Chemistry Reference
In-Depth Information
This study revealed, once again, that despite the extremely high quality of powder
data now obtainable at high pressures using angle-dispersive techniques at synchro-
tron sources, the unavoidable problems of peak overlap remains, and can be such
that structural identification can remain ambiguous. The study also revealed, how-
ever, that recrystallisation can occur at phase transitions, even in hard metals, some
1,500 K below their melting temperatures, suggesting that single-crystal studies may
be possible in other systems hitherto thought to be inaccessible to such techniques.
4.3 Single-Crystal Studies of Na Above 100 GPa
The group I metal sodium crystallises in the bcc structure at ambient conditions, and
has a simple electronic structure that is well explained by the nearly-free electron
model. Early predictions that sodium would undergo a metal-insulator transition
at pressure close to 100 GPa [
247
] were followed by powder diffraction studies to
above 100 GPa, which revealed a series of transitions to low-symmetry structures
[
39
,
89
,
248
] similar to those seen previously in Li [
38
]. In addition, studies of the
melting temperature found that, after reaching a maximum of ~1,000 K at 31 GPa,
it reduced strongly, reaching ~300 K at 118 GPa. Such a low melting temperature
above 100 GPa is unique to sodium, and enabled us to grow high quality single-
crystals of the numerous complex crystal structures that exist in the vicinity of the
melting minimum [
90
].
Within a
2 GPa range about the minimum at 118 GPa, our diffraction studies
revealed six different crystalline phases, four of which are unique to sodium [
91
].
The simplest of these is an orthorhombic structure with space group
Pnma
and eight
atoms per unit cell (oP8-Na), which has since also been found in potassium above
54 GPa [
100
]. But the other four structures are extremely complex, with tetragonal,
orthorhombic, monoclinic and triclinic symmetries, and with approximately 52,
120, 512 and 90 atoms, respectively, per unit cell [
91
]. Determining the lattice types
and lattice parameters of these complex structures would not have been possible
using powder methods. However, the availability of high quality single-crystals of
each phase meant that we were able to obtain this information with confidence,
although full solution of these four structures will require further experiments.
On further pressure increase at 300 K, the oP8 phase of Na transforms at
125 GPa to another form [
120
], accompanied by a marked decrease in optical
reflectivity. A diffraction pattern from this phase is shown in Fig.
16
, and contains
characteristic lines of diffuse scattering. This, and the highly-distinctive spacing of
the planes of Bragg reflections, reveal this phase to have an incommensurate
host-guest composite structure, of the type found previously in Sc, and in a number
of other elemental metals [
87
]. The host-guest structure of Na, as seen down the
crystallographic
c
-axis, is shown in Fig.
17
, and comprises a 16-atom body-centred
tetragonal host framework, isostructural with that found previously in Rb and K
[
249
,
250
], but different to the 8-atom host structure found in Sc, Ba, Sr, Bi, Sb and
As (compare Fig.
17
to Fig.
12
). As in Sc, the host framework has channels along
