Novel metal oxides prepared by ingenious synthetic routes
- PDF / 2,085,377 Bytes
- 15 Pages / 594 x 810 pts Page_size
- 43 Downloads / 296 Views
-o-o r; (a) O.S
Fraction of Oxygen-atom lost
10
£
(b)
.•'
E
0 0
-£
FIG. 9. Thermogravimetric curves of (a) reduction of LaNiO 3 in pure hydrogen and (b) oxidation of La 2 Ni 2 O 5 in oxygen (the corresponding curves for LaCoO 3 -La 2 Co 2 O 5 are shown in the inset).
s o
~l
1.5
1.00
1 500
1 600
1
i
A00
500
1 700
™
l 800
i 900
1
600
700
1
1
800
900
Temperature(K)
286 http://journals.cambridge.org
J. Mater. Res., Vol. 1, No. 2, Mar/Apr 1986 Downloaded: 12 Mar 2015
IP address: 128.218.248.200
Rao eta/.: Novel metal oxides
(a)
, 6O
7O
.. 5O
iff 4O 2«(d«g) ColU
3O
2O
IO
FIG. 10. Plot of x-ray powder diffraction patterns of (a) La 2 Co 2 0 5 , (b) LaCoO 3 obtained by oxidation of La2Co2O5, and (c) La 2 Ni 2 O 5 .
•
1
•'"£
*
' " • • • *
FIG. 11. Electron diffraction pattern and the lattice image of nominal La2Co2O3 recorded with electron beam parallel to [100] of brownmiUerite.
J. Mater. Res., Vol. 1, No. 2, Mar/Apr 1986 http://journals.cambridge.org
Downloaded: 12 Mar 2015
287 IP address: 128.218.248.200
Rao eta/.: Novel metal oxides
[no]
X
xx x
x xn n
•
•
• 100 000
010
X nX
FIG. 12. Vacancy ordering in perovskite related La 2 Ni 2 O 5 . (a) Proposed structure for La 2 Ni 2 O 5 . Projection of the proposed structure (left), expected electron diffraction pattern (center) and electron diffraction pattern observed for La 2 Ni 2 O 5 (right) in (b) [001], (c) [110], and (d) [111] directions (©— lanthanum; •—nickel; •—oxygen, and O—oxygen vacancy).
(b) 002
220
(c) 220
000
© • © • > • • • ) o © o 0
(d)
LaO 0 5 Do_5 ( • = vacancy) and NiO 2 stacked alternately parallel to the c direction, giving rise to square-planar and octahedral coordinations of Ni 2 + . The topotactic relationship between La 2 Ni 2 O 5 and LaNiO 3 may be expressed as (200),||(100) c and (002) ( ||(001) c . Lattice images of La 2 Ni 2 O 5 recorded with the electron beam parallel to [001] direction (Fig. 13) show regular fringes of 7.7 A periodicity as expected for the proposed structure. The difference between the structures of La 2 Co 2 O 5 and La 2 Ni 2 O 5 can be traced to the difference in site-preference energies of Co 2 + and Ni 2 + . While Ni 2 + has a strong preference for square-planar coordination, Co 2 + has a tetrahedral site preference. Both the ions exhibit relatively high octahedral site preference. While a majority of La 2 Ni 2 O 5 crystals examined in the electron microscope exhibited features described above, a few crystals showed different features (Fig. 14). The diffraction pattern and the image shown in Fig. I4(a) are reminiscent of disordered {102} crystallographic shear (CS) planes in slightly reduced WO 3 , while those shown in Fig. 14(b) are similar to {103} CS 2 2 2 3 in 288 http://journals.cambridge.org
W n O 3B _ 2 , exhibiting regions of constant width ( ~ 15 A) between the CS planes. This would correspond to La^Ni n O 3 n _ 2 with n = 14. It is likely that slightly reduced LaNiO 3 accommodates oxygen vacancies through the CS mechanism. If so, this would be th
Data Loading...
