Band structure calculations on Ti-enhanced NaAlH4 - HZG

37 Pages·2006·1.73 MB·English

by  Ole Martin Løvvik

FuncHy 2006:
First principles modelling
for predicting and
understanding novel
complex hydrides
Ole M. Løvvik
Centre for Materials Science and Nanotechnology, University of Oslo, Norway
Institute for Energy Technology, Kjeller, Norway
Alanates for hydrogen storage
Accessible hydrogen wt%:
(cid:132)
LiALliHAlH4 5.3 2.7
4
NaNAalHAlH4 3.7 1.9
4
KAKlHAlH4 2.9 1.4
4
Li LAi3lAHlH6 5.6
3 6
NaNaA3lAHlH6 3.0
3 6
K AK3lHAlH6 2.0
3 6
MMgg(A(AllHH4))2 7.0
4 2
CCa(aA(AlHlH4))2 3.0 3.0
4 2
0.0 2.0 4.0 6.0 8.0
Crystal structure
predictions
E.g. Na AlH : Seven representative systems
(cid:132) 3 6
Full relaxation; allows to leave space group/symmetry
(cid:132)
Space # Type # atoms Model system
group
P-1 2 Triclinic 20 Ti NiS
3 6
P2 /n 14 Monoclinic 20 K Fe(CN)
1 3 6
P2 /n 14 Monoclinic 20 Na AlF (α)
1 3 6
Pna2 33 Orthorombic 40 Li AlF
1 3 6
Immm 71 Tetragonal 20 Na AlF (β)
3 6
R-3 148 Rhombohedral 20 Ti NiS
3 6
Fm-3m 225 Cubic 40 K MoF
3 6
Calculation details
Band-structure DFT
(cid:132)
VASP
(cid:132)
Generalized gradient appr. (GGA) PW91
(cid:132)
Projector augmented wave (PAW) method
(cid:132)
Spin polarization allowed
(cid:132)
Cut-off energy 780 eV
(cid:132)
Overall convergence: ~1 meV (0.01 kJ/mol)/unit cell
(cid:132)
Predicted crystal structures
Compound Predicted Expt. O. M. Løvvik, S. M. Opalka, H. W.
(cid:132)
Brinks, B. C. Hauback, Phys. Rev.
structure structure
B 69 (2004) 134117)
LiAlH P2 /c P2 /c
O. M. Løvvik, O. Swang, Europhys.
4 1 1
(cid:132)
Lett. 67 (2004) 607.
NaAlH I4 /a I4 /a
4 1 1
O. M. Løvvik, Phys. Rev. B. 71
(cid:132)
KAlH Pnma Pnma
4 (2005) 144111.
Li AlH R–3 R–3 O. M. Løvvik, O. Swang, J. Alloys
3 6 (cid:132)
Comp. 404-406 (2005) 757-761.
Na AlH P2 /n P2 /n
3 6 1 1
O. M. Løvvik, P. N. Molin, Phys.
(cid:132)
K AlH P2 /n
Rev. B. 72 (2005) 073201.
3 6 1
Mg(AlH ) P–3m1 P–3m1 O. M. Løvvik, O. Swang, S. M.
(cid:132)
4 2
Opalka, J. Mater. Res. 20 (2005)
Ca(AlH ) Pbca
3199 (Invited paper).
4 2
Thermodynamic stability
Formation enthalpy:
(cid:132)
H (M AlH ) =
form n (n+3)
E(M AlH ) – n E(M) – E(Al) – (n+3)/2 E(H )
n (n+3) 2
Compound LiAlH NaAlH KAlH Li AlH Na AlH K AlH
4 4 4 3 6 3 6 3 6
Formation
(kJ/mol H ) -55.5 -54.9 -70.0 -102.8 -69.9 -78.5
enthalpy 2
Ca(AlH )
4 2
Non-layered structure
AlH tetrahedra,
4
Al-H distance 162 pm
Ca
Al Ca 8-coordinated,
Ca-H distance 220 pm
H
H wt. density: 5.9%
H vol density: 70 kg/m3
Space group: Pbca
• O. M. Løvvik, Phys. Rev.
B. 71 (2005) 144111.

Ca(AlH ) density of states (DOS)
4 2
1
3
Ca Al
2
0.5
1
S
O 0 0
D
H Total DOS
0.2 100
0.1 50
0 0
-5 0 5 -5 0 5
E (eV) E (eV)