Chemistry Reference
In-Depth Information
A number of closely related fluorite-like phases of PbSnF
4
with orthorhombic,
tetragonal and monoclinic structures were synthesized by different methods. The
tetragonal ordered
-PbSnF
4
phase (P4/nmm) is stable at ambient temperatures and
couldbepreparedbysolid-statesynthesis of constituent binary fluorides [6]. The
structure of
-PbSnF
4
remains tetragonal at high temperatures [28]. The phase
transitions
!
ß
!
occur at 608 and 672
J
, respectively; its melting point is
693 K [29].
The lattice parameters of
-PbSnF
4
are related to those of the parent fluorite structure by
a
¼
a
fl
/
p
2
,c
¼
2a
fl
. The stabilization of the layered structure is generally attributed to
5s
2
electron lone-pairs of Sn
2
þ
cations. They point to the interlayer space between the two
Sn
2
þ
sheets, creating cation-centred pseudo-octahedral SnF
5
E units (where E represents
the lone pair) [29].
F(1)
Sn
F(2)
F(4)
Pb
F(3)
Figure 14.10 Structure of
a
-PbSnF
4
showing the cation layers and the four fluoride site;
nominal vacant F(1) site; the mobile F(2) and F(4) sites between Sn and Pb layers; and static
F(3) sites between two Pb layers. (Reprinted with permission from [30] Copyright (2008)
American Chemical Society.)
It was shown by powder neutron diffraction,
19
F field-cycling NMR and MD simulation
that the static F(3) sites are located between two Pb layers in
-PbSnF
4
and the mobile F(2)
and F(4) sites are between Sn and Pb layers [6,29,30].
The time-averaged anion density and two-dimensional diffusion within
-PbSnF
4
have
recently been successfully reproduced within MD simulations, which essentially treat the
'lone-pairs' as an extreme manifestation of cation polarizability. As a consequence, the
structural behaviour of
-PbSnF
4
can be interpreted in terms of a higher 'effective'
polarizability of the smaller Sn
2
þ
ion [6]. The authors of [30] recently proposed a detailed
mechanism for conductivity of this compound.
