Thermoelectric Properties of Pnma and Rocksalt SnS and SnSe

Round 1

Reviewer 1 Report

REFEREE’S REPORT: “Thermoelectric Properties of Pnma and Rocksalt SnS and SnSe” J. M. Flitcroft , I. Pallikara , J. M. Skelton.

The peer-reviewed manuscript reports a combination of theoretical predictions and calculations with experimental results of thermoelectric properties for various modifications of SnS (Se) phases. The authors raise and discuss a very important task of the thermoelectric community - the theoretical calculation of electrical and transport properties of thermoelectric materials and their relationship with structural modifications of potential thermoelectric materials for future use. This report contains valuable theoretical calculations that can be recommended for publication in Solids in its current form.

Remarks:

• It would be good if the authors connected the directions (xx, yy and zz) with the crystallographic axes and presented it in Fig. 1. Since the first discussion of anisotropic properties is shown in Fig. 5, but the explanation is presented in Fig. 8.
• Can the authors add to the discussion a specific idea (substituted elements, dopping, etc) of how to stabilize the salt-rock structure, more promising in terms of thermoelectricity?
• page 2, line 55 - "to a more symmetric Cmcm phase" it would be good to change to "to a higher symmetric Cmcm phase".

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

This is a good piece of theoretical work dealing with the thermoelectric properties of orthorhombic and cubic forms of SnS and SnSe, which complements earlier works by the same authors on the same topic.  The study has been carried out competently and deserves publication in Solids.  I have only relatively minor suggestions to make before publication proceeds.

1)  The paper is too long.  Some shortening in some parts should be made without, I guess, losing the take-home message of the paper.  Some data could be transferred in SI and some comments could be shortened.

2)  Structure and Lattice Dynamics.  

1. a) The optimised orthorhombic structures of SnS and SnSe are slightly smaller than the experimental ones. I agree with the authors that this was expected. However, the contraction is anisotropic with a rather large mismatch for the c parameter.  Is there any reason for that?
2. b) The equilibrium structure of RS SnS shows an imaginary mode at q = X, which becomes real under compression. However, weak imaginary modes are still observed for compressed SnS structure close to Gamma point. This indicates that it is also unstable thermodynamically. The authors should comment on that.

3) Electronic Structure and Transport Properties.  I agree with the authors that the protocol they used gives reasonable predictions of the electronic transport properties in these systems.  They mention that their model does not consider the effect of temperature on the structure through, e.g., thermal expansion at finite temperature.  One way to estimate this effect could be to perform calculations on the experimental structure (even if we know that the experimental structure is not an energy minimum on the DFT-potential energy curve).

4) Minor points.

1. a) 2 (bottom). ‘PbTe’ instead of ‘PnTe’
2. b) Electronic Structure and Transport Properties. The ‘technical’ part concerning Boltzmann transport theory should be transferred in Section 2.  This would facilitate the reading for those who are interested only in the results.
3. c) Lattice Thermal Conductivity. Same remark for the equations about the computation of Kappa (lattice).

Author Response

Please see the attachment.

Author Response File: Author Response.pdf