Ab initio study of Nb2SC and Nb2S2C:: Differences in coupling between the S and Nb-C layers

Using ab initio calculations, we have studied the structurally related compounds Nb2SC and Nb2S2C. In Nb2S2C (space group R (3) over barm, prototype Bi2Te3), S atoms are nearest neighbours, while in Nb2SC (space group P6(3)IMMC, prototype Cr2AlC) this is not the case. The calculated equilibrium volume for these two phases deviates by 1.6-3.7% to previously-published experimental data and the bulk modulus-to-c(44) ratios obtained are 1.5 and 5.9, respectively. These results indicate a resemblance of Nb2S2C to hexagonal BN and graphite. Furthermore, we have demonstrated that the uniform compression method is adequate for estimating the elastic properties of Nb2SC, a so-called MAX phase. It is our ambition that these calculations will stimulate further experimental research on these compounds. (c) 2005 Elsevier Ltd. All rights reserved.