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High resolution electron microscope contrast from substituted diamond-type lattices
Micron,
Vol.]3, No.3, pp.247-248, Printed in Great Britain
1982.
0047-7206/82/030247-02503.00/0 Pergamon Press Ltd.
HIGH RESOLUTION ELECTRON MICROSCOPE CONTRAST FROM SUBSTITUTED DIAMOND-TYPE LATTICES
P. Goodman, A. Olsen and H.J. Whitfield CSIRO, Division of Chemical Physics P.O. Box 160, Clayton, Victoria, Australia 3168.
I.
Experiment In this investigation we exploit the resolution now available in JEOL 200 CX microscopes to examine tetrahedrally co-ordinated sulphides and selenides giving projections requiring 2 ~ resolution for individual atom-site resolution. Ternary compounds were examined to explore different atomic species within the structure. A variet~ of compOoUnds ABS 2 could be prepared having the chalcopyrite structure (tetragonal, a ~ 5.5 A, c ~ ii A, u ~ ~; Wyckoff, 1964). This proved the ideal test specimen. In [0103 orientation the 3 atom sites are separated in projection; with isomorphous replacement in sites A or B different site potentials could be examined. II.
Results (i) Fig. 3 shows the E010~ projection of the chalcopyrite structure (Wyckoff, 1964). Electron microscope images were taken of the structures CuTIS 2 and AgGaS 2. In both cases, at a certain Af (defect-of-focus) range, these specimens presented images having the character of separately resolved atom sites. These images were characteristic for these two cases (e.g. Fig. I). The intensities within the resolved unit cell were markedly different for CuTIS 2 and AgGaS 2 . (ii) Images calculated using a special purpose computer* reproduced the observed images satisfactorily (e.g. Fig. 2). Next, the computer was used to trace the origin of contrast differences. Fig. 4 shows the results of calculations made for i00 ~ crystal thickness, for the Af range which yields the characteristic (Fig. 2 - type) image, for CuTIS2, and for CuGaxTll_xS 2 where site B is a mixed-atom site on average. Another variable in the chalcopyrite structure is the sulphur-atom co-ordinate. Taking u = ~ as "ideal", displacements occur from this position according to the A, B atomic radii (Wyckoff, 1964). Displacements for CuTiS 2 are indicated in Fig. 3. In Fig. 4, calculations of IB/I A are shown for CuTIS 2 (a) assuming u = ~, (b) retaining u = ~ but changing site B Occupancy according to Cu(GaxTll_x)S 2 (0 < x < %), (c) retaining x = 0, but with realistic sulphur shifts (u = 0.19). In this way it can be shown convincingly that the displacements of the light sulphur atoms produce a much greater effect on image contrast than simple substitution into an "ideal" lattice. III. Discussion The foregoing study of ABS 2 was extended to include AgGaS2, and then followed by a study of more complex sulphide-selenides. These structures can be thought of as a substituted diamond lattice. With ideal co-ordinates, and all atoms equivalent, peaks in Figs. 1 and 2 would be equal. Observed differences in contrast are found to arise primarily from atom shifts caused by the size effect of substitution, rather than directly to substitution itself. This contrast mechanism can be applied more generally. (a) "Lattice waves" observed during microscopy of CuTIS 2 (Fig. 5), to be passing continuously through the image with strong condenser illumination, can now be understood as resulting from continuous but microscopic sulphur migration out of the lattice. Opticaldensity waves can be caused by a displacive mechanism, rather than a bulk translation of matter which would have led to a much more rapid decomposition. (b) Atom-co-ordinate effects (departure from an ideal geometric lattice) will give a first-order contribution to distinct site appearance in larger sulphide-selenide structures such as the twinned diamond type: CuAsSe0.sS0. 2 (see Fig. 6). 247
248
*
P. Goodman}
A. Olse~ and H. J. %~q~itfield
The assistance of Dr. D.F. Lynch in image computation
Reference:
Wyckoff,
R.W.G., Crystal Structures,
is gratefully
acknowledged.
Vol. 2 (1964), pp.336-339.
/
Fig. 1.
Experimental
V
image from CuTIS 2
Fig. 2.
Computed
image to scale of Fig. 1
I%A 18 x=O
/
/
/
A
%
~4
/ x
/
/
I0
("• /"1- S ( i d e ~ L '. position)
'-S(shifted position)
6
_
d
×=0
× =
Fig. 3.
"25
~010~ projection of the chalcopyrite structure
-600
-700
-800
Fig. 4.
Calculated image-contrast plotted against Af.
Fig. 6.
Experimental image from CuAsSe0.sS0.2
I
I i
Fig. 5.
"Lattice waves" attributed sulphur-atom migration
to
results
Vol.]3, No.3, pp.247-248, Printed in Great Britain
1982.
0047-7206/82/030247-02503.00/0 Pergamon Press Ltd.
HIGH RESOLUTION ELECTRON MICROSCOPE CONTRAST FROM SUBSTITUTED DIAMOND-TYPE LATTICES
P. Goodman, A. Olsen and H.J. Whitfield CSIRO, Division of Chemical Physics P.O. Box 160, Clayton, Victoria, Australia 3168.
I.
Experiment In this investigation we exploit the resolution now available in JEOL 200 CX microscopes to examine tetrahedrally co-ordinated sulphides and selenides giving projections requiring 2 ~ resolution for individual atom-site resolution. Ternary compounds were examined to explore different atomic species within the structure. A variet~ of compOoUnds ABS 2 could be prepared having the chalcopyrite structure (tetragonal, a ~ 5.5 A, c ~ ii A, u ~ ~; Wyckoff, 1964). This proved the ideal test specimen. In [0103 orientation the 3 atom sites are separated in projection; with isomorphous replacement in sites A or B different site potentials could be examined. II.
Results (i) Fig. 3 shows the E010~ projection of the chalcopyrite structure (Wyckoff, 1964). Electron microscope images were taken of the structures CuTIS 2 and AgGaS 2. In both cases, at a certain Af (defect-of-focus) range, these specimens presented images having the character of separately resolved atom sites. These images were characteristic for these two cases (e.g. Fig. I). The intensities within the resolved unit cell were markedly different for CuTIS 2 and AgGaS 2 . (ii) Images calculated using a special purpose computer* reproduced the observed images satisfactorily (e.g. Fig. 2). Next, the computer was used to trace the origin of contrast differences. Fig. 4 shows the results of calculations made for i00 ~ crystal thickness, for the Af range which yields the characteristic (Fig. 2 - type) image, for CuTIS2, and for CuGaxTll_xS 2 where site B is a mixed-atom site on average. Another variable in the chalcopyrite structure is the sulphur-atom co-ordinate. Taking u = ~ as "ideal", displacements occur from this position according to the A, B atomic radii (Wyckoff, 1964). Displacements for CuTiS 2 are indicated in Fig. 3. In Fig. 4, calculations of IB/I A are shown for CuTIS 2 (a) assuming u = ~, (b) retaining u = ~ but changing site B Occupancy according to Cu(GaxTll_x)S 2 (0 < x < %), (c) retaining x = 0, but with realistic sulphur shifts (u = 0.19). In this way it can be shown convincingly that the displacements of the light sulphur atoms produce a much greater effect on image contrast than simple substitution into an "ideal" lattice. III. Discussion The foregoing study of ABS 2 was extended to include AgGaS2, and then followed by a study of more complex sulphide-selenides. These structures can be thought of as a substituted diamond lattice. With ideal co-ordinates, and all atoms equivalent, peaks in Figs. 1 and 2 would be equal. Observed differences in contrast are found to arise primarily from atom shifts caused by the size effect of substitution, rather than directly to substitution itself. This contrast mechanism can be applied more generally. (a) "Lattice waves" observed during microscopy of CuTIS 2 (Fig. 5), to be passing continuously through the image with strong condenser illumination, can now be understood as resulting from continuous but microscopic sulphur migration out of the lattice. Opticaldensity waves can be caused by a displacive mechanism, rather than a bulk translation of matter which would have led to a much more rapid decomposition. (b) Atom-co-ordinate effects (departure from an ideal geometric lattice) will give a first-order contribution to distinct site appearance in larger sulphide-selenide structures such as the twinned diamond type: CuAsSe0.sS0. 2 (see Fig. 6). 247
248
*
P. Goodman}
A. Olse~ and H. J. %~q~itfield
The assistance of Dr. D.F. Lynch in image computation
Reference:
Wyckoff,
R.W.G., Crystal Structures,
is gratefully
acknowledged.
Vol. 2 (1964), pp.336-339.
/
Fig. 1.
Experimental
V
image from CuTIS 2
Fig. 2.
Computed
image to scale of Fig. 1
I%A 18 x=O
/
/
/
A
%
~4
/ x
/
/
I0
("• /"1- S ( i d e ~ L '. position)
'-S(shifted position)
6
_
d
×=0
× =
Fig. 3.
"25
~010~ projection of the chalcopyrite structure
-600
-700
-800
Fig. 4.
Calculated image-contrast plotted against Af.
Fig. 6.
Experimental image from CuAsSe0.sS0.2
I
I i
Fig. 5.
"Lattice waves" attributed sulphur-atom migration
to
results