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ARISING FROM L.-D. Zhao et al. Nature 508, 373-377 (2014); doi:10.1038/nature13184
Several groups have been unable to reproduce the record high thermoelectric figure of merit ZT of SnSe reported in ref. 1. Zhao et al.1 measured an ultralow thermal conductivity (< 0.4 W m-1 K-1 at 923 K) and consequently record high values of ZT [asymptotically =] 2.6 ± 0.3 and ZT [asymptotically =] 2.3 ± 0.3 at 923 K along the b and c directions, respectively, in their singlecrystalline SnSe. However, after careful analysis of the data of ref. 1, we deduce that their samples are not fully dense and thus not truly single crystalline, implying that their reported thermal conductivities are not intrinsic to SnSe. This warrants further investigation into intrinsic thermal transport in SnSe single crystals and its use as a thermoelectric material. There is a Reply to this Comment by Zhao, L.-D. et al. Nature 539, http://dx.doi.org/10.1038/nature19833 (2016).
In ref. 1, the total thermal conductivity [kappa] of single-crystalline SnSe was calculated using the relation [kappa] = DCp[rho], where D is the thermal diffusivity, Cp is the specific heat capacity at constant pressure, and [rho] is the density. Although the authors did not list the density of their SnSe crystals, they did provide the diffusivity, specific heat and total thermal conductivity data along the three major crystallographic directions (source data for figure 2d, and extended data figure 6a and b, of ref. 1). We extracted the [rho] value for their crystals using the above relation, and found their single-crystalline SnSe samples to be of much lower density (around 5.43 g cm-3) when compared to the theoretical density [rho]th reported in the literature (Table 1). In their Reply to this Brief Communication Arising, Zhao et al. confirmed that the experimentally measured density of their samples was indeed lower than [rho]th. The [rho] value of single-crystalline SnSe estimated from the powder X-ray diffraction studies were in the range 6.13-6.18 g cm-3 (Joint Committee on Powder Diffraction Standards (JCPDS) card numbers 01-089-232, 01-089-233 and 01-089-235)2, while neutron diffraction studies3 reported a [rho] value of about 6.18 g cm-3 (JCPDS card number 01-071-3877). In addition, electron diffraction studies reported a [rho] value of 6.07 g cm-3 (JCPDS card number 01-075-2123)4. In other words, the [rho] value...