Chemistry Reference
In-Depth Information
These difficulties are partially solved by examining the compound Mn
2
GeO
4
.
This olivine-like phase [
17
] is transformed into a modified spinel (
I
mma) at 6.4 GPa
and 1,513 K, and a second phase (
bam), of the Sr
2
PbO
4
type, is obtained at the
denser phase
P
mma) by increasing
temperature, following the behaviour of the data collected in Scheme
1
.
In what follows, the cation arrays of the three phases of Mn
2
GeO
4
will be
discussed in connection with the Mn
2
Ge alloy. Only one phase has been reported
for Mn
2
Ge. It is Ni
2
In type (
P
bam transforms into the modified spinel (
I
important issue here is that this structure corresponds to the cation array of the
Because the oxide is also stable as a modified spinel structure (Imma), and also
in view of that the Ni
2
In-type structure is only stable above 1,063 K, the question
arises whether Mn
2
Ge could undergo a Ni
2
In
P
MgCu
2
-type transition, giving
therefore support to the existence of the modified spinel Mn
2
GeO
4
. As far as we
know, such a cubic Laves phase has never been reported. However, related Laves
phases of compositions MnNi
1.3
Ge
0.7
and MnNi
1.55
Ge
0.45
have been synthesized
!
P
6
3
/mmc) and the latter is MgCu
2
type (
d
3
m
).
Although these compounds do not have the stoichiometry of the cation array of
Mn
2
GeO
4
, the pseudo-formula
F
C
-Mn
2
Ge can be easily obtained by applying the
EZKC
in the following way.
MnNi
1.55
Ge
0.45
can be reformulated as MnNi(1)Ni(2)
0.55
Ge
0.45
. Following the
pseudo-atom
, if we consider that Ni(1) donates three electrons to Ni(2)
0.55
,
Ni(1)
3+
would become pseudo-Mn [
EZKC
-Mn(1)], whereas [Ni(2)
0.55
]
3
is converted
into the hypothetical Ni(2)
5.5
cation. This formal charge is much greater than that
needed to convert Ni(2) into
C
-Ge (four electrons). Thus, with the reported
stoichiometry, the pseudo-formula Mn
2
Ge cannot be attained. However, if we
admit small variations in the composition of the alloy,
C
the pseudo-formula
C
-Mn
2
Ge is possible. This assumption is made on the basis of the lack of accuracy
in the composition of many of these alloys and considering also that, in many
instances, the powder diffraction data short of the accuracy needed to obtain
reliable values of the site occupancy factors [
96
].
Thus, the slightly modified formula of Mn
1.1
Ni
1.50
Ge
0.4
could also be re-written
as Mn
1.1
Ni(1)
0.9
Ni(2)
0.6
Ge
0.4
. Proceeding in the same manner, the electron transfer
from Ni(1) to Ni(2) would give rise to a
-Mn
2
Ge formula, corresponding to the
We must also mention that the structures of the isoelectronic compounds
FeGeMo and VCoSi are hexagonal Laves phases (MgZn
2
type), and that the related
compound ZrV
0.5
Mn
0.5
Ni adopts the cubic MgCu
2
-type structure. An adequate
electron transfer, involving V, Mn and Ni atoms, entails that ZrV
0.5
Mn
0.5
Ni can
be formulated as
C
C
-ZrCoMn, coincident with the “real alloy” ZrCoMn, a cubic
Laves phase [
97
].
We have discussed so far the structural relationships of the pair Mn
2
Ge/
Mn
2
GeO
4
, making special emphasis on the well-known relationships olivine-spinel
