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Intercalation of the amalgams KHg and RbHg into graphite: Reaction mechanisms and thermal stability
2 (1980) 191 - 196 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands S y n t h e t i c Metals,
191
I N T E R C A L A T I O N OF THE AMALGAMS KHg AND RbHg INTO GRAPHITE: REACTION MECHANISMS AND T H E R M A L STABILITY
PHILIPPE LAGRANGE, MOHAMED EL MAKRINI, DANIEL GUERARD and ALBERT HEROLD L a b o r a t o i r e de Chimie Mindrale A p p l i q u d e - CO 1 4 0 - 5 4 0 3 7 N a n c y Cddex (France)
(Received June 7, 1980)
Summary The intercalation o f the amalgams MHg (M = K, Rb) allows the synthesis o f the first stage MHgC4 and the second stage MHgCs ternary compounds. The intercalant is formed b y two layers of alkali metal surrounding an intermediate plane of mercury atoms. We have shown that the intercalation occurs in two steps. (i) A quasiselective intercalation with a continuous change of stage where only the alkali metal penetrates between the graphite planes. This step leads to the MCs c o m p o u n d with some mercury atoms in the metallic sheets. (ii) A simultaneous cooperative intercalation: alkali metal and mercury penetrate together in the occupied interlayers with the ratio Hg: M = 2 : 1 to build the three layered intercalant MHgM. The formation of the second stage compounds from the MCs binaries involves an addition o f mercury atoms with a reorganization o f the metallic layers in the graphite interval. On the other hand, the intercalation isobar and isotherm curves show that the MHgC4 compounds have a very small stability domain when t h e y are in presence o f an excess of free amalgam in a temperature gradient. When this gradient exceeds a few degrees, these ternary compounds decompose in the binary MCs compounds.
1. Introduction The amalgams KHg and RbHg intercalate into graphite and lead to the first stage MHgC4 and second stage MHgC8 ternary compounds [1, 2]. The intercalated layers are very thick, e . g . , the space between the carbon sheets increases from 3.35 to 10.16 £ in the case of potassium and 10.76 £ with rubidium. We have shown that these sheets are polylayered: t h e y are constit u t e d b y t w o alkali metal planes in epitaxy on the adjacent layers and the mercury atoms o c c u p y prismatic sites in the medium plane [3].
192 2. Mechanism of intercalation of the amalgam KHg We observed the intercalation mechanisms on samples of pyrographite placed in sealed glass tubes in the presence of the saturated vapor in equilibrium with an excess of the amalgam KHg at about 200 °C. The X-ray study of samples isolated after increased times of reaction show that the intercalation takes place in two successive steps. The first step is a quasi-selective intercalation by stage (Figs. 1 and 2): the potassium penetrates almost alone between the graphite planes, according to the reaction: 12nC + Kvapor -* KC12n
(stage n)
KC36 + I/2Kvapor -~ 3/2KC24
(2nd stage)
KC24 + 2Kvapor -~ 3KCs
(Ist stage).
In fact, the intercalation is not completely selectivebecause of the low solubility of mercury in the potassium sheets. The second step is a simultaneous cooperative intercalation: both mercury and potassium penetrate in the intervals already occupied by a single
KC24
+
KC 8
KCs+KHgC4
± B
22"
la"
14°
10o
6o
2" oKCs
Fig. 1. The two steps of the intercalation into graphite of K H g (anticathode: Mo). (1) Quasi-selective intercalationof potassium. (2) Simultaneous cooperative intercalationof potassium and mercury.
193 K
K
o
o
o
o
o
o ,,.., o
c) 0
o
o
o ~-. ~ 'f
~
"/-o
o o
o
~
o
~
0
o o
0 0
o °o° ~
o o°o
o Hg
o
o
+ ° Hg
K o
Hg
o
° °
o
o ~
o
o
~
2
0
0
°o
f oo °
o
o
o
o
0
0
0
0
o
o
0 °o
o
o
o
°o °o
o . / ' _ _
0
o °o
0
0
o
/
o o
°o 0
" - - - -
Oo
o
o
o
o Hg
K
Fig. 2. The two steps of the intercalation into graphite of KHg. (1) Quasi-selective intercalation of potassium. (2) Simultaneous cooperative intercalation of potassium and mercury.
potassium layer. They intercalate simultaneously to build the trilayers K-Hg-K between the carbon planes: two mercury atoms penetrate at the same time as one potassium atom, according to the reaction: KCs + Kvap + 2Hgvap --> 2KHgC4
(lst stage).
This last step (Figs. 1 and 2) allows passage from the 1st stage (single graphitide) to another first stage compound (mercurographitide). This is an isostage process. This process is directly observable through the various colors of the compounds which appear successively: the gray initial sample turns blue (KC36, KC24), then pale yellow (KCs) and finally pink-copper-like (KHgC4). One must note that the blue second stage, KHgCs, does not appear during the formation of the KHgC 4 mercurographitide. One can imagine this second stage compound as a transition step between KCs and KHgCs, as KC24 always appears before KCs. The same kind of results are obtained during the intercalation of the amalgam RbHg.
3. Mechanism o f addition o f mercury in the phases KCs and RbC s A calculated amount of pure mercury intercalates in the MCs compounds according to the reaction: MCs + Hgvap -* MHgCs. In contrast to the simultaneous cooperative intercalation previously described, which was an isostage process, this addition mechanism involves
194
a change of stage. It proceeds from the yellow 1st stage MCs to the blue second stage mercurographitide MHgCs. The model of the folded sheets [4] allows the mechanism to be described as follows: under the action of mercury, the alkali metal congregates on half the carbon sheet surface and leads to a double sheet of this metal, stabilized by the presence of the intermediate layer of mercury (Fig. 3). It may be noted that this process is similar to the hydrogenation of the MCs phases which leads to the formation of the blue second stage compounds MH2/3Cs [5].
4. Thermal stability of the mercurographitides MHgC4 Placed in an isotherm oven and in the absence of free KHg amalgam, the KHgC4 compound does not decompose below 450 °C. However, if the compound is in presence of the KHg amalgam, it decomposes at a temperature as low as 300 °C, and at 350 °C, it disappears completely to give the KCs phase which contains small amounts of mercury in the intercalated layer and which is denoted KCs(Hg). If, finally, the compound is placed in the presence of an excess of the KHg amalgam, in a temperature gradient, it decomposes more rapidly into KCs(Hg). A two-zone furnace allows a cold temperature, T, and a hot temperature, T', to be maintained constant as required. A glass tube, sealed under vacuum, contains a large excess of the KHg amalgam in the cold zone and a few weighted pieces of pyrographite* in the hot zone. At each experiment, the weight uptake is determined to draw intercalation isotherm or isobar curves.
Hg o
o o
"-~ 0
o o o °°
f
o 0
o
o
o
0
0
0
0
0
~o. °o
°o
0
//
0
~ 0
~ 0
0
0
0
0
J
"
o o
o
°° °°
°° 0
0
0
--
O
O
O
Hg
Fig. 3. A d d i t i o n o f m e r c u r y t o KC s g r a p h i t i d e .
* P G C C L f r o m C a r b o n e L o r r a i n e o r H O P G was k i n d l y given b y Dr A. W. M o o r e o f U n i o n C a r b i d e Co.
195
4.1. Intercalation isobar The cold zone containing the amalgam is maintained at a constant temperature o f about 180 °(3. The hot zone is maintained at a temperature T' which changes from one experiment to another. The isobar curve is shown in Fig. 4. For a value of A T = ( T ' - - T) = 0, one obtains the mercurographitide KHgC4. By contrast, as soon as A T > 0, one obtains the KCs (Hg) compound with a weight uptake of about one-tenth. The second step, which corresponds to the KC24 (Hg) second stage compound, starts at a value of AT around 45 - 50 °C. These results indicate the very low thermodynamic stability of the KHgC4 mercurographitides placed in a temperature gradient with an excess of amalgam.
500
~oo
30C 20O 10oI 5(; 0
".
_,
10
20
30
40
50
60
70
80 AT= T'-T
Fig. 4. Isobaric intercalation of potassium and mercury into graphite.
500
400
300 200
I00 dO
0
10
20
30
40
50
60
70 AT~T~T
Fig. 5. Isothermal intercalation of potassium and mercury into graphite.
196 4.2. I n t e r c a l a t i o n i s o t h e r m
With t h e h o t z o n e at a c o n s t a n t t e m p e r a t u r e o f a b o u t 250 °C, t h e t e m p e r a t u r e T o f t h e cold z o n e is varied f r o m o n e e x p e r i m e n t t o a n o t h e r u p t o t h e m a x i m a l value T = T'. T h e e x p e r i m e n t a l results (intercalation i s o t h e r m ) are given in Fig. 5. In this case, o n e m a y n o t e t h e low t h e r m o d y n a m i c stability o f t h e m e r c u r o g r a p h i t i d e w h i c h t r a n s f o r m s into KCs (Hg) as s o o n as A T is diff e r e n t f r o m zero. Finally, t h e s e c o n d step, c o r r e s p o n d i n g t o t h e second stage c o m p o u n d K C ~ (Hg), appears f o r a value o f A T near 45 °C.
References 1 M. El Makrini, P. Lagrange, D. Gu6rard and A. H6rold, C. R. Acad. Sci., Sdrie C, 288 (1979) 303. 2 P. Lagrange, M. E1 Makrini, D. Gu6rard and A. H6rold, Physica, 99 B (1980) 473. 3 P. Lagrange, M. E1 Makrini, D. Gu6rard and A. H6rold, Carbon '80 (Baden-Baden), p. 111. 4 N. Daumas and A. H6rold, C. R. Acad. Sci., Sdrie C, 268 (1969) 373. 5 P. Lagrange and A. H6rold, Carbon, 16 (1978) 235.
191
I N T E R C A L A T I O N OF THE AMALGAMS KHg AND RbHg INTO GRAPHITE: REACTION MECHANISMS AND T H E R M A L STABILITY
PHILIPPE LAGRANGE, MOHAMED EL MAKRINI, DANIEL GUERARD and ALBERT HEROLD L a b o r a t o i r e de Chimie Mindrale A p p l i q u d e - CO 1 4 0 - 5 4 0 3 7 N a n c y Cddex (France)
(Received June 7, 1980)
Summary The intercalation o f the amalgams MHg (M = K, Rb) allows the synthesis o f the first stage MHgC4 and the second stage MHgCs ternary compounds. The intercalant is formed b y two layers of alkali metal surrounding an intermediate plane of mercury atoms. We have shown that the intercalation occurs in two steps. (i) A quasiselective intercalation with a continuous change of stage where only the alkali metal penetrates between the graphite planes. This step leads to the MCs c o m p o u n d with some mercury atoms in the metallic sheets. (ii) A simultaneous cooperative intercalation: alkali metal and mercury penetrate together in the occupied interlayers with the ratio Hg: M = 2 : 1 to build the three layered intercalant MHgM. The formation of the second stage compounds from the MCs binaries involves an addition o f mercury atoms with a reorganization o f the metallic layers in the graphite interval. On the other hand, the intercalation isobar and isotherm curves show that the MHgC4 compounds have a very small stability domain when t h e y are in presence o f an excess of free amalgam in a temperature gradient. When this gradient exceeds a few degrees, these ternary compounds decompose in the binary MCs compounds.
1. Introduction The amalgams KHg and RbHg intercalate into graphite and lead to the first stage MHgC4 and second stage MHgC8 ternary compounds [1, 2]. The intercalated layers are very thick, e . g . , the space between the carbon sheets increases from 3.35 to 10.16 £ in the case of potassium and 10.76 £ with rubidium. We have shown that these sheets are polylayered: t h e y are constit u t e d b y t w o alkali metal planes in epitaxy on the adjacent layers and the mercury atoms o c c u p y prismatic sites in the medium plane [3].
192 2. Mechanism of intercalation of the amalgam KHg We observed the intercalation mechanisms on samples of pyrographite placed in sealed glass tubes in the presence of the saturated vapor in equilibrium with an excess of the amalgam KHg at about 200 °C. The X-ray study of samples isolated after increased times of reaction show that the intercalation takes place in two successive steps. The first step is a quasi-selective intercalation by stage (Figs. 1 and 2): the potassium penetrates almost alone between the graphite planes, according to the reaction: 12nC + Kvapor -* KC12n
(stage n)
KC36 + I/2Kvapor -~ 3/2KC24
(2nd stage)
KC24 + 2Kvapor -~ 3KCs
(Ist stage).
In fact, the intercalation is not completely selectivebecause of the low solubility of mercury in the potassium sheets. The second step is a simultaneous cooperative intercalation: both mercury and potassium penetrate in the intervals already occupied by a single
KC24
+
KC 8
KCs+KHgC4
± B
22"
la"
14°
10o
6o
2" oKCs
Fig. 1. The two steps of the intercalation into graphite of K H g (anticathode: Mo). (1) Quasi-selective intercalationof potassium. (2) Simultaneous cooperative intercalationof potassium and mercury.
193 K
K
o
o
o
o
o
o ,,.., o
c) 0
o
o
o ~-. ~ 'f
~
"/-o
o o
o
~
o
~
0
o o
0 0
o °o° ~
o o°o
o Hg
o
o
+ ° Hg
K o
Hg
o
° °
o
o ~
o
o
~
2
0
0
°o
f oo °
o
o
o
o
0
0
0
0
o
o
0 °o
o
o
o
°o °o
o . / ' _ _
0
o °o
0
0
o
/
o o
°o 0
" - - - -
Oo
o
o
o
o Hg
K
Fig. 2. The two steps of the intercalation into graphite of KHg. (1) Quasi-selective intercalation of potassium. (2) Simultaneous cooperative intercalation of potassium and mercury.
potassium layer. They intercalate simultaneously to build the trilayers K-Hg-K between the carbon planes: two mercury atoms penetrate at the same time as one potassium atom, according to the reaction: KCs + Kvap + 2Hgvap --> 2KHgC4
(lst stage).
This last step (Figs. 1 and 2) allows passage from the 1st stage (single graphitide) to another first stage compound (mercurographitide). This is an isostage process. This process is directly observable through the various colors of the compounds which appear successively: the gray initial sample turns blue (KC36, KC24), then pale yellow (KCs) and finally pink-copper-like (KHgC4). One must note that the blue second stage, KHgCs, does not appear during the formation of the KHgC 4 mercurographitide. One can imagine this second stage compound as a transition step between KCs and KHgCs, as KC24 always appears before KCs. The same kind of results are obtained during the intercalation of the amalgam RbHg.
3. Mechanism o f addition o f mercury in the phases KCs and RbC s A calculated amount of pure mercury intercalates in the MCs compounds according to the reaction: MCs + Hgvap -* MHgCs. In contrast to the simultaneous cooperative intercalation previously described, which was an isostage process, this addition mechanism involves
194
a change of stage. It proceeds from the yellow 1st stage MCs to the blue second stage mercurographitide MHgCs. The model of the folded sheets [4] allows the mechanism to be described as follows: under the action of mercury, the alkali metal congregates on half the carbon sheet surface and leads to a double sheet of this metal, stabilized by the presence of the intermediate layer of mercury (Fig. 3). It may be noted that this process is similar to the hydrogenation of the MCs phases which leads to the formation of the blue second stage compounds MH2/3Cs [5].
4. Thermal stability of the mercurographitides MHgC4 Placed in an isotherm oven and in the absence of free KHg amalgam, the KHgC4 compound does not decompose below 450 °C. However, if the compound is in presence of the KHg amalgam, it decomposes at a temperature as low as 300 °C, and at 350 °C, it disappears completely to give the KCs phase which contains small amounts of mercury in the intercalated layer and which is denoted KCs(Hg). If, finally, the compound is placed in the presence of an excess of the KHg amalgam, in a temperature gradient, it decomposes more rapidly into KCs(Hg). A two-zone furnace allows a cold temperature, T, and a hot temperature, T', to be maintained constant as required. A glass tube, sealed under vacuum, contains a large excess of the KHg amalgam in the cold zone and a few weighted pieces of pyrographite* in the hot zone. At each experiment, the weight uptake is determined to draw intercalation isotherm or isobar curves.
Hg o
o o
"-~ 0
o o o °°
f
o 0
o
o
o
0
0
0
0
0
~o. °o
°o
0
//
0
~ 0
~ 0
0
0
0
0
J
"
o o
o
°° °°
°° 0
0
0
--
O
O
O
Hg
Fig. 3. A d d i t i o n o f m e r c u r y t o KC s g r a p h i t i d e .
* P G C C L f r o m C a r b o n e L o r r a i n e o r H O P G was k i n d l y given b y Dr A. W. M o o r e o f U n i o n C a r b i d e Co.
195
4.1. Intercalation isobar The cold zone containing the amalgam is maintained at a constant temperature o f about 180 °(3. The hot zone is maintained at a temperature T' which changes from one experiment to another. The isobar curve is shown in Fig. 4. For a value of A T = ( T ' - - T) = 0, one obtains the mercurographitide KHgC4. By contrast, as soon as A T > 0, one obtains the KCs (Hg) compound with a weight uptake of about one-tenth. The second step, which corresponds to the KC24 (Hg) second stage compound, starts at a value of AT around 45 - 50 °C. These results indicate the very low thermodynamic stability of the KHgC4 mercurographitides placed in a temperature gradient with an excess of amalgam.
500
~oo
30C 20O 10oI 5(; 0
".
_,
10
20
30
40
50
60
70
80 AT= T'-T
Fig. 4. Isobaric intercalation of potassium and mercury into graphite.
500
400
300 200
I00 dO
0
10
20
30
40
50
60
70 AT~T~T
Fig. 5. Isothermal intercalation of potassium and mercury into graphite.
196 4.2. I n t e r c a l a t i o n i s o t h e r m
With t h e h o t z o n e at a c o n s t a n t t e m p e r a t u r e o f a b o u t 250 °C, t h e t e m p e r a t u r e T o f t h e cold z o n e is varied f r o m o n e e x p e r i m e n t t o a n o t h e r u p t o t h e m a x i m a l value T = T'. T h e e x p e r i m e n t a l results (intercalation i s o t h e r m ) are given in Fig. 5. In this case, o n e m a y n o t e t h e low t h e r m o d y n a m i c stability o f t h e m e r c u r o g r a p h i t i d e w h i c h t r a n s f o r m s into KCs (Hg) as s o o n as A T is diff e r e n t f r o m zero. Finally, t h e s e c o n d step, c o r r e s p o n d i n g t o t h e second stage c o m p o u n d K C ~ (Hg), appears f o r a value o f A T near 45 °C.
References 1 M. El Makrini, P. Lagrange, D. Gu6rard and A. H6rold, C. R. Acad. Sci., Sdrie C, 288 (1979) 303. 2 P. Lagrange, M. E1 Makrini, D. Gu6rard and A. H6rold, Physica, 99 B (1980) 473. 3 P. Lagrange, M. E1 Makrini, D. Gu6rard and A. H6rold, Carbon '80 (Baden-Baden), p. 111. 4 N. Daumas and A. H6rold, C. R. Acad. Sci., Sdrie C, 268 (1969) 373. 5 P. Lagrange and A. H6rold, Carbon, 16 (1978) 235.