semiconductors NaSnAs and NaSnP, in which the measured minimum lattice thermal conductivities are 0.62 and 0.58 W/m/K. The values are close to the calculated minimum of 0.46 and 0.40 W/m/K, respectively, and obviously lower than that for their closely related compound $\mathrm{NaS}{\mathrm{n}}_2\mathrm{A}{\mathrm{s}}_2$, which has fewer lone-pair electrons. Such low thermal conductivities in NaSnAs and NaSnP are due to double lone $s^2$ electron pairs that are present in both Sn sites and As or P sites. Double lone-pair electrons can induce greater Grüneisen parameters $\gamma$ and lower thermal conductivity further than that in $\mathrm{NaS}{\mathrm{n}}_2\mathrm{A}{\mathrm{s}}_2$. Our findings provide hints for exploring new materials with low thermal conductivity.

• Received 19 November 2016
• Revised 19 February 2017

DOI:https://doi.org/10.1103/PhysRevB.95.165201