Ab initio study of incommensurately modulated crystals

We present an ab initio study of the average structure of some incommensurately (IC) modulated materials: K2SeO4, Mo2S3, AuTe2 (calaverite) and Pb2MgTeO6 (elpasolite structure). All the calculations are done using the local density approximation (LDA) and/or the general gradient approximation (GGA) of the density functional theory (DFT). The electronic band structures, the corresponding density of states and the valence electron density distributions are determined for all the materials. Three-dimensional representations of the valence electron density distributions help to identify the important structural groups present in the structures and the chemical bonds between the atoms. Partial valence electron density maps are generated in order to link the electronic bands to different atomic orbitals or to linear combinations of atomic orbitals. K2SeO4 and Pb2MgTeO6 are insulators. For both materials the electron band structure is made of weakly dispersive electronic bands. Mo2S3 and AuTe2 present a metallic character. Consequently for these two materials the Fermi surface has been also analyzed in order to find the instabilities and possible nesting of the Fermi surface. Results of the ab initio determination of the average structure are also reported. The accuracy of these results, compared to the experimental data, is very material-dependent, the final results varying within 1% and 10% of the experimental data. For AuTe2 the LDA gave 6–8% differences, while the GGA improved considerably the quality of the results, reducing the differences of up to 3–4%. For Mo2S3, LDA is within 2% of the experiment, while for the other materials the agreement lies between the one for AuTe2 and the one for Mo2S3.