Elastic constants of 2H-MoS2 and 2H-NbSe2 extracted from measured dispersion curves and linear compressibilities

Recent neutron data on the dispersion curves and X-ray measurements of the linear compressibilities of the 2H polytypes of MoS2 and NbSe2 have been used to obtain approximate values of the five independent elastic constants of these materials. In the case of NbSe2 sufficient information is available to over-determine the elastic constants and the results are self consistent within estimated uncertainties, although the uncertainties are especially large for c33 and c11. Additional related considerations such as Debye temperatures and model calculations of c33, and c44 are also made. It is found that there is significant and unexplained disagreement between the value of the low temperature specific heat Debye temperature of NbSe2 and the value determined on the basis of the elastic constants, but that the model predictions of c33 and c44 are in satisfactory agreement with the values extracted from the neutron data for both MoS2, and NbSe2.