2950 Inorganic Chemistry, Vol, 16, No. 11, 1977
A. Lerf and R. Schollhorn
Contribution from the Zentralinstitut fur Tieftemperaturforschung, D-8046 Garching, West Germany, and the Anorganisch-chemisches Institut der Universitat, D-4400 Munster, West Germany
Solvation Reactions of Layered Ternary Sulfides AxTiS2, AxNbS2, and A,TaS2 A. LERF and R. SCHOLLHORN* Received April 6, 1977
AIC702415
Layered ternary phases A,MS2 (A = alkali metal; M = Ti, Nb, Ta) undergo topotactic solvation reactions at ambient temperature in inorganic and organic solvents of adequate polarity. The reaction products are characterized by their polyelectrolyte behavior; they are built up by negatively charged two-dimensional [MX2]’- lattice units with mobile and exchangeable solvated cations in the interlayer space. Two well-defined hydration stages are found in the case of water as the solvent for alkali and alkaline earth cations. They are described in terms of a model which is based on the hydration properties of the cations. A series of known intercalation compounds of the binary transition metal dichalcogenides is shown to be closely related to the solvated ternary phases with respect to basic structure and chemical reactivity. The compounds investigated represent a novel class of electronically conducting crystalline polyelectrolytes, whose physical properties can be varied by topotactic exchange reactions at room temperature. solid salt matrix with water the corresponding hydrated phases were Introduction obtained. Analysis was performed by flame spectrophotometry (alkali Ternary chalcogenides of transition metals with strongly and alkaline earth cations) and wet methods (transition metal and electropositive elements, Le., alkali and alkaline earth metals, sulfur). All reactions with these compounds were carried out at room are often characterized by highly anisotropic structures, the temperature under an inert atmosphere. Crystallographic data were transition metal and the chalcogen forming frequently distinct obtained from polycrystalline samples and single crystals by powder one- or two-dimensional lattice elements. This has to be and Weissenberg methods using Cu K a radiation. For the investigation explained in terms of the fact that the bonding between the of fully hydrated phases the preparations were kept-if not otherwise transition element and the chalcogen atoms tends to exhibit stated-in contact with 1 M electrolyte equilibrium solution. Aqueous sulfuric acid solutions of defined concentrations served for maintaining considerable covalency in character, whereas the electropositive defined water vapor pressure over polycrystalline samples in closed group 1A and 2A elements prefer ionic bonding. The resulting vessels. In order to reach the true equilibrium states it was necessary “structural segregation” leads in many cases to chain and layer to store the compounds for several days or weeks under these contype compounds, e.g., KFeS2’or A,TiSZa2The anisotropy with ditions. Cation exchange was performed by treating the samples respect to bonding and structure affects not only the physical repeatedly with 1 M solutions of halides or sulfates of the cations to properties but also the chemical reactivity of these solids. We be introduced until quantitative reaction was achieved. For the were able to show in previous studies that many of the layered preparation of organic solvated phases the hydrates or unhydrated ternary compounds AXMA,,,(A = group 1A or 2A metal, M ternary phases were treated with an excess of the purified and dried = transition metal, X = chalcogen) may easily undergo organic reagent. Colloid solutions were formed by high-speed stirring solvation reactions with polar inorganic and organic l i q ~ i d s ~ , ~ of suspensions of the solvated compounds in the corresponding liquid phase. Preparations of solvated phases by cathodic reduction of the according to eq 1 All these reactions proceed in a topotactic A,M,X,
ty(s01v)
-).
Atx(solv)y [M,X,IX-
(solv = solvent) (1)
way: the polar molecules are taken up into the interlayer space solvating the electropositive ions; the transition metal chalcogenide layers remain unchanged as negatively charged matrix elements. As a consequence of the high mobility of the solvated cations in the interlayer space ion-exchange reactions can be performed with ambient electrolyte solutions. The solvated phases are thus a novel class of inorganic polyelectrolytes which exhibit semiconductor or metallic
proper tie^.^ The ternary compounds of group 4B and 5B metals are of special interest with respect to their high reactivity and relative stability; the phases known so far are nonstoichiometric compounds with large phase rangeO2v5Recently we reported on a method to prepare ternary chalcogenides of these elements with defined stoichiometry from alkali halide meltsa6 We found that the corresponding hydrated forms of N b and Ta phases are superconducting, the transition temperature T, being dependent on the nature of the exchangeable cations and on the solvate phasea7 A reliable interpretation of the physical properties of these compounds requires a thorough understanding of the chemical properties and stability as well as the knowledge of the detailed structures of these phases. In this communication we present our results on a study of the solvation properties of the compounds Ao.5TiSz,Ao.5NbSz,and Ao.3TaS2. Methods The ternary phases A,MX2 were prepared from alkali halide melts according to the procedure described previously.6 On removal of the * To whom correspondence should be addressed at the Anorganischchemisches Institut der Universitat, D-4400 Miinster, West Germany.
binary chalcogenides and by reaction of alkali metal intercalation compounds with solvents were described in earlier reports.3 4 8 13,19 8
8
8
Results and Discussion Hydrated Phases. On contact with water vapor or with liquid water, the ternary alkali phases Ao,5TiS2,A. 5NbS2,and Ao,3TaS2undergo spontaneous hydration according to eq 1. The water molecules are taken up into the lattice and crystalline hydrates A+,(H20),[MX2]“ are obtained. The reaction is a topotactic process which can easily be demonstrated by investigating single crystals of the ternary phases which turn over to single crystals of the hydrated compounds. All phases can be indexed on a hexagonal base; the lattice parameters of the alkali series are given in Table I. As a result of the hydration process, the interlayer spacing d between successive MX2 sheets increases, whereas the a-axis values remain almost constant. A comparison with the a-axis parameters of the corresponding binary phases Tis2 (a = 3.40 A), 2H-NbS2 (a = 3.31 A), and 2H-TaS2 ( a = 3.32 A) indicates that the deformation of the layers is very small. The metallic appearance of the ternary phases is retained in the hydrated forms, the colors observed were golden yellow for Ti compounds and blue to gray for N b and Ta compounds. Water content found analytically for Kt and Na’ phases corresponded to y N 0.7 (K), y N 1.8 (Na) for Ta hydrates and y N 0.5 (K), y N 1.6 (Na) for N b and Ti hydrates. From the data given in Table I it is seen that two sets of interlayer spacings are found corresponding to ca. 9 A and ca. 12 A. In order to be able to compare the values from the Ti, Nb, and Ta series it is necessary to account for the small differences in the thickness of the chalcogenide layers: the use of the interlayer distance doof the binary phases Tisz (5.71 A), NbS2 (5.95 A), and TaS2 (6.05 A) as the van der Waals
Inorganic Chemistry, Vol. 16, No. 11, 1977 2951
Solvation Reactions of Layered Ternary Sulfides
Table I. Hexagonal Lattice Constants, Interlayer Spacings d (A), and Interlayer Heights Ad (A) of Hydrated Alkali Phases
a
C
d
Ad
a
c
d
Ad
a
c
d
Ad
3.38 3.43 3.45 3.45 3.46 3.46
11.40 11.43 8.86 8.83 8.91 9.19
11.40 11.43 8.86 8.83 8.91 9.19
5.69 5.72 3.15 3.12 3.20 3.48
3.35 3.34 3.35 3.34 3.33 3.34
22.82 23.37 17.81 17.95 17.96 18.36
11.41 11.69 8.91 8.98 8.98 9.18
5.46 5.74 2.96 3.03 3.03 3.23
3.33 3.33 3.35 3.34 3.33 3.32
23.29 23.63 18.18 17.94 18.18 18.53
11.65 11.82 9-09 8.97
5.60 5.77 3.04 2.92 3.04 3.22
A+
LP Na" K+ NH,'" Rb' cs+ a Obtained by
9.09 9.27
cation exchange.
Table 11. Interlayer Spacings d (A) and Interlayer Heights Ad (A) of Hydrated Alkaline Earth Phases AZtx1, (H,O), [MS,IX' Obtained by Cation Exchange from the Corresponding Sodium Compounds M = Ti M=Nb M=Ta A2+
d
Ad
d
Ad
d
Ad
Mgz+
11.94 11.68 11.73 11.81
6.23 5.97 6.02 6.10
11.74 11.77 11.84 11.90
5.79 5.82 5.89 5.95
11.98 11.88 11.95 11.99
5.93 5.83 5.90 5.94
12
11
~~
Caa+ SI,+ Baa+
10
9 ...I I d
.I.... ... A?
AxMXz
A;
(H2O)y [MX21X-
A> (H?O)z[ M X d X -
Figure 1. Scheme of hydration reaction of ternary layered chalcogenides A,MS2 (delocalized negative charges on the MX2 layers are omitted).
height of the layers is a reasonable assumption as was proved in earlier investigations. If we now calculate the difference Ad = d - do for the compounds listed in Table I, we arrive at values for the height of the space between the dichalcogenide layers, With the exception of Cs+, whose diameter is significantly larger than that of water, these distances range between 2.9 and 3.2 8, and between 5.6 and 5.8 A; they obviously compare rather well to once and twice the van der Waals diameter of a water molecule (ca. 2.8 A). We found similar values before on related layered hydrates; they have been interpreted by us3'498in terms of a structural model which assigns monolayers of water to hydrates with Ad of ca. 3 A and bilayers for hydrates with Ad of ca. 5.5-6 A; Le., the size of the water molecules is obviously responsible in both cases for the interlayer distance observed (Figure 1). In the case of bilayers of water we were able to show earlier by onedimensional Fourier synthesis that the cations are located in the central plane between the two H 2 0 sheet^.^ Hydrates with bilayers of water are also formed by the alkaline earth derivatives A2+,/2(HzO),[MS2]x-(A = Mg2+,Ca*+,Sr2+,Ba2+; M = Ti, Nb, Ta) which we obtained from the alkali forms by cation exchange. Interlayer distances are presented in Table 11. In order to explain the two distinct sets of interlayer distances observed for the hydrates it is necessary to elucidate the role of the exchangeable cation. In the alkali series a simple relationship is apparent: all hydrates of cations with ionic radii > 1 A show interlayer distances corresponding to monolayers of water, whereas bilayers are found for cation radii
