The crystal structures of Pb1/3TaS2 and Sn1/3NbS2 were determined using single-crystal X-ray diffraction. The space group is P6(3)22 and the unit cell dimensions are: a = 5.759(1), c = 14.813(1)Angstrom and a = 5.778(1), c = 14.394(1)Angstrom, for the Pb and Sn compounds, respectively. The post-transition metal atoms occupy one-third of the trigonal-antiprismatic holes between sandwiches NbS2 and TaS2. The Nb and Ta atoms are in trigonal-prismatic coordination by sulfur atoms. The stacking of sandwiches is the same as in 2H disulfides. The arrangement of the post-transition metal atoms is different for the two compounds. A bond valence calculation showed Sn and Pb to be divalent.
Ab initio band structure calculations were performed for Sn1/3NbS2 using the localized spherical wave method, and for Pb1/3TaS2 with the augmented spherical wave method with spin-orbital interactions included. The calculations show that the rigid band model is approximately valid for the electronic structures; the main difference with those of 2H-NbS2 and 2H-TaS2 being the presence of Sn 5s and 5p (Pb 6s and 6p) bands and a larger S 3p/Nb(Ta) 4d (5d) gap in the intercalates (1.0 eV for Sn1/3NbS2, 1.3 eV for Pb1/3TaS2). The Sn 5s (ph 6s) bands are at the bottom (bonding) and top (antibonding) of the valence bands which range from about -7 to about 0 eV. The conduction bands are composed of Nb 4d(z)2 or Ta 5d(z)2 orbitals hybridized with S 3p. These bands are filled to about 0.3 holes per Nb (Ta), corresponding to a donation of two electrons per Sn (Pb).
- BOND-VALENCE PARAMETERS