COORDINATION STATESOF Ni(I1)
IN
1601
MOLTENZnClrKCl
Short-Range Order in Fused Salts. I.
Coordination States of Nickel(I1) in
Molten Zinc Chloride-Potassium Chloride Mixtures'
by C. A. Angel1 and D.M. Gruen Chemistry Division, Argonne National Laboratory, Argonne, Illinois
(Received December 6,1966)
Absorption spectra of Ni(I1) in molten KC1-ZnC12 mixtures have been obtained as a function of temperature and of melt composition in the range 4000-40,000 cm-l. Spectra have been obtained over the entire composition range from pure liquid ZnClz to pure liquid Observed specKC1. The extremes in the temperature range studied were 250 to 900'. tral changes have been interpreted to indicate the existence of (a) a two-species equilibrium involving Ni(I1) in octahedral and distorted tetrahedral sites, and (b) transformation by a continuous distortion mechanism of an octahedral Ni(I1) species to a highly distorted tetrahedral species. The effect of C1- polarization by Zn(I1) and of volume changes on the coordination behavior of Ni(I1) is discussed. A simple technique for handling and weighing small samples of highly hygroscopic materials is described.
Introduction The elucidation of short-range order in fused salts on the basis of 3d-ion absorption spectra has been discussed in previous publication^.^*^ It is well known that the visible absorption spectra of these ions are strongly dependent on the ligand field in which they find themselves. Since the ligand field is determined to a good first approximation by the number and configuration of the nearest neighbor anions, the spectra observed in a given melt can provide detailed information on the coordination state of a 3d ion and can reveal the dependence of this state on the properties of the solvent. The present study will show in detail how the absorption spectrum of Ni(I1) is affected by the forces determining the short-range order in ionic melts and the means by which changes in the local order can be brought about. Spectral studies have s h o ~ nthat, ~ , ~in alkali chloride melts at high temperatures, the dipositive 3d ions are surrounded by four chloride ions in tetrahedral or distorted tetrahedral symmetry, while in the molecular AI&& melt the 3d ions are octahedrally coordinated. Of particular interest are certain intermediate situations observed, for example, in the LiCl-KC1 eutectic in the range 400-700" where temperaturedependent equilibria involving both tetrahedral and octahedral coordination states of a number of 3d ions
have been shown to occur.2 The importance of solvent composition has been illustrated by studies in A1Cl3-KC1 melts, where profound changes in coordination take place over relatively small ranges of composition. I n order to obtain more detailed information on the various factors influencjng coordination changes, we have sought other fused salt solvents in which effects due to temperature and composition could be systematically investigated. Our previous work has resulted in the recognition of the strong dependence of the coordination behavior of the 3d ions on the polarizing power of the melt cations, This suggests that one might vary the polarizing power in a controlled fashion in order to influence the energetics of coordination number changes. In particular, it would appear that the conditions for the concurrent existence of four- and six-coordinated 3d ion species in measurable concentrations are most readily fulfilled in melts whose cation charge-to-radius ratios, z + / T + , are intermediate between those of alkali ions, such as K(1) with z + / T + = 0.8 and Al(II1) with x + / T + = 6.0. Thus, MgClz ( z + / T + = 3.2), ZnClz ( z + / T + = 2.7), or (1) Based on work performed under the auspices of the U. S. Atomic Energy Commission. (2) D. M. Gruen and R. L. McBeth, Pure Appl. Chem., 6,23 (1963). (3) H.A. @ye and D. M. Gruen, Inorg. Chem., 4, 1173 (1965).
Volume 70, Number 6 May 1966
C. A. ANGELLAND D. M. GRUEN
1602
I
t
I
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I
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I
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I
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12
IO
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6
Figure 1. Spectra of Ni(I1) in KC1-ZnCl2 melts at 320'.
KCl contents are given in mole per cent.
Figure 2. Spectra of Ni(I1) in KC1-ZnCl melts at 700'.
KC1 contents are given in mole per cent.
the divalent transition metal chlorides themselves (z+/T+ = 2.7-2.4) would appear to be suitable cornponents in binary solvent systems with either AIC13 or KC1 as the second component. Of these, ZnClz is of interest, since many binary systems involving ZnClz have unusually low liquidus temperatures. Such systems therefore offer wide temperature intervals The Journal of Phywiud Chemistry
I
as well as suitable ranges of solvent cation potentials forstudy. For the purposes of the present investigation, the Ni(I1) ion is particularly advantageous since the energy separations between absorption bands in octahedral and tetrahedral crystal fields are large enough to prevent extensive overlapping. As will be seen, this
COORDINATION STATESOF Ni(I1)
IN
MOLTENZnCl2-KC1
1603
100 90 80 w 70
>
L I-
t
60
am 50 (r
U
a: 40
4
.30 20 IO
32
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IO
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cm-' x
Figure 3. Spectra of Ni(I1) in 45.3 mole % KC1-54.7 mole % ZnC1Z melt as a function of temperature in the range 250-700O.
circumstance enables one to draw important distinctions concerning the manner in which the Ni(I1) configuration changes in the ZnCI2-KC1 system chosen for the present study.
Results Spectra of Ni(I1) in the ZnCl2-KCl system are shown as a function of composition in Figures 1 and 2 and as a function of temperature at fixed compositions (45.3, 52.5, and 58 mole yoKC1) in Figures 3-5. Spectra at 34, 60, 68, 80, and 90 mole % KC1 have also been studied as a function of temperature and will be referred to in the Discussion section. The spectra given in Figures 1-5 were selected because they best illustrate the important features encountered in this system. Measurements at 320" (Figure 1) and at 700" (Figure 2) show that the spectra of Ni(I1) in molten ZnC12-KC1 depend in a complicated way both on the composition and temperature of the solvent. For example, in going from pure ZnClz to -6.5 mole % KC1, pronounced spectral changes occur at the fixed temperature of 320". Presumably, the structure of pure liquid ZnClz changes drastically on addition of small amounts of KC1, and these changes are reflected in the Ni(I1) spectrum. A detailed discussion of the spectra in this composition range will be given in a subsequent paper. The spectral changes observed at 320" on further addition of KC1 (up to 45 mole %) can be described as due primarily to a reduction in the intensity of the 6.5 mole yo spectrum. At still higher
KC1 contents (52.5 mole %), the spectrum shifts to lower energies and the oscillator strength increases. On the other hand, the spectrum of Ni(I1) in liquid ZnCL at 700" (curve marked 0 in Figure 2) is affected only to a minor extent by KC1 additions up to 45 mole %. These observations are in accord with other evidence4J which shows that ZnClz is much less associated at 700" than a t 320". Comparisons of 320 and 700" spectra a t fixed compositions reveal the pronounced effect of temperature on the energies and intensities of the absorption maxima. Temperature effects are presented in greater detail in Figures 3-5 for the fixed compositions 45.3, 52.5, and 58.0 mole yoKCl. The spectra at fixed compositions can be divided into two classes. The first class extends over the composition range 0 to about 50 mole % KC1. Isosbestic points are absent from this class of spectra as shown by the 45.3 mole % KC1 composition (Figure 3). The spectra shift to lower energies with increasing temperature. However, the intensities may increase or decrease with temperature depending on composition. (Compare Figures 1 and 2.) The second class of spectra which can be studied over the composition range 52-58 mole yo KC1 is exemplified by the 52.5 and 58 mole yo compositions (4) F. R. (1957). (5)
Duke and R. A. Fleming, J. Wectrochem. SOC.,104, 251
D. E. Irish and T. F. Young, J. Chem. Phys., 43, 1766 (1965).
Volume 70, Number 6 May 1.966
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C. A. ANGELLAND D. M. GRUEN
Figure 4. Spectra of Ni(I1) in 52.5 mole % KC1-47.5 mole % ZnClz melt as a function of temperature in the range 250-900".
'"I 90
Figure 6. Phase diagram for the KC1-ZnCl2 binary system. Figure 5. Spectra of Ni( 11) in 58 mole % KC1-42 mole % ZnClZ melt as a function of temperature in the range 255-700".
shown in Figures 4 and 5, respectively. These spectra are characterized by the presence of an isosbestic point for each composition. With increasing temperature, the absorption maxima decrease in energy while increasing in intensity. The 250-400" temperature region in which many of the spectral changes described in the foregoing paragraphs occurred is not available for study a t compositions with KC1 contents greater than -58 mole yo. This is due to the increase in the liquidus temperature as shown in Figure 6, which gives the KC1ZnC12 phase diagram determined by Shatilo and Ugai.g The J o u r d of Physical Chemistry
At the higher temperatures accessible to measurement in the 60-100 mole % KC1 region, the effects of temperature on the spectra are not very pronounced. Furthermore, the appearances of the spectra do not alter greatly with composition in the range 60-100 mole % KC1.
Discussion Two of the spectral types seen in Figures 1-5 have been identified in previous work2 on crystals of known structure as due to Ni(I1) ions in either octahedral or tetrahedral chloride ion environments. In Figure 7, the spectra of Ni(I1) in molten 68.4% KC1-31.6% ZnClz at 550" (curve lb) and in molten 45.3% KC1~
(6) V. A. Shatilo and Y'a A. Ugai, Zh. Fiz. Khim., 23, 744 (1949).
COORDINATION STATES OF Ni(I1)
IN
1605
MOLTENZnClzKCl
room temperature are annealed a t a higher temperature. The peaks in the region 12,000-19,000 cm-1 in quenched crystals (Figure 8) are apparently due to Ni(I1) in a severely distorted tetrahedral site. The distorted tetrahedral spectrum of Ni(I1) in KZnzC15 is quite distinct from that of Ni(I1) in CszZnC14or K2ZnCl4in which distortions from tetrahedral symmetry are
Figure 7. Comparison spectra of Ni(I1) in melts and crystals. l a and 2a are crystal spectra taken a t 550" of Ni(I1)-doped CszMgClr and CsMgCls, respectively. l b and 2b are melt spectra in 68.4% KC1-31.60j, ZnCls a t 550' and in 45.3% KC1-54.7% ZnClz a t 250', respectively.
54.7% ZnClz at 250" (curve 2b), are compared with the 550" spectra of Ni(I1) in the crystal lattices of CszMgCh (curve la) and CsMgCla (curve 2a), respectively. Since in theat crystals, Ni(I1) is known to substitute for Mg(I1) at sites of essentially undistorted tetrahedral and octahedral symmetry, it is reasonable to conclude that the same configurations account for the melt spectra. However, not all the spectra shown in Figures' 1-5 can be resolved in terms of these two coordination states. At least one other Ni-C1 coordination state is required to permit detailed discussion of the spectral changes which are observed on varying the temperature and composition of the melts. One possible configuration is square-planar NiC142-, and to obtain its spectrum in crystals, we attempted to substitute Ni(I1) into the square-planar site of K2PtCl4. These attempts were unsuccessful. The spectral identification procedure based on comparison of melt spectra with s.oectra in host lattices of known structure therefore cannot as yet be employed for the squareplanar NiC14!!- configuration. A crystal spectrum different from either the octahedral or tetrahedral spectrum of Ni(I1) has been obtained, however, by quenching a 213 mole % KC1-67 mole % ZnClz melt containing a low concentration of Ni(I1). This melt has the composition of the congruently melting compound KZnzC16 which occurs in the phase diagram (Figure 6). The visible absorption spectrum of the crystalline compound doped with Ni(I1) is shown in Figure 8. The absorption band at 22,300 cm-l is due to Ni(I1) in an octahedral site and is in fact the only absorption band observed if the melt is allowed to solidify slowly or if crystals obtained by quenching to
The principal new feature of the Ni(I1) spectrum in KZnzCls is the appearance of an absorption peak at 16,600 cm-'. Unfortunately, the crystal structure of KZnzCl5is not known, but it is reasonable to suppose that in this compound, Zn(I1) retains tetrahedral symmetry by linking tetrahedra through chlorine bridges. This is made plausible by the fact that the Co(I1) spectrum in KZnzC15is that of the well-characterized undistorted tetrahedral CoC14*- configuration. Because Ni (11) gains considerably more crystalfield stabilization energy by distorting than does Co(II), it is not unexpected to find the site symmetry about Ni(I1) to be highly distorted. Apparently, the lattice energy of the compound KZnzClj is such as to allow distortion to occur whereas little distortion can occur in the case of a substitutional Si(I1) impurity in the KzZnC1,lattice. The configurations responsible for the high-temperature spectra of the 45.3 mole % KCl (Figure 3 ) and 52.5 mole yo KC1 (Figure 4) compositions are seen to transform to an octahedral configuration at temperatures below 300". I n the 58 mole % KCl melt (Figure 5) a transformation similar to that seen in Figures 3 and 4 appears to occur but is not complete at the lowest temperatures at which measurements can be made at this composition. An interpretation of these spectral changes in terms of different Yi(I1) species will now be given. It is easily shown that an equilibrium among two distinct light absorbing species results in an isosbestic (constant absorption) point at the wavelength at which the molar absorbances of the two species are equal. Isosbestic points at 19,000 cm-' are, in fact, seen in Figure 4 between 320 and 600" and in Figure 5 between 255 and 600°, respectively. In Figure 3 (45.3 mole % KCl), however, no evidence of an isosbestic point is found. It appears that here, and also a t higher ZnClz contents (spectra not shown), the transformation occurs as a result of a continuous distortion of the chloride ion environment of Ni(I1) with changing temperatures. Which of these two situations occurs depends, of course, on the energies of the states of intermediate configuration relative to those of the end (7) D. M. Gruen and R. L. McBeth, J . Phys. Chem., 6 3 , 393 (1959).
Volume 70,Sumber 6
May 1966
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members. Presumably, a two-species equilibrium occurs if intermediate configurations are of higher energy, i e . , if there is an energy barrier to be crossed in the transformation. An interesting feature of the continuous distortion spectra is that the spectral bands of the intermediate configurations are no broader than those of the end mernbers. Qualitatively, this can be interpreted to imply that the free energy along the distortion path changes rapidly with respect to kT so that virtually all of the Ni(I1) ions in the melt have the same intermediate configuration. One may regard the additional broadness of the peak at 500” (Figure 3) as the result of a flattening of the free-energy path, resulting in :L limited spreading out of configurations. Somewhere between 45.3 and 52.5% KC1 this “flattening-out” of the free-energy path must pass over to a “col” separating “valleys,” resulting ultimately in a two-species equilibrium. It thus appears that the tmo-species equilibrium region should be preceded on either side of its compositional range by regions of progressive distortion. Indications of the presence of such regions may, in fact, be seen in the spectral changes between 250 and 320” and between 700 and 900” in Figure 4. As noted above, the spectra (Figures 3 and 4) in melts at temperatures below 300” show that Ki(I1) ends up in an octahedral configuration. It should be pointed out that the energy of the most intense octahedral band (which is presumably an inverse function of the Ni-C1 distance) changes slightly with the temperature and with composition of the solvent. The spectra of Ni(I1) in high-temperature melts are also clearly dependent on the solvent composition. The difference between the high-temperature spectra in Figures 4 and 5 lies in the relative intensities of the peaks at 1600 and 1720 cm-l. These high-temperature spectra differ from that of the undistorted tetrahedral species only in the presence of the 1720-cm-’ peak. Their similarity to the crystal spectrum of Figure 7 is obvious. The absorption at 1720 cm-1 of the “distorted tetrahedral” species decreases as the KC1 content of the solvent increases, ie., as the composition approaches the K2ZnC14composition at 67% KC1. The manner in which the measurements shown in Figure 5 were obtained is of interest to the understanding of the spectra involving a distorted tetrahedral species. The liquidus temperature for this composition (589;/, KCI) IS 380” (Figure 6). However, in a remarkable instance of supercooling, the spectral changes could be followed, over a period of more than 1 hr, down to a temperature of 255’. At this temperature the sample suddenly crystallized to a homogeneous translucent mass. Inspection of the phase diagram The J O U T of ~ Physical Chmistry
C. A. ANGELLAND D. M. GRUEN
0.5
r-----l
x)
28
8
I
I
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I
1
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Figure 8. Room-temperature spectrum of Ni(I1)-doped KZntC15. Sample obtained by quenching from the melt.
(Figure 6) shows that this is the temperature at which the peritectic compound KsZn2C1, (60 mole % KCl), becomes thermodynamically stable. It is tempting to speculate that a substantial energy barrier hinders ’ KC1) phase nucleation of the K2ZnC14 (67 mole % while there is no energy barrier to the nucleation of the K3Zn2C17phase in the 58 mole % KCl melt. The structure of KzZnC14contains isolated ZnC142- ions. Since liquid of composition 2ZnC12.3KC1 contains insufficient chloride to form ZnCldZ-,there may be a tendency to form Zn2Ch3-by corner linking of tetrahedra. Although there is no direct spectroscopic evidence, it is not unreasonable to suppose the existence of Zn2C173-inthe liquid.8 The energy barrier to nucleation of the K2ZnC14phase on this view would involve the Zn-C1-Zn bond energy. We therefore suggest that the spectrum of the 58% KCl high-temperature end member is due to Ni(I1) with a tetrahedral chloride environment distorted as a result of being corner linked to another tetrahedron. For statistical reasons, this will almost always be a [ZnCld] tetrahedron. Presumably, the Ni(I1) in such an environment is closer to the three unshared ligands than to the fourth shared ligand. At higher ZnC12contents, there will be a tendency to share additional corners. There can, of course, also be sharing of edges or faces. The varying degrees of distortion from tetrahedral symmetry to be expected on the basis of these considerations are presumably responsible for the complicated spectral changes Peen in Figures 1-5. The change from a two-species equilibrium to a continuous distortion transformation encountered in (8) It may be noted that Raman spectra studies of this system [see, e . g . , J. R . Moyer, J. C. Evans, and G. Y - S . Lo, J . Ekctrochem. Soc., 113, 158 (1966)l have so far been interpreted without the postulation of an average Zn-C1 coordination state represented by the ZnzCh- formula, near the 2ZnCIz3.KC1 composition, although the linking of [ZnCla] tetrahedra by common chlorides has been used in previous interpretations of pure molten ZnClz Raman spectra.6
COORDINATION STATESOF Ni(I1)
IN
MOLTENZnC12-KC1
the present study can help to provide a better understanding of charge distribution in ionic melts. With increasing ZnClz content beyond the 60% KC1 composition, the Ni(I1) ions are increasingly coordinated to chloride ions which are already members of [Zn,Cl,] tetrahedral groupings. Polarization of the chloride ions by the zinc ions lowers the charge density on the chloride ions, thus decreasing the ligand repulsion energies which tend to give symmetrical configurations. For this reason, distortions due to ligand field stabilization energies tend to become more pronounced with increasing ZnClz content, a trend seen, for example, in comparing the high-temperature spectra of Figures 3 and 4. Distortions appear to be energetically more favored in the liquid than in the crystal lattice (Figure 8) and are seen to depend sensitively on the balance between crystal field energies and electrostatic repulsion energies of the ligands. The ligand charge density at the 43 mole % KC1 composition is apparently small enough for the Ni(I1) site to be distorted to such an extent that the energy barrier between the distorted tetrahedral site and the octahedral site is negligible. As observed above, this is the condition which permits the continuous transformation mechanism exemplified in Figure 5 to occur. Support for this conclusion may be derived from the fact that in LiC1-KC1 solutions, where the chloride ligands are less polarized than in the ZnClz-coiitaining melts, an octahedral-tetrahedral equilibrium has been observedg which can be shown to involve an almost undistorted tetrahedral species. It remains to comment on the reasons for the change of configuration and coordination number with temperature. The simplest view is that the octahedral configuration is a result of the 16-kcal octahedral site stabilization energy.2 The decrease of the coordination number with increasing temperature is due to the addition of therma! energy which overcomes the crystal field energy and allows the anion-ligand repulsion energies to assert themselves. However, not only the internal energy of the system but also its volume changes with temperature. The effect of the volume factor on the coordination state of an ion is elucidated by the following argument. It is well known that the increase in volume on fusion of ionic crystals is the result of a decrease in coordination number rather than an increase in nearest neighbor separations’ This ’s taken to indicate that proportion O f holes, vacant quasi-lattice sites, or, less specifically, an amount of free volume has been intraduced into the structure. If the temperature is raised, the liquid expands and, as the internuclear distances do not appear to increase significantly, the expansion is presumably accomplished by an imm?aSe
’
1607
in “hole” or “free” volume. If the number of holes, as well as their mean size, increases, then the coordination number must decrease further. Conversely, if the free volume is reduced by lowering the temperature (or increasing the pressure), the coordination number must increase, If the crystallization temperature is reduced either by supercooling or by adding a second component, the tendency to increase the coordination number should be maintained unless the properties of the liquid change in some unexpected way. I n practice it is found that if crystallization is severely inhibited, the liquid eventually becomes a glassy solid at a density slightly less than that of the corresponding crystalline solid, It is possible to relate this transformation to the effective disappearance of free volume or hole volume from the liquid. The coordination number should, therefore, reach a maximum value, approximating that of the crystalline material, at the glass transition temperature. These considerations are consistent with the fact that the present system, in which Ni(I1) has for the first time been observed to undergo a complete change from four coordination to six coordination with changing temperature, is one in which the liquidus temperatures are unusually low, and in which a glass transformation is known to occur.12 Indeed, these properties of the solvent system influenced its choice for this study. In conclusion, we note that, except for the low-temperature results, the principal spectra observed in this study may be matched by spectra observed in previous studies in different solvents such as alkali halides7~~~* LiC1-KCl12~9* and MgC12-KCl.13b The effects discussed above and their structural consequences are therefore apparently of general interest, rather than being specific to the ZnCGKC1 system. It is hoped that further studies with other 3d ions in these low-melting solvents, and also optical studies at pressures up to 2 kbars, will throw further light on the factors determining local structure in melts. (9) (a) C. R.Boston and G. P. Smith, J. Phys. Chern., 62,409 (1958); (b) C. K.J@rgensen,Mol. Phys., 1, 410 (1958). (10) D.Turnbull and M. H. Cohen, J . Chem. Phys., 34, 120 (1961). (11) It is probably more fundamentally correct, and is even more intuitively reasonable, to relate the glass transformation to the effective disappearance from the liquid of configurational entropy, as suggested by J. H. Gibbs and E. A. Dimarzio [ibid.,28, 373 (1958)l. The configurational entropy in a fused salt must be intimately related to the free volume, and the views expressed here could be rephrased in terms of the loss of entropy in passing from a fourcoordinate state t o the more ordered six-coordinate octahedral configuration. In this view, however, the volume change itself becomes a reflection of the changing entropy content of the liquid. (12) I. Schulz, Naturwiss., 44, 536 (1957); C. A. Angel1 J . P h w Chem., 68, 1917 (1964). (13) (a) G. P. Smith and C. R. Boston, J . Chem. Phys., 43, 4051 (1965); (b) D.M. Gruen and H. A. Bye, unpublished results.
Volume 70,Number 6 M a y 1966
C. A. ANGELLAND D. M. GRUEN
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Experimental Section Materials. AR grade KC1 and, separately, AR grade ZnClz containing some water were fused under dry HC1 gas and, in the case of ZnCL, HCl gas was passed through the melt for 1-3 hr to ensure the removal of all water. After filtration, the melts were pelletized by dropping into liquid nitrogen held in storage vessels which were subsequently sealed after the Nz had boiled off. Procedure. As our current program requires the variation of composition in binary systems where both components may be hygroscopic, we have developed a convenient technique for transferring weighed quantities of beads of hygroscopic salt from storage vessel to cell without recourse to drybox handling. This system, which will be described below, is so simple to use that the potential difficulty of changing composition in doubly hygroscopic binary systems is no obstacle to the detailed examination of composition effects in such systems. I n the present work, the initial survey was made in two series of runs, one commencing with pure ZnC1, doped with NiC12 (-lo-* M ) to which KC1 beads were systematically added up to 45.3 mole % KCI, and the other commencing with pure KC1 plus NiCh, to which ZnCl, beads were added until the KCl content had been reduced to 52.5%. The total Ni in each series was determined by analysis after the completion of the series. The composition region of special interest, 52.&W% KC1, was examined in greater detail in a subsequent series. Spectra of the melts were taken using a Cary 14H spectrophotometer and furnace design which have been described in previous publications." The spectra of solidified melts were taken using a Cary 14 ambienb temperature spectrophotometer. Flat sections of the solid salt were used, as the intensities were not sufficient to use KBr pellet techniques successfully. Density. Density data necessary to obtain molar absorptivities from the optical densities of the solutions were obtained by use of a simple quartz densitometer. The data, and derived information on molar volume changes in this system, will be reported elsewhere. Technique for Transferring Weighed Samples of Hygroscopic Materiak The system used in the transfer of beads from storage vessel, A, to weighing bottle, XY, is illustrated in Figure 9. A similar system is used to complete the transfer of weighed material to the optical cell. The principle of the transfer technique lies in being able to open and close the weighing bottle, Figure 9a, by rotating the cap X, relative to the body, Y, and thus to align or disalign the
Ibl
Figure 9. Apparatus for transfer of dry ZnCI2 beads from storage vessel to weighing bottle. Wire-mesh basket suspended from cap B of vessel A contains silica gel or molecular sieve.
two holes in the cone and sleeve as illustrated. Since this action can easily be performed from outside a sealed system, a dry atmosphere can be maintained during the transfer process. The procedure is as follows. The weighing bottle XY, with the holes aligned, is placed in the antechamber section E, and the porthole is closed with the cap F. The square ends of the weighing bottle are now located in simiiarly square recesses in the antechamber E, and cap F is located so that rotation of cap F about the sleeve joint opens or closes the weighing bottle as required. The antechamber E is then attached to the vessel A at the sleeve joint D and secured by springs. By operating the two-way tap G, which connects the antechamber to a vacuum pump and to a source of dry nitrogen gas, the antechamber is evacuated and refilled with dry nitrogen several times. The widebore Teflon tap C on the side arm of the storage vessel A has been kept closed up to this point. The approximate number of beads required for the desired composition change is now tipped into the side arm and transferred to the weighing bottle by opening the tap C. The weighing bottle is then closed by a rotation of antechamber cap F, tap C is closed, and the weighing bottle may be removed through the porthole and transferred to the balance for exact weight determination. A device similar to the antechamber section is then used to transfer the beads from the weighing bottle to the melt in the optical cell. The only additional (14)
D.M.Gruen and R. L. MeBeth, J . PAW. C b . . W,57
(1962).
COORDINATION STATESOF Ni(I1)
IN
MOLTENZnClTKCl
feature is the connection of this device to the cell by means of a ball joint. This is necessary because the cell is usually in a vertical position in the furnace when the addition is made, so that a means of tipping the weighing bottle to let the beads run out must be provided. Using ZnC12 glass beads, it is easy to detect shortcomings in transfer technique, since the beads are perfectly clear when kept dry, but quickly “frost” over on the surface on contact with traces of moisture. With due care it was possible to carry out the transfer from storage vessel to optical cell without any “frosting” being observed. As these transfers would normally be performed by drybox handling and weighing techniques, i t is notable that to prevent the occurrence
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of a similar frosting of their ZnC12 glass samples, Goldstein and Nakonecanyi16 found it necessary to first flush out their drybox for at least 3 days. ZnClz is an extremely hygroscopic substance so that the efficacy of the system has been proven under severe conditions. The use of beads, which are easy to prepare, is generally to be recommended in the manipulation of small quantities of salts, as the transfer operations are cleaner and more direct, and 100% transfer of weighed materials is always attained. (15) M. Goldstein and M. Nakonecznyi, Phy8. Chem. GhSSe8, 6 , 126 (1966).
Volume 70, Number 6 May 1066
