K. Kadir, D. Noréus / Journal of Alloys and Compounds 477 (2009) 149–151
151
Table 3
Inter-atomic distances and unit cell dimensions in (Å) of NaGa2Al2 compared to compounds with the ThCr2Si2 type structure.
Formula
Unit cell (Å)
Metal-distance(Å)
c/a
V (Å3)
Reference
NaGa2Al2
a = 4.1817(5)
c = 11.388(2)
3.2060(3)
3.5323(4)
2.4778(4)
2.6377(3)
2.9569(4)
199
2.7
Na–Ga1
Na–(Al,Ga2)
Ga1–Ga1
Ga1–Al
Al–(Al,Ga2)
a = 4.278(1)
c = 11.015(2)
202
194
[5]
[6]
CaGa2Al2
2.57
Ca–Ga1
3.268
3.487
2.475
2.622
3.025
Ca–(Al,Ga2)
Ga1–Ga1
Ga1–Al
Al–(Al,Ga2)
a = 4.203(3)
c = 10.955(2)
CeGa2Al2
2.6
Ce–Ga1
3.219
3.452
2.476
2.582
2.972
Ce–(Al,Ga2)
Ga1–Ga1
Ga1–Al
Al–(Al,Ga2)
We also performed a differential thermal analysis (DTA) scan up
to 773 K under Ar gas atmosphere, whereby no enthalpy change was
observed, which could indicate release of hydrogen or a structural
phase transformation. This indicates that NaGa2Al2 is rather sta-
ble intermetallic compound that is not willing to absorb hydrogen.
This is also understandable with respect to the limited numbers
of electrons that can be donated from the single Na atom to the
Al–Ga polyhedra. The existence of this stable non-hydrogen absorb-
ing phase will also be limiting for the search of new quaternary
hydrides when trying to modify alanates by Ga additions.
Acknowledgments
We are grateful to Håkan Rundlöf for collecting the neutron
diffraction pattern at the R2 reactor in Studsvik and to Dr. Lars
Eriksson for helpful discussions and supplying us with Grusplot
and Pfilm programs. Partial funding by the European Commission
DG Research (contract SES6-2006-518271/NESSHY) is gratefully
acknowledged by the authors.
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4. Conclusion
In this study we have shown that NaAl2Ga2 can easily be pro-
duced by sintering Na or NaH with Ga and Al metals in a 1:2:2
atomic ratio. Also if the initial composition of the reactants varied
around this ratio, the 1:2:2 compound easily formed and always
with the same unit cell axes lengths. The NaAl2Ga2 showed no
reaction with hydrogen gas up to 5 MPa pressures at temperatures
between room temperature up to 723 K.
Although the inter-atomic distances Na–Al and Ga–Al are
slightly long they are comparable to a number of compounds with
similar structure type and without transition metal that has been
reported to crystallize in the same type of structure like CaAl2Ga2
and CeAl2Ga2. No disproportion or phase transformation of the
alloy was observed by X-ray diffraction after differential ther-
mal analysis up to 773 K under Ar gas atmosphere. All this taken
together indicate that NaAl2Ga2 is a fairly stable ternary compound,
the existence of which will create difficulties to attempts to improve
hydrogen storage properties of NaAlH4/Na3AlH6 by gallium doping.