Lone pairs in Group 14 ceramics

The electronic structure of SnX (X=O, S, Se, Te)

A good example of the anion dependence of the lone pair activity is SnX (X=O, S, Se, Te). SnO has the litharge structure with strongly distorted Sn sites, while SnS and SnSe show less distortion and SnTe is perfectly symmetric (rock salt). DFT calculations show that as you proceed down the series the anion states become higher in energy reducing their interaction with the low energy Sn 5s states. This reduces the Sn 5s states at the top of the conduction band generated in the anti-bonding combination with the anion states. The stronger the contribution of the Sn 5s states to the top of the conduction band the stronger the lone pair activity - as it is these antibonding states that interaction with the Sn 5p states causing the asymmetry in the structure. Hence the lone pair activity reduces as you proceed down group 16.

related references:

84. Walsh A. and Watson G.W.
    Influence of the anion on lone pair formation in Sn(II) monochalcogenides: A DFT study.
    Journal of Physical Chemistry B 109, 18868-18875 (2005).
    


74. Walsh A.J. and Watson G.W.
    Electronic structures of rock salt, litharge and herzenbergite SnO by density functional theory
    Physical Review B, 70, 235114 (2004).
    


45. Watson G.W.
    'The structure and electronic structure of SnO'
    Journal of Chemical Physics 114, 758-763 (2001).
    

The electronic structure of PbO and PbS.

DFT calculation of PbO (litharge) and PbS (rocksalt) show that the distorted structure of PbO is not caused directly by the Pb 6s states in a non bonding lone pair. The primary location of the Pb 6s states is 7 eV below the VBM and the resulting electron density from these states show that it is not responsible for the asymmetry. The asymmetry comes from states at the top of the VB which are formed by the anti-bonding combination of the Pb 6s and O 2p states mixing with the Pb 6p with the resulting electron density giving rise the strong asymmetry. It is only through this indirect coupling the Pb 6s and Pb6p through their interaction with the O 2p that the asymmetry can form.

For PbS the anion state are at a higher energy and hence the coupling to the low energy Pb 6s states is much weaker. This leads to reduced Pb 6s states at the top of the VB and hence their is less stabilisation by mixing in the Pb 6p states and less asymmetry. Since to form the asymmetric structure the coordination of the cation must reduce the loss of bonding can no be made up for in the case of this weak stabilisation and hence PbS adopts the symmetric rock salt structure.

The revised theory of lone pairs predicted on the basis of DFT calculations has been supported by experimental XPS data.

related references:

89. Payne D.J., Egdell R.G., Walsh A., Watson G.W., Guo J., Glans P.A., Learmonth T., Smith K.E.
    Electronic origins of structural distortions in post-transition metal oxides: Experimental and theoretical evidence for a revision of the lone pair model
    Physical Review Letters 96, 157403 (2006)
    


77. Walsh A. and Watson G.W.
    The origin of the stereochemically active Pb(II) lone pair: DFT calculations on PbO and PbS
    Journal of Solid State Chemistry 178, 1422-1428 (2005)
    


69. Walsh A. and Watson G.W.
    The formation and absence of asymmetric electron densities in PbO and PbS: the end of the Pb 6s2 lone pair myth
    Proceedings of 3rd International Conference Computational Modelling and Simulation of Materials, Part A, 157-164 (2004)
    


30. Watson G.W. and Parker S.C.
    'The origin of the lone pair of a -PbO from first principles calculations.'
    Journal of Physical Chemistry B 103, 1258-1262 (1999).
    


27. Watson G.W., Parker S.C., and Kresse G.
    'Ab initio calculation of the origin of the distortion of a-PbO.'
    Physical Review B 59, 8481-8486 (1999)