Chemistry Reference
In-Depth Information
10 The Question of the Charge Transfer Between
Cations: Arguments Accounting for the Structures
of the Group 14 Elements
The three phases of Ca
2
GeO
4
(filled-wurtzite, olivine and Ba
2
SnO
4
type), described
above, might help to answer a crucial question, formulated in two previous works
. This concept considers the possibility of a charge
transfer between cations, even if they are of the same kind. Remember that this
idea served to postulate the underlying zincblende resonance structures in anti-
fluorites such as Li
2
O, Li
2
S, Na
2
S or (Li
3
Al)N
2
, so that Li
2
O could be regarded as
Li
+
(
EZKC
-BeO).
This explanation provides a rational understanding of many structures in terms
of simple skeletons of the elements of Group 14, which underlie in many com-
pounds [
14
]. Although the idea could be questioned on the basis of commonly
accepted chemical arguments, it could find support in the structures of, for example,
double alkali sulphates, chromates, etc., such as LiKSO
4
. Remember that in these
double sulphates, the two unlike alkali atoms are separated in two crystallographic
different sites.
Thus, while in Li
2
SO
4
, which at HT adopts the anti-fluorite structure, all the Li
atoms are located at tetrahedral S
4
voids, in the compounds Cs[LiCrO
4
] and K
[LiMoO
4
] only the Li atoms are centering Mo
4
tetrahedra. Additionally, the Cs
atoms are located at (1/2, 0, 0), forming a NaCl-type substructure with the Cr(Mo)
atoms. The result is that an electron transfer from Cs to Li, the [Li
1
Mo] subarray, i.
e. the [
C
-BeMo] subnet, conforms to the zincblende-type structure. In this context,
while the implicit existence of a zincblende network, in the HT-Li
2
SO4 structure,
could be questioned by the simultaneous existence of both Li
+
and Li
, its occur-
rence seems doubtless in the chromates and molybdates.
The phase-transitions observed in Ca
2
GeO
4
are, however, of great interest in the
interpretation of this phenomenon. Even if the olivine
C
Ba
2
SnO
4
transition, at HP,
is important, probably the most relevant feature is the direct HT olivine
!
!
filled-
The crucial aspect here is that the final structure is the one adopted by the elements
of Group 14, as well as by IV-IV, III-V and II-VI binary compounds. Among others,
we can mention the hexagonal diamond (londsdaleite), CSi, AlP, BN, GaN, BeO,
ZnO and wurtzite (ZnS) itself!
Thus, the wurtzite structure formed by the pair (CaGe) (Fig.
29a
) can be
explained in the light of the
EZKC
[
14
] if one Ca atom donates two electrons to
the (CaGe) pair, becoming either [
-ScAs], that is, a pseudo-(IV-IV)
or a -(III-V) compound. In this respect, we must recall the wurtzite-like structure of
GaN and the zincblende-type structure of ScP! This interpretation is consistent with
the principle establishing the tendency of any structure to form four-connected
We remarked above that in the case of anti-fluorites such as Li
2
S and Na
2
S,
which undergo the reversible transition Ni
2
In
C
-TiGe] or [
C
,
