Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids - Inorganic 3D Structures - page 188

This is one of the crucial points of our discussion, i.e. when the compounds
Mn 2 GeO 4 and Ca 2 GeO 4 transform under pressure into structures of the Sr 2 PbO 4
type and Ba 2 SnO 4 type, respectively, the cations are not forming capricious struc-
tures, but they are preparing the partial substructures which remain after the
decomposition. When the HP phase of Ca 2 GeO 4 (Ba 2 SnO 4 type) is formed, we
are producing blocks of the perovskite CaGeO 3 which contains implicit the HP
phase of the CaGe alloy (CsCl type), but simultaneously, fragments of the rocksalt
CaO structure are also building up. Both components separate in an ordered manner
when the pressure increases. Thus, the compounds of the Ba 2 SnO 4 -type structure
can be contemplated as the intergrowth of phases which are just those in which the
compounds decompose when the pressure increases. The decomposition products
already existed!
We wish to finish this chapter mentioning the recent HP experiments on Na and
K. Two new phases ( o P8 and h P4) have been reported for these elements. In the
case of Na, the o P8 phase ( nma), isolated at 117 GPa [ 130 ], transforms at 200 GPa

into the hexagonal h P4 phase (

P

6 3 /mmc) [ 131 ] . The same structures were observed
for K [ 132 , 133 ] in the range of 20-50 GPa. Interestingly, the work on h P4-K
includes calculations of the electron localization function (ELF) at 25 GPa, showing
maxima located at the 2 d sites (1/3, 2/3, 1/4), of the space group P6 3 /mmc. The ELF
maxima contain approximately two electrons and have been interpreted as Lewis
pairs (LPs) acting as pseudo-anions [ 133 ] . Note that the same sites are occupied by
the S atoms in the HP phase of Na 2 S (Fig. 1c )[ 7 ]. These results confirm that, at
elevated pressures, the valence electrons can form LPs converting the metals into
true ionic structures. This outcome confirms the validity of the AIMM model
(Anions in Metallic Matrices Model) [ 134 ] . The model predicts that anions, like
O 2 ,S 2 , are formed when the O, S atoms ( foreign atoms ) locate in the vicinity of
either bonding pairs or lone pairs of a metallic skeleton [ 63 , 64 , 134 ].
The stabilization of these high-pressure phases of Na and K also gives support to
an intuitive hypothesis that considered the metallic subnets, in compound, as
metastable structures of the parent metal [ 3 , 110 ]. This idea was expressed as
follows: If high pressure gives rise to a redistribution of the electrons and hence
to a phase transition, similar results could be obtained if electrons are redistributed
by the presence of

P

.
These predictions become a categorical statement after the work of Marqu´s
et al. [ 133 ] where, for the first time, it is shown that, at very high pressures, the
elemental K adopts the h P4 structure, a phase topologically identical to that of the K
atoms in the HP K 2 S and in the HT K 2 SO 4 . Thus, the structure of K remains in the
series K

foreign atoms

K 2 SO 4 , indicating that a third journey seems to exist, i.e. that of
the metals. The reader can find illustrative drawings in [ 135 ] .
Along this chapter, we have remarked some of the most unusual features
observed in crystal structures in last few years. In some cases we have referred to
crucial aspects of these structures which have been hidden for almost one century.
We could say more properly that our discussion has dealt with structural features
which are rarely discussed in crystal chemical works. We are aware that many other
insights are implicit in our work. They were not discussed in depth for the sake of

! K 2 S !