Lone-pair cations

Lone pair cations exhibit external pairs of electrons which do not participate in the bonds but can influence dramatically the geometry of the structures (52). This is the case of cations like Bi(III), Pb(II) or T1(I) whose 6s2 lone pairs have been shown to present an important stereochemical activity. Such cations which can be found in the rock salt type layers are capable of influencing the oxygen framework and may consequently affect the superconducting properties of the layered cuprates. 

Table 8.1 Typical geometries of selected lone-pair cations in their most distorted form...

The ligands of a lone-pair cation lie on the surface of a sphere. When TU is surrounded by weakly bonding oxyanions, it lies at the centre of the sphere, forming nine bonds of 297 pm each (O.llvu, Fig. 8.2(a)). When it bonds to strongly bonding anions, as in TI3BO3, TU moves about 70 pm away from its centre to form three primary bonds of 266 pm (0.33 vu), and six secondary bonds of 324-372 pm (Fig. 8.2(b)). In the process the radius of the coordination sphere increases from 297 to 322 pm in accordance with the distortion theorem (Rule 3.6). 

The discovery of the tin(II) phosphate family led to the search for other systems containing lone pair cations, such as antimony(III). This indeed has turned out to be feasible, and an extensive range of antimony(III) phosphates and fluorophos-phates has been discovered [65]. As with the tin compounds, the structures appear to be open but the presence of lone pairs protruding into the cavities reduces their porosity. 

The B-site displacement idea is too simplistic to explain the behaviour of a majority of ferroelectrics, and other features of the perovskite structure must be taken into account in explaining the formation of arrays of permanent switchable dipoles. Among these additional aspects are dipoles due to A-cation displacements, especially for lone pair cations such as Pb or Bi, octahedral tilting and irregularities in the positions of the oxygen ions that make up the BOg polyhedra. 

The only compound truly isomorphous with red PbO is the common modification of SnO, crystalline GeO being unknown. X-ray studies of the structure of SnO at room temperature and pressure up to lOOkbar were reported by Vereshchagin et al [247]. A linear decrease of the lattice constants as well as of the axial ratio c/a was found up to 40 kbar where a first-order phase transition accompanied by a 7% volume decrease was observed. The high-pressure phase has not, as one might perhaps expect, the orthorhombic PbO structure (which appears to be about 2% denser than the tetragonal PbO structure) but is claimed to adopt the hexagonal wurtzite structure. A tetrahedral coordination for a lone pair cation, however, appears to be rather peculiar. The assumption of a second high-pressure phase with a rocksalt structure, on the other hand, seems to be quite plausible. 

The values added in brackets refer to an ideal cubic close-packing of Sn atoms. The reduction in symmetry is paraded by a reduction of the number of bonded Sn—O contacts and an enhancement of the non-metallic character. The two-dimensional structure thereby transforms into a molecular structure with ring-shaped Sn202 molecules. This transition is quite analogous to the sequences from three-dimensional to molecular structures observed in compounds with other lone-pair cations such as Bi —> P and Te —> S. The four equivalent Sn—O distances of 2.21 A in tetragonal SnO go over into two sets of pairs in the orthorhombic modification, namely... 

If we neglect the H atom in LiOH then its structure corresponds to the anti-PbO type. Since the axial ratio and the free positional parameter of this tetragonal structure can vary in a certain range, different isopuntal structure [9] types are possible. Thus for c/a = V2 and z(anion) = j, a cubic close-packing of the anions results with the cations in tetrahedral holes. An axial ratio c/a = 1/V2 and an anion parameter z =, on the other hand, correspond to the CsCl type with coordination number 8. As follows from Table 56, LiOH approximates a cubic close-packing with Li in deformed tetrahedral coordination. The position of the lone electron pair of PbO is here taken by H (corrected O—H distance 0.98 A [325] similar to the lone pair-cation distance). The electron density corresponds to Li 0 ° H and one electron smeared between the layers [1012]. In Table 55, LiOH is compared with chemically related compounds. Lithium amide has a closely related structure in which the layers of tetrahedral cation sites are alternately I and i occupied (5T1 + IT2 and ti, respectively) instead of the completely occupied and completely empty layers of LiOH. This is obviously a consequence of the weaker dipole character of NHJ. LiF, with no dipole moment, crystallizes in the rocksalt structure. The structure of LiSH is similar to chalcopyrite whereas that of the hydrosulfides and hydroselenides of Na, K and Rb is a rhombohedrally deformed rocksalt type. 

Figure 1.3 ORTEP (50% probability ellipsoids) diagram for lone-pair cation...

Figure 1.15 Hypothetical polarisation reversibility in an MO3E (M = lone-pair cation E = lone-pair) polyhedron...

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