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Alkali metal and alkali metal hydroxide intercalates of 2s-tantalum disulfide
1. Phys. Chem. Solids Pergamon Press Ltd.,
Vol.40, pp.91 I-914 1979.
Printed
in Great Britain
ALKALI METAL AND ALKALI METAL HYDROXIDE INTERCALATES OF 2s-TANTALUM DISULFIDE YASUSHI KANZAKI, M~TSUHARU KONUMA, EMKO YAMADA and OSAMU MATSUMOT~ Department of Chemistry, College of Science and Engineering, Aoyama Gakuin University, Chitosedai, Setagaya-ku,
Tokyo 157, Japan (Received 18 December, 1978; accepted in revised fonn 17 April, 1979)
Ah&act-The alkali metal intercalates of the layered compound Zs-tantalum disulfide were prepared from the respective hexamethylphosphoric triamide solutions of the metals. The c-lattice parameters of the intercalates increased with increase in the crystallographic radii of the metals. All intercalates prepared were superconductors, and the transition temperatures increased as the crystallographic radii of the metals became larger. The intercalates reacted with water to produce hydrogen gas and changed to different intercalates. These had properties similar to those of the corresponding alkali metal hydroxide intercalates prepared from aqueous solutions of the metal hydroxides. The alkali metal hydroxide intercalates, on the other hand, were found to be classified into two groups in terms of the c-lattice parameters; one having c-lattice parameters around 23.8 A and the other 18 A. Lithium and sodium hydroxide intercalates belong to the former type, and potassium, rubidium and cesium hydroxide intercalates, including ammonium hydroxide, to the latter. Dried lithium and sodium hydroxide intercalates were also classified in the latter group. In the former case the disulfide was found to intercalate the cations, conserving the ice-lie structure of the surrounding water molecules. In the latter, the cations were intercalated in their naked or primary hydrated states, and the interlayer distances were governed by cointercalated hydroxide ions. The observed superconducting transition temperatures were similar for the intercalates with c-lattice parameters around 18 A irrespective of the particular cation.
INTRODUCXION
The layered dichalcogenides of the transition metal groups IV to VI, such as titanium, tantalum and molybdenum, are known to form intercalation compounds with alkali and alkali earth metals and with their hydroxides. More recently the hydration reaction of alkali metal and alkali earth metal ions intercalated in the van der Waals gaps of the dichalcogenides has been of interest. Subba Rao et cil. [l] found that the water molecules were intercalated simultaneously with lithium or sodium hydroxide intercalation in aqueous solutions. Seiyama et al. [2] investigated the dehydration reaction of the hydroxide intercalates of 2s-tantalum disulfide by means of therm0 gravimetry and differential thermal analysis. They found a stepped dehydration of sodium hydroxide intercalate. Recently Lerf et al. [3] found that the alkali and alkali earth metal hydroxide intercalates of 2s-transition metal disulfides could be classified into two groups depending on the charge-radius ratio of the intercalated cations. They interpreted the hydration process from a different point of view. They developed their discussion on the assumption that the alkali metals were hydrated topotactically, this being accompanied by a charge transfer reaction between alkali metals and the host disulfide matrix. In a previous investigation we have surveyed the 911
intercalation reaction of 2s-tantalum disulfide with sodium and potassium metals prepared from the hexamethylphosphoric triamide (HMPA) solutions of the metals and with their hydroxides in aqueous solutions [4]. The sodium and potassium metal intercalates treated with water were found to have properties similar to those of the corresponding hydroxide intercalates as regards X-ray analysis and superconducting properties. In this paper the work is extended to the intercalates of 2s-tantalum disulfide with all alkali metals and their hydroxides except for francium. The results are compared with those resulting from the well known hydration process of alkali metal and alkali earth metal ions in aqueous solutions. EXPERIMENTAL
Experimental procedures were fundamentally those described previously [4]. Hexagonal 2s-tantalum disulfide was prepared from trigonal Is-tantalum disulfide by annealing for 4 days at 600°C followed by a 2 week anneal at 500°C. The Is-tantalum disulfide was prepared by heating a mixture of the respective elemental powders for 2 h at 900°C. Commercially available lithium, sodium and potassium metals and high purity rubidium and cesium metals were used. Except for lithium, they were placed in glass capillaries and purifted in uacuo by a three step distillation in a glass tube. The metals were then dissolved in HMPA in oacub [5]. Lithium metal was dissolved in
YASUSHI KANZAKIet al.
912
radii of alkali metals. Intuitively speaking, the increases in the c-lattice parameters for the intercalates of the 2s-polymorphs are expected to be proportional to the radii of the alkali metals and a used at a concentration of 0.006 M and the disulfide w-as factor of four larger [3]. The observed increases, treated with these for 3 hr. 0.7-4.9A, were noticeably smaller than fourtimes the ionic radii of the metals, 2.72-6.8 A. or RESULTS AND DISCUSSION the atomic radii, 6.08-10.48 A. Detailed consideration of the increases in the c-lattice parameters X-Ray diffraction data and the superconducting transition temperature onsets, T,, for all the 2swill be given in a subsequent paper [12], together with those of the hydroxide intercalates. tantalum disulfide intercalates investigated are The superconducting transition temperatures insummarized in Table 1. All intercalates retained their 2s-hexagonal crystal lattices, and the a-lattice creased with increase in the interlayer distance parameters were almost invariant irrespective of except for the rubidium intercalate. This tendency is similar to the result obtained in molybdenum the intercalation process. disulfide intercalates [7]. It is not certain whether Alkali metal intercalates the result for the rubidium intercalate in the present experiment is real or whether it is some kind of A systematic investigation of the alkali metal experimental artefact. The cesium intercalate has intercalation of 2s-tantalum disulfide has not yet the highest T, value of those for intercalation combeen conducted as far as is known. Preparation of pounds of 2s-tantalum disulfide reported so far. alkali metal intercalates and the measurements of their physical properties must be carried out in the Alkali metal hydroxide intercalates absence of oxygen and moisture. Several methods have been proposed so far to satisfy these condiThere have been several systematic studies on tions [6-91; e.g. the use of alkali metal vapors at the alkali and alkali earth metal hydroxide intercahigh temperatures [6] and the use of alkali metal lates of 2s-tantalum disulfide [l, 3,8,9,13,14]. As solutions of aprotic polar solvents such as liquid revealed in these studies, the alkali metal ions are ammonia [7] seem appropriate for the present pursupposed to be hydrated in the lattice of the host pose. In this investigation HMPA solutions of alkali layered compounds. Evidence for this supposition is the anomalous behavior observed in lithium and metals [lo] were used. Intercalation of the solvent molecules into tantalum disulfide was not observed sodium hydroxide intercalates, as seen in Table 1. in the solutions as well as in the pure HMPA Two significant stages of intercalation were obsolvent. In the case of the ammonia solution of served; one having a c-lattice parameter of 23.8 A lithium for molybdenum disulfide, cointercalation and the other 18 A. The former was obtained only of the ammonia molecules has been observed [7]. in rather humid conditions, above 10 torr of Hz0 The van der Waals radius of the HMPA molecule for the sodium hydroxide intercalate [3], and is estimated to be 4.1 A [ 111. The interlayer dischanged to the latter following slight drying or tance increased with increase in the crystallographic heating [2]. On the other hand, potassium, HMF’A directly without further ptication. 2s-tantalum disulfide was treated with the HMPA solutions of the metals for a few hours to complete the intercalation reaction. Reagent grade alkali metal hydroxides were
Table
1. X-ray
diffraction analysis and T, onset value data for intercalated TaS,
3.31 3.30
T&* -Li
12.1
12.8
0.8 0.7
4.1
-m
3.34
14.4
2.3
4.2
-K
3.32
16.2
4.1
5.6
-P.tl
3.32
16.7
4.6
2.3
-cs
3.36
17.2
5.1
8.1 4.7
-Li i 5
3.30
17.6
5.5
p
3.34
18.0
5.9
4.1
.8
3.32
18.0
5.9
4.7
-P.b
:
3.30
18.1
6.0
5.0
-cs
r
3.31
18.5
6.4
4.7
3.31
23.8
11.7
3.7
-LiOH*
7
11.6
5.5
4.7
-NaOH
3.33
23.8
11.7
5.1
-NaOH'
3.33
18.0
5.9
4.1
-Na -K
$
-LiOH
-KOH
3.33
18.0
5.9
4.8
-RtoH
3.34
18.5
6.4
4.4
-CBOH
3.34
18.6
6.5
5.0
-NH40H
3.30
18.2
6.1
3.4
*: Dried
with
silica
gel.
fll
Alkali metal and alkali metal hydroxide intercalates of 2s-tantalum disulfide rubidium, cesium and ammonium hydroxide intercalates did not show such anomalous behavior. They showed only one stage with a c-lattice parameter of 18 A. To clarify the above phenomenon a different procedure for producing alkali metal hydroxide intercalates was adopted. The alkali metal intercalates were exposed to humid air or treated with water. If such a procedure is carried out for the pure alkali metals, alkali metal hydroxides will be produced accompanying the evolution of hydrogen. As expected, in alkali metal intercalates such a procedure gave products similar to those of alkali metal hydroxide intercalates. The results are shown in Table 1. In addition, gas evolution was observed when the alkali metal intercalates were treated with water. This finding shows that the alkali metals intercalated in the layered compounds reacted with water molecules and produced metal hydroxides and hydrogen gas. This fact is not consistent with Lerf et al.‘s assumption [3] that the charge transfer reaction accompanying the hydration reaction takes place between the alkali metal and the matrix of the host layered compound. If the latter were correct, gas (hydrogen) evolution would not be observed. T, values observed for these water-treated metal intercalates were consistent with those of hydroxide intercalates, as seen in Table 1. The T, values measured here for the hydroxide intercalates which had c-lattice parameters around 18 8, are not so sensitive to the particular alkali metal as those of the metal intercalates. This fact suggests that the intercalation of alkali metal hydroxides in aqueous solutions is governed preferentially by hydroxide ions over alkali metal cations as will be mentioned below. Our interpretation as well as our results differs from that proposed by Sernetz et al. [8]. They plotted T, values against the crystallographic ionic radii of the intercalated cations and obtained a maximum at an ionic radius of 1.1 A. The plots look rather artificial, for the two groups with c-lattice parameters of 18 and 23.8 A in lithium and sodium hydroxide intercalates result from the difference in the number of water molecules cointercalated. The crystallographic radius as abscissa makes no sense. A better procedure is to make a plot for the groupofintercalateswithasimilarc-latticeparameter of 18 A. The lithium and sodium hydroxide intercalates with a c-lattice parameter of 23.8 8, should be discussed separately. Sernetz et al.‘s T, value for the cesium hydroxide intercalate, 2.75 K, is noticeably lower than ours. Hydration
reaction
Several dehydration stages are known from previous studies of the hydroxide intercalates [2,3]. Two of these stages are of particular note, and show well-resolved X-ray ditfraction patterns with a hexagonal crystal lattice which has an u-lattice parameter similar to that of the host disulfide. One is classified as a group with c-lattice parameters
913
around 23.8A-lithium, sodium and alkali earth metal [3] hydroxide intercalates belong to this group. The other group has c-lattice parameters around 18 A-the intercalates of potassium, rubidium, cesium and ammonium hydroxides belong to this group. Dried lithium and sodium hydroxide intercalates also belong to this group. According to thermogravimetric measurements [2], the number of water molecules cointercalated into 2s-tantalum disulfide are about eight for each hydroxide molecule iu the sodium hydroxide intercalate with a c-lattice parameter of 23.8 A, and three to four per hydroxide for the potassium hydroxide intercalate. Lerf et al. gave an interpretation of the phenomenon. They classified the intercalates into two groups. One has charge-radius ratio, e/r, greater than unity, where e is the charge of an intercalated cation and r the crystallographic ionic radius of the cation. This group has a c-lattice parameter of 23.8 A. The other group has e/r values lower than unity and a c-lattice parameter of 18 A. They related qualitatively the charge-radius ratio to the hydration energy of the cations with water molecules. Their interpretation appears somewhat intuitive and did not suceed in giving any physical meaning to the choice of unity in classifying the two groups. A different interpretation is proposed here as follows. In Table 2 the well known B-coefficients of the Jones-Dole equation [15] for the viscosity of aqueous electrolyte solutions are listed [16]. The equation is q/q,,= l+Ac”‘+Bc where rl and ~9 are the viscosities of the electrolyte solution and the pure solvent, respectively, c the concentration of electrolyte, and A and B empirical constants which depend on the solvent and the kind of electrolyte. The coefficient B has been associated with ion-solvent interactions. Comparing the B-coefficients with the present phenomenon in question, it is found that the intercalates for the cations with positive B-coefficients belong to the group with a c-lattice parameter of 23.8A. Such cations are known to increase the “ice-like” structure of water over that of the pure solvent. On the other hand, the intercalates for cations with negative B-coefficients belong to the group with a clattice parameter of 18 A. These cations decrease the “ice-like” structure of water. One defect of our assumption is the fact that the B-coefficient of the hydroxide ions is positive, which means that the c-lattice parameters should be around 23.8 8, if our assumption were applicable to the hydroxide ions. However, this difficulty is eliminated if the following special situations for the hydroxide ions are properly taken into consideration; (i) the intercalation reaction of the alkali metal ions occurs only in basic (hydroxide) solutions, (ii) mass transfer of the hydroxide ions is ascribed to proton transfer
914
YASU.~I KANzAKl et al. Table 2. Crystallographic ionic radii (re), effective ionic radii (Tic) and B-coefficients of alkali and alkali earth metal cations in aqueous solutions at 25°C [16]
Li+
0.68
3.7
+0.150
Na+
0.97
3.3
+0.086
K+
1.33
2.5
-0.007
Rb+
1.52
2.4
-0.030
C8+
1.70
2.4
-0.045
NH4+
1.45
2.5
-0.007
0.65
4.4
+0.395
0.99
4.2
+0.285
1.13
4.2
+0.265
1.35
4.1
+0.220
OH-
1.45
2.8
+0.122
H20w30+)
1.40
2.85
through hydrogen bonding, and thus the positive B-coefficient of the hydroxide ions results from the average of a rapid motion of protons, and (iii) water molecules are oriented by anions such that the direction of their dipoles is opposite to that of the cations. A structural model of the hydroxide intercalates may be offered on the basis of the above consideration although it seems still premature to regard this as a completely realistic model. According to the above qualitative relation between hydration reactions in the lattice and those in aqueous solutions, it seems more reasonable to consider the effective ionic radii of the ions in aqueous solutions than to consider the crystallographic ionic radii. The effective ionic radii of ions are listed in Table 2 in comparison the crystallographic ionic radii. The cations which gave the intercalates with a c-lattice parameter of 23.8 8, have larger effective ionic radii than that of the hydroxide ions. Since such cations are known to be effecting in promoting “ice-like” structures and have large effective ionic radii, it is reasonable to ascribe the large c-lattice parameters to the “ice-like” structure of water molecules which are hydrating cations in the lattice. Such an assumption appears to be consistent with the experimental hydration number [2]. However it is still ambiguous why the c-lattice parameters are invariant irrespective of the effective ionic radii of cations. The monolayer and bilayer water molecule model which is based on the e/r values stated above [3] seems to be too much of an artifact. Such a problem will be solved when the proper quantum mechanical analyses are carried out for the dispersion forces which act between S-M-S slabs and play a most important role in the intercalation reaction. The extension of the interlayer distance in the intercalates which have a c-lattice parameter of 18 8, is also insensitive to the effective ionic radii of cations. This insensitivity undoubtedly results from
(+0.069)
the fact that the extensions depend on the hydroxide ions rather than the cations. As seen in Table 2, the effective ionic radius of the hydroxide ions is larger than those of cations with negative Bcoefficients. A slight increase in the c-lattice parameters is seen in this group and coresponds to the increase in the crystallographic ionic radii of the cations. The fact results from ion-paring of hydroxide ions with the naked or partially hydrated cations in the lattice [12].
REFEltJmcEs
1. Subba Rao, G. V., Shafer, M. W. and Tsang, J. C., J. Phys. Chem. 79, 553 (1975). 2. Saito, Y., Makino, M., Yamazoe, N. and Seiyama, T. Extended Abstracts of 35th Fall Meeting of the Chemical Society of Japan, Sapporo (1976). 3. Lerf, A. and Schijllhom, R., Inorg. Chem. 16,2950 (1977). 4. Matsumoto, O., Yamada, E., Kanzaki, Y. and Konuma, M., J. Phys. Chem. Solids 39, 191 (1978). Chem. 5. Kanzaki, Y. and Aoyagui, S., J. Ektroanaf.
36,297 (1972). 6. Omloo, W. P. F. A. M. and Jellinek, F., J. LessCommon Metals 20, 121 (1970).
7. Subba Rao, G. V., Shafer, M. W., Kawarazaki,
S. and Toxen, A. M., J. Solid State Chem. 9, 323 (1974) 8. Sernetz, F., Lerf, A. and Schijllhorn, R., Mat. Res. Bull. 9, 1597 (1974). 9. SchGUhorn, R. and Mayer, H., Mat. Res. Bull. 9,1237 (1974). 10. Normant, H., Angew. Chem. 79, 1029 (1967). 11 Fujinaga, T., Izutsu K. and Sakura, S., Nippon Kagaku Kaishi, 1973, 191. 12. Kanzaki, Y., Konuma, M. and Matsumoto, O., in preparation. 13. Whittingham, M. S., Mat. Res. Bull. 9,1681 (1974). 14. Schijllhorn, R. and L.erf, A., J. Las-Common Metals, 42,89 (1975). 15. Jones, G. and Dole, M., J. Am. Chem. Sot. 51,295O
(1929). 16. Kaminsky,
M., Disc. Faraday
Sot. 24, 171 (1957).
Vol.40, pp.91 I-914 1979.
Printed
in Great Britain
ALKALI METAL AND ALKALI METAL HYDROXIDE INTERCALATES OF 2s-TANTALUM DISULFIDE YASUSHI KANZAKI, M~TSUHARU KONUMA, EMKO YAMADA and OSAMU MATSUMOT~ Department of Chemistry, College of Science and Engineering, Aoyama Gakuin University, Chitosedai, Setagaya-ku,
Tokyo 157, Japan (Received 18 December, 1978; accepted in revised fonn 17 April, 1979)
Ah&act-The alkali metal intercalates of the layered compound Zs-tantalum disulfide were prepared from the respective hexamethylphosphoric triamide solutions of the metals. The c-lattice parameters of the intercalates increased with increase in the crystallographic radii of the metals. All intercalates prepared were superconductors, and the transition temperatures increased as the crystallographic radii of the metals became larger. The intercalates reacted with water to produce hydrogen gas and changed to different intercalates. These had properties similar to those of the corresponding alkali metal hydroxide intercalates prepared from aqueous solutions of the metal hydroxides. The alkali metal hydroxide intercalates, on the other hand, were found to be classified into two groups in terms of the c-lattice parameters; one having c-lattice parameters around 23.8 A and the other 18 A. Lithium and sodium hydroxide intercalates belong to the former type, and potassium, rubidium and cesium hydroxide intercalates, including ammonium hydroxide, to the latter. Dried lithium and sodium hydroxide intercalates were also classified in the latter group. In the former case the disulfide was found to intercalate the cations, conserving the ice-lie structure of the surrounding water molecules. In the latter, the cations were intercalated in their naked or primary hydrated states, and the interlayer distances were governed by cointercalated hydroxide ions. The observed superconducting transition temperatures were similar for the intercalates with c-lattice parameters around 18 A irrespective of the particular cation.
INTRODUCXION
The layered dichalcogenides of the transition metal groups IV to VI, such as titanium, tantalum and molybdenum, are known to form intercalation compounds with alkali and alkali earth metals and with their hydroxides. More recently the hydration reaction of alkali metal and alkali earth metal ions intercalated in the van der Waals gaps of the dichalcogenides has been of interest. Subba Rao et cil. [l] found that the water molecules were intercalated simultaneously with lithium or sodium hydroxide intercalation in aqueous solutions. Seiyama et al. [2] investigated the dehydration reaction of the hydroxide intercalates of 2s-tantalum disulfide by means of therm0 gravimetry and differential thermal analysis. They found a stepped dehydration of sodium hydroxide intercalate. Recently Lerf et al. [3] found that the alkali and alkali earth metal hydroxide intercalates of 2s-transition metal disulfides could be classified into two groups depending on the charge-radius ratio of the intercalated cations. They interpreted the hydration process from a different point of view. They developed their discussion on the assumption that the alkali metals were hydrated topotactically, this being accompanied by a charge transfer reaction between alkali metals and the host disulfide matrix. In a previous investigation we have surveyed the 911
intercalation reaction of 2s-tantalum disulfide with sodium and potassium metals prepared from the hexamethylphosphoric triamide (HMPA) solutions of the metals and with their hydroxides in aqueous solutions [4]. The sodium and potassium metal intercalates treated with water were found to have properties similar to those of the corresponding hydroxide intercalates as regards X-ray analysis and superconducting properties. In this paper the work is extended to the intercalates of 2s-tantalum disulfide with all alkali metals and their hydroxides except for francium. The results are compared with those resulting from the well known hydration process of alkali metal and alkali earth metal ions in aqueous solutions. EXPERIMENTAL
Experimental procedures were fundamentally those described previously [4]. Hexagonal 2s-tantalum disulfide was prepared from trigonal Is-tantalum disulfide by annealing for 4 days at 600°C followed by a 2 week anneal at 500°C. The Is-tantalum disulfide was prepared by heating a mixture of the respective elemental powders for 2 h at 900°C. Commercially available lithium, sodium and potassium metals and high purity rubidium and cesium metals were used. Except for lithium, they were placed in glass capillaries and purifted in uacuo by a three step distillation in a glass tube. The metals were then dissolved in HMPA in oacub [5]. Lithium metal was dissolved in
YASUSHI KANZAKIet al.
912
radii of alkali metals. Intuitively speaking, the increases in the c-lattice parameters for the intercalates of the 2s-polymorphs are expected to be proportional to the radii of the alkali metals and a used at a concentration of 0.006 M and the disulfide w-as factor of four larger [3]. The observed increases, treated with these for 3 hr. 0.7-4.9A, were noticeably smaller than fourtimes the ionic radii of the metals, 2.72-6.8 A. or RESULTS AND DISCUSSION the atomic radii, 6.08-10.48 A. Detailed consideration of the increases in the c-lattice parameters X-Ray diffraction data and the superconducting transition temperature onsets, T,, for all the 2swill be given in a subsequent paper [12], together with those of the hydroxide intercalates. tantalum disulfide intercalates investigated are The superconducting transition temperatures insummarized in Table 1. All intercalates retained their 2s-hexagonal crystal lattices, and the a-lattice creased with increase in the interlayer distance parameters were almost invariant irrespective of except for the rubidium intercalate. This tendency is similar to the result obtained in molybdenum the intercalation process. disulfide intercalates [7]. It is not certain whether Alkali metal intercalates the result for the rubidium intercalate in the present experiment is real or whether it is some kind of A systematic investigation of the alkali metal experimental artefact. The cesium intercalate has intercalation of 2s-tantalum disulfide has not yet the highest T, value of those for intercalation combeen conducted as far as is known. Preparation of pounds of 2s-tantalum disulfide reported so far. alkali metal intercalates and the measurements of their physical properties must be carried out in the Alkali metal hydroxide intercalates absence of oxygen and moisture. Several methods have been proposed so far to satisfy these condiThere have been several systematic studies on tions [6-91; e.g. the use of alkali metal vapors at the alkali and alkali earth metal hydroxide intercahigh temperatures [6] and the use of alkali metal lates of 2s-tantalum disulfide [l, 3,8,9,13,14]. As solutions of aprotic polar solvents such as liquid revealed in these studies, the alkali metal ions are ammonia [7] seem appropriate for the present pursupposed to be hydrated in the lattice of the host pose. In this investigation HMPA solutions of alkali layered compounds. Evidence for this supposition is the anomalous behavior observed in lithium and metals [lo] were used. Intercalation of the solvent molecules into tantalum disulfide was not observed sodium hydroxide intercalates, as seen in Table 1. in the solutions as well as in the pure HMPA Two significant stages of intercalation were obsolvent. In the case of the ammonia solution of served; one having a c-lattice parameter of 23.8 A lithium for molybdenum disulfide, cointercalation and the other 18 A. The former was obtained only of the ammonia molecules has been observed [7]. in rather humid conditions, above 10 torr of Hz0 The van der Waals radius of the HMPA molecule for the sodium hydroxide intercalate [3], and is estimated to be 4.1 A [ 111. The interlayer dischanged to the latter following slight drying or tance increased with increase in the crystallographic heating [2]. On the other hand, potassium, HMF’A directly without further ptication. 2s-tantalum disulfide was treated with the HMPA solutions of the metals for a few hours to complete the intercalation reaction. Reagent grade alkali metal hydroxides were
Table
1. X-ray
diffraction analysis and T, onset value data for intercalated TaS,
3.31 3.30
T&* -Li
12.1
12.8
0.8 0.7
4.1
-m
3.34
14.4
2.3
4.2
-K
3.32
16.2
4.1
5.6
-P.tl
3.32
16.7
4.6
2.3
-cs
3.36
17.2
5.1
8.1 4.7
-Li i 5
3.30
17.6
5.5
p
3.34
18.0
5.9
4.1
.8
3.32
18.0
5.9
4.7
-P.b
:
3.30
18.1
6.0
5.0
-cs
r
3.31
18.5
6.4
4.7
3.31
23.8
11.7
3.7
-LiOH*
7
11.6
5.5
4.7
-NaOH
3.33
23.8
11.7
5.1
-NaOH'
3.33
18.0
5.9
4.1
-Na -K
$
-LiOH
-KOH
3.33
18.0
5.9
4.8
-RtoH
3.34
18.5
6.4
4.4
-CBOH
3.34
18.6
6.5
5.0
-NH40H
3.30
18.2
6.1
3.4
*: Dried
with
silica
gel.
fll
Alkali metal and alkali metal hydroxide intercalates of 2s-tantalum disulfide rubidium, cesium and ammonium hydroxide intercalates did not show such anomalous behavior. They showed only one stage with a c-lattice parameter of 18 A. To clarify the above phenomenon a different procedure for producing alkali metal hydroxide intercalates was adopted. The alkali metal intercalates were exposed to humid air or treated with water. If such a procedure is carried out for the pure alkali metals, alkali metal hydroxides will be produced accompanying the evolution of hydrogen. As expected, in alkali metal intercalates such a procedure gave products similar to those of alkali metal hydroxide intercalates. The results are shown in Table 1. In addition, gas evolution was observed when the alkali metal intercalates were treated with water. This finding shows that the alkali metals intercalated in the layered compounds reacted with water molecules and produced metal hydroxides and hydrogen gas. This fact is not consistent with Lerf et al.‘s assumption [3] that the charge transfer reaction accompanying the hydration reaction takes place between the alkali metal and the matrix of the host layered compound. If the latter were correct, gas (hydrogen) evolution would not be observed. T, values observed for these water-treated metal intercalates were consistent with those of hydroxide intercalates, as seen in Table 1. The T, values measured here for the hydroxide intercalates which had c-lattice parameters around 18 8, are not so sensitive to the particular alkali metal as those of the metal intercalates. This fact suggests that the intercalation of alkali metal hydroxides in aqueous solutions is governed preferentially by hydroxide ions over alkali metal cations as will be mentioned below. Our interpretation as well as our results differs from that proposed by Sernetz et al. [8]. They plotted T, values against the crystallographic ionic radii of the intercalated cations and obtained a maximum at an ionic radius of 1.1 A. The plots look rather artificial, for the two groups with c-lattice parameters of 18 and 23.8 A in lithium and sodium hydroxide intercalates result from the difference in the number of water molecules cointercalated. The crystallographic radius as abscissa makes no sense. A better procedure is to make a plot for the groupofintercalateswithasimilarc-latticeparameter of 18 A. The lithium and sodium hydroxide intercalates with a c-lattice parameter of 23.8 8, should be discussed separately. Sernetz et al.‘s T, value for the cesium hydroxide intercalate, 2.75 K, is noticeably lower than ours. Hydration
reaction
Several dehydration stages are known from previous studies of the hydroxide intercalates [2,3]. Two of these stages are of particular note, and show well-resolved X-ray ditfraction patterns with a hexagonal crystal lattice which has an u-lattice parameter similar to that of the host disulfide. One is classified as a group with c-lattice parameters
913
around 23.8A-lithium, sodium and alkali earth metal [3] hydroxide intercalates belong to this group. The other group has c-lattice parameters around 18 A-the intercalates of potassium, rubidium, cesium and ammonium hydroxides belong to this group. Dried lithium and sodium hydroxide intercalates also belong to this group. According to thermogravimetric measurements [2], the number of water molecules cointercalated into 2s-tantalum disulfide are about eight for each hydroxide molecule iu the sodium hydroxide intercalate with a c-lattice parameter of 23.8 A, and three to four per hydroxide for the potassium hydroxide intercalate. Lerf et al. gave an interpretation of the phenomenon. They classified the intercalates into two groups. One has charge-radius ratio, e/r, greater than unity, where e is the charge of an intercalated cation and r the crystallographic ionic radius of the cation. This group has a c-lattice parameter of 23.8 A. The other group has e/r values lower than unity and a c-lattice parameter of 18 A. They related qualitatively the charge-radius ratio to the hydration energy of the cations with water molecules. Their interpretation appears somewhat intuitive and did not suceed in giving any physical meaning to the choice of unity in classifying the two groups. A different interpretation is proposed here as follows. In Table 2 the well known B-coefficients of the Jones-Dole equation [15] for the viscosity of aqueous electrolyte solutions are listed [16]. The equation is q/q,,= l+Ac”‘+Bc where rl and ~9 are the viscosities of the electrolyte solution and the pure solvent, respectively, c the concentration of electrolyte, and A and B empirical constants which depend on the solvent and the kind of electrolyte. The coefficient B has been associated with ion-solvent interactions. Comparing the B-coefficients with the present phenomenon in question, it is found that the intercalates for the cations with positive B-coefficients belong to the group with a c-lattice parameter of 23.8A. Such cations are known to increase the “ice-like” structure of water over that of the pure solvent. On the other hand, the intercalates for cations with negative B-coefficients belong to the group with a clattice parameter of 18 A. These cations decrease the “ice-like” structure of water. One defect of our assumption is the fact that the B-coefficient of the hydroxide ions is positive, which means that the c-lattice parameters should be around 23.8 8, if our assumption were applicable to the hydroxide ions. However, this difficulty is eliminated if the following special situations for the hydroxide ions are properly taken into consideration; (i) the intercalation reaction of the alkali metal ions occurs only in basic (hydroxide) solutions, (ii) mass transfer of the hydroxide ions is ascribed to proton transfer
914
YASU.~I KANzAKl et al. Table 2. Crystallographic ionic radii (re), effective ionic radii (Tic) and B-coefficients of alkali and alkali earth metal cations in aqueous solutions at 25°C [16]
Li+
0.68
3.7
+0.150
Na+
0.97
3.3
+0.086
K+
1.33
2.5
-0.007
Rb+
1.52
2.4
-0.030
C8+
1.70
2.4
-0.045
NH4+
1.45
2.5
-0.007
0.65
4.4
+0.395
0.99
4.2
+0.285
1.13
4.2
+0.265
1.35
4.1
+0.220
OH-
1.45
2.8
+0.122
H20w30+)
1.40
2.85
through hydrogen bonding, and thus the positive B-coefficient of the hydroxide ions results from the average of a rapid motion of protons, and (iii) water molecules are oriented by anions such that the direction of their dipoles is opposite to that of the cations. A structural model of the hydroxide intercalates may be offered on the basis of the above consideration although it seems still premature to regard this as a completely realistic model. According to the above qualitative relation between hydration reactions in the lattice and those in aqueous solutions, it seems more reasonable to consider the effective ionic radii of the ions in aqueous solutions than to consider the crystallographic ionic radii. The effective ionic radii of ions are listed in Table 2 in comparison the crystallographic ionic radii. The cations which gave the intercalates with a c-lattice parameter of 23.8 8, have larger effective ionic radii than that of the hydroxide ions. Since such cations are known to be effecting in promoting “ice-like” structures and have large effective ionic radii, it is reasonable to ascribe the large c-lattice parameters to the “ice-like” structure of water molecules which are hydrating cations in the lattice. Such an assumption appears to be consistent with the experimental hydration number [2]. However it is still ambiguous why the c-lattice parameters are invariant irrespective of the effective ionic radii of cations. The monolayer and bilayer water molecule model which is based on the e/r values stated above [3] seems to be too much of an artifact. Such a problem will be solved when the proper quantum mechanical analyses are carried out for the dispersion forces which act between S-M-S slabs and play a most important role in the intercalation reaction. The extension of the interlayer distance in the intercalates which have a c-lattice parameter of 18 8, is also insensitive to the effective ionic radii of cations. This insensitivity undoubtedly results from
(+0.069)
the fact that the extensions depend on the hydroxide ions rather than the cations. As seen in Table 2, the effective ionic radius of the hydroxide ions is larger than those of cations with negative Bcoefficients. A slight increase in the c-lattice parameters is seen in this group and coresponds to the increase in the crystallographic ionic radii of the cations. The fact results from ion-paring of hydroxide ions with the naked or partially hydrated cations in the lattice [12].
REFEltJmcEs
1. Subba Rao, G. V., Shafer, M. W. and Tsang, J. C., J. Phys. Chem. 79, 553 (1975). 2. Saito, Y., Makino, M., Yamazoe, N. and Seiyama, T. Extended Abstracts of 35th Fall Meeting of the Chemical Society of Japan, Sapporo (1976). 3. Lerf, A. and Schijllhom, R., Inorg. Chem. 16,2950 (1977). 4. Matsumoto, O., Yamada, E., Kanzaki, Y. and Konuma, M., J. Phys. Chem. Solids 39, 191 (1978). Chem. 5. Kanzaki, Y. and Aoyagui, S., J. Ektroanaf.
36,297 (1972). 6. Omloo, W. P. F. A. M. and Jellinek, F., J. LessCommon Metals 20, 121 (1970).
7. Subba Rao, G. V., Shafer, M. W., Kawarazaki,
S. and Toxen, A. M., J. Solid State Chem. 9, 323 (1974) 8. Sernetz, F., Lerf, A. and Schijllhorn, R., Mat. Res. Bull. 9, 1597 (1974). 9. SchGUhorn, R. and Mayer, H., Mat. Res. Bull. 9,1237 (1974). 10. Normant, H., Angew. Chem. 79, 1029 (1967). 11 Fujinaga, T., Izutsu K. and Sakura, S., Nippon Kagaku Kaishi, 1973, 191. 12. Kanzaki, Y., Konuma, M. and Matsumoto, O., in preparation. 13. Whittingham, M. S., Mat. Res. Bull. 9,1681 (1974). 14. Schijllhorn, R. and L.erf, A., J. Las-Common Metals, 42,89 (1975). 15. Jones, G. and Dole, M., J. Am. Chem. Sot. 51,295O
(1929). 16. Kaminsky,
M., Disc. Faraday
Sot. 24, 171 (1957).