Ab initio study of incommensurately modulated crystals

We present an ab initio study of the average structure of some incommensura tely (IC) modulated materials: K2SeO4, Mo2S3, AuTe2 (calaverite) and Pb2MgT eO6 (elpasolite structure). All the calculations are done using the local d ensity 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 distribut ions are determined for all the materials. Three-dimensional representation s of the valence electron density distributions help to identify the import ant structural groups present in the structures and the chemical bonds betw een the atoms. Partial valence electron density maps are generated in order to link the electronic bands to different atomic orbitals or to linear com binations of atomic orbitals. K2SeO4 and Pb2MgTeO6 are insulators. For both materials the electron band structure is made of weakly dispersive electro nic bands, Mo2S3 and AuTe2 present a metallic character. Consequently for t hese two materials the Fermi surface has been also analyzed in order to fin d the instabilities and possible nesting of the Fermi surface. Results of t he ab initio determination of the average structure are also reported. The accuracy of these results, compared to the experimental data. is very mater ial-dependent, the final results varying within 1% and 10% of the experimen tal data. For AuTe2 the LDA gave 6-8% differences, while the GGA improved c onsiderably 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 mat erials the agreement lies between the one for AuTe2 and the one for Mo2S3. (C) 2001 Elsevier Science B.V. All rights reserved.