J. Phys. Chem. 1984, 88, 417-420
417
Unit Cell Correlation Effects in LiNaSO, and LiKSO, Roger Frech* and Dale Teeterst Department of Chemistry, University of Oklahoma, Norman, Oklahoma 73019 (Received: June I , 1983)
The polarized Raman spectra of the internal optic modes of single-crystal LiNaSO, and LiKS0, are measured from 25 "C to temperatures above the phase transition. The observed temperature dependence of the modes in LiNaSO, is attributed to a breakdown of correlation coupling in the unit cell due to the increasing orientational disorder of the sulfate ions and is compared to the behavior observed in LiKS0,. The effect of the lithium ion-oxygen atom interactions on the frequencies of the vibrational multiplet components is discussed.
Introduction The high-temperature phase of lithium sodium sulfate has been described as a plastic phase,' that is, a phase characterized by extensive orientational disorder of the sulfate ions.2 This phase also exhibits fast ion conduction, or superionic cond~ctivity,~ and it has been suggested that the ionic conductivity of the cubic phase is dynamically enhanced by rotational motion of the sulfate ions4 The temperature dependence of the modes primarily involving the motion of the sulfate ion in the room temperature phase provides useful information about the sulfate ion in the hightemperature phase, particularly as the phase transition temperature is approached. In addition, information about the motion of the sulfate ion may in turn provide some insight into the problem of the dynamics of the mobile ion species in the plastic phase. The Raman spectra of the low-frequencymodes which originate in the motion of the sulfate ions have been measured at various temperatures in LiNaSO, and LiKSOe5 In this paper the Raman spectra of the high-frequency internal optic modes of LiNaS0, are reported at various temperatures and compared with similar modes in LiKSO,. The high-temperature phase of LiKSO, is not a plastic phase and does not exhibit fast ion conductivity,providing a useful counterpart to LiNaSO, in a comparative spectroscopic study. Lithium sodium sulfate crystallizes in the P31c (G,) space group with six molecular units in the hexagonal ce11.6 The sulfate groups are located on the threefold axes on three sets of crystallographicallyinequivalent sites, with one pair of ions on Wyckoff a sites and the remaining pairs on two sets of Wyckoff b sites. The lithium and sodium ions are reported to be in general positions. The irreducible representations of the internal optic modes are rht= 9A1 9A2 18E. The Al and E modes are both infrared and Raman active, while the A2 are not optically observable modes. At 518 OC LiNaSO, is reported to undergo a transition into a body-centered cubic phase;' however, in this study the phase transition is observed at 530 O C . Lithium potassium sulfate forms a bimolecular unit cell in the P63 (c",)space group.8 Both the sulfate ions and the lithium ions are found on Wyckoff b sites of threefold symmetry while the potassium ions are located on Wyckoff a sites. The irreducible = 3A 3B representations of the internal optic modes are rint + 3E1 + 3E2. The A and E, modes are Raman and infrared active while the E, modes are only Raman active and the B modes are not optically observable. At 436 OC LiKSO, undergoes a transition into an orthorhombic phase.9
+
+
+
Experimental Section Single crystals of LiNaSO, were grown by slow evaporation from an aqueous solution at 70 OC containing equimolar proportions of Li2S04.H20and Na2S04 to which sulfuric acid had been added to bring the solution to pH 2. Single crystals of L i m o 4 were grown by slow evaporation from an aqueous solution at 32 OC containing equimolar amounts of Li2SO4-H2Oand K2S04. Current address: Department of Engineering and Applied Sciences, University of Tulsa, 600 S. College Av., Tulsa, OK 74104.
Raman spectra were recorded on a Spex Ramalog 5 spectrometer with a 3-cm-' spectral bandpass by using the 488.0-nm line of an argon ion laser for excitation at 600 mW. The Raman spectra at temperatures below 25 "C were obtained by using an Air Products Displex Model CSA-202E closed-cycle refrigeration system equipped with a DMX- 1E vacuum shroud. Temperatures could be controlled to fO.l "C. Raman spectra at temperatures greater than 25 OC were recorded with the crystal in a hightemperature cell similar to that described by Quist,Iobut modified for compatibility with the optics of the Spex Raman system. With this apparatus the temperature could be controlled to within 1 OC. Results Lithium Sodium Sulfate (LiNaSO,). A symmetry-based vibrational analysis of LiNaS04 predicts that there should be three internal optic modes originating in the v 1 intramolecular motion of the sulfate ion." The frequencies of these modes at room temperature are 970, 999, and 1028 cm-'. The frequencies reported here differ by 1 or 2 cm-' from those in ref 11 because the crystals used in this study have been annealed by slow heating to 300 OC and slowly cooled resulting in small frequency shifts. The temperature-dependent Raman scattering spectra of these modes are shown in Figure 1. The modes are observed to broaden with increasing temperature. The frequency of the 970-cm-I mode increases to a value of 977 cm-I just before the phase transition. The mode at 999 cm-' changes very little in frequency, slowly decreasing to a value of 995 cm-' by 525 OC. The 1028-cm-I mode is the most temperature dependent, decreasing to a value of 1009 cm-I at 425 OC. At this temperature the 1028-cm-I mode has merged with the rather broad band due to the modes originating at 970 and 999 cm-' and can no longer be distinguished. The 970-cm-I mode decreases somewhat in intensity relative to the 999-cm-I mode just below the phase transition. After the phase transition occurs one broad band at 990 cm-l is observed which changes only slightly with temperature up to 625 OC. The temperature-dependent frequency data are shown in Figure 2. The v3 spectral region contains modes of both AI and E symmetry. The Raman spectra of the E factor group components of v 3 are shown in Figure 3 at various temperatures. Many of the bands in this region are weak and difficult to observe, broadening and merging with more intense components as the temperature increases. The intense band at 1136 cm-' (25 "C) ( I ) L. F. Polishchuk and A. K. Bogdanova, Russ. J . Phys. Chem., 51, 1195 (1977). (2) For an excellent collection of recent work in plastic phase crystals see "The Plastically Crystalline State", J. N. Sherwood, Ed., Wiley, New York, 1979. (3) A. Josefson and A. Kvist, Z. Naturforsch. A, 24, 466 (1969). (4) L. Nilsson, J. 0. Thomas, and B. C. Tofield, J . Phys. C, 13, 6441 (1980). ( 5 ) D. Teeters and R. Frech, Phys. Reu. B, 26, 5897 (1982). (6) B. Morosin and D. L. Smith, Acta Crysrullogr., 22, 906 (1967). (7) K. Schroeder and A. Kvist, 2. Nururforsch. A , 23, 773 (1968). (8) A. J. Bradley, Phil. Mug., 49, 1225 (1925). (9) K. Schroeder, Thesis, University of Gothenburg, Goteborg, Sweden, 1975. ( I O ) A. S. Quist, App. Specfrosc.,25, 82 (1971). (11) D. Teeters and R. Frech, J . Chem. Phys., 76, 799 (1982)
0022-3654 184 I ~ O R R - O 1~7en1 50 10 0 1984 American Chemical Societv
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
418
Frech and Teeters LiNaS04 X(Y X)Y
n
VI c
.-
C
a
1
h L
kooc
0 L
c
.-
n L
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>
-
I-
cn
z W
I-
-z
~
5 I
I
990
950
I
1030
0
c
FREQUENCY (crn" Figure 1. Raman scattering spectra of the uI region in LiNaSO,. Full scale intensity at 25 O C is 2 X 10' counts/s and at all other temperatures is 1 X los counts/s. The spectrum at 600 OC is above the phase tran-
sition.
,---
1270
1190
1110
1030
F R E Q U E N C Y (cm" Figure 3. Raman scattering spectra of the u3 region in LiNaS04. Full scale intensity is 1 X lo4 counts/s. The spectrum at 550 O C is above the
phase transition. 7
1010-
2 0
990-
W
t-
5 1000--
-
asymmetric band centered at 1108 cm-' is observed which does not seem to exhibit polarization properties. In the v4 spectral region two A, components are observed, one at 634 cm-' (25 "C) which decreases in frequency to 633 cm-l at 425 O C , and the other at 660 cm-', which decreases to 644 cm-' at 525 O C . An E component observed at 644 cm-I (25 "C) in an x(zx)y experiment does not shift in frequency from 25 to 425
D O 0 0 0
0 0 0 0
0
0
0 0 0 0 0
2
980-
2
9 7 0 ~ ~ o ' o o o o o o o o o o oO
O
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960-
-
950
I
1
8
1
I
1
1
I
I
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1
1
I
1
1
I
After the phase transition a broad symmetric band is observed in all polarizations at 629 cm-'. This band is seen at temperatures up to 825 O C with little change in frequency. In the v2 region only the E mode at 483 cm-' was studied. This mode broadens with increasing temperature and the frequency smoothly decreases to 465 cm-' at temperatures just below the phase transition. After the phase transition the band further broadens and decreases in frequency to 455 cm-I at 550 O C . The frequency then increases slightly to a value of 458 cm-' at 625 OC. An interesting generalization can be made from the preceding data. In a vibrational multiplet originating in a particular intramolecular motion of the sulfate ion, the highest-frequency A, component exhibits the largest decrease in frequency with increasing temperature. This is most obvious in the data for the u l region as shown in Figure 1 and previously discussed. In the u1 region all of the expected three Al components are observed and the conclusion is unambiguous. In the case of u3 and v4 only two of the expected three Al components are seen. However, in each case it is the highest frequency A I component of a given vibrational multiplet which is the most temperature dependent. There seems to be no obvious generalization possible about the temperature dependence of the E components within a multiplet except to note that in all cases the frequency shift in any E
Unit Cell Correlation Effects in LiNaSO, and LiKSO, TABLE I: Internal Optic Mode Frequencies (in cm-') in LiKSO, at Various Temperatures
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 419 TABLE 11: Frequencies of the ul Modes in Various Sulfate Crystals
temp, 'c
mode
25
1013 463 1118 1119 1119 623 635 635
2s 0 1010 45s 1112 1112 1114 618 631 632
space
400 998 452 1108 1109 1109 619 6 26 628 ~~
component is significantly smaller than the shift in the highest frequency A, component of that multiplet. Lithium Potassium Sulfate (LiKSO,). The polarized Raman spectra of single-crystal LiKS0, were measured at various temperatures and representative data points are given in Table I. The largest frequency shift observed in any component is the A component of ulr which decreases 15 cm-' from 25 to 400 'C. In the v 3 region the A component decreases as much as the E, and E, components, and in the u4 region the E, and E, components decrease more in frequency than does the A component over the same temperature range. In general, the temperature dependence of the internal optic modes in LiKS0, is less than that observed in LiNaSO,.
Discussion The differences in the frequencies of the various components in a particular vibrational multiplet depend on the intermolecular or interatomic forces which correlate the vibrational motion of the molecules or atoms within the unit cell. As the temperature is changed, the nature of the correlation coupling forces may change, thus affecting the observed structure of a vibrational multiplet. This is most directly seen in the u1 region of LiNaSO, as illustrated in Figure 1 where all three A, components are observed. As the temperature is increased, the highest frequency component markedly decreases in frequency, the middle component decreases only slightly, while the lowest frequency component increases in frequency. At sufficiently high temperatures the components begin to merge into one broad band with the expected thermal broadening, but the frequencies of the individual components change at different rates, suggesting that the correlation coupling is breaking down. This is most dramatic in the case of the lowest frequency A I component (970 cm-I at 25 "C) which actually increases in frequency to a value of 977 cm-I below the phase transition. The breakdown of the correlation coupling within a unit cell is also evident in the temperature dependence of the E components of u3 as shown in Figure 3. The various components display very different frequency shifts over the same temperature interval, ranging from a decrease of 23 cm-I for the mode at 1 172 cm-l to 7 cm-' for the mode at 1136 cm-l. The 1067-cm-I component decreased only 3 cm-I over a slightly smaller temperature interval. A formalism for the vibrational multiplet structure resulting from the coupling of vibrationally induced dipoles has been previously describedI2 and applied to several sulfate crystals.13 This model may be expected to provide reasonable results only in the case of internal optic modes with large dipole moment derivatives, such as the u3 components where anion-anion dipolar coupling seems to be the dominant correlation mechanism. However, when a calculation was made of the ul vibrational multiplet structure, using this formalism in an attempt to explain the unusual shifts, the results were unsatisfactory. The calculation correctly predicted the number of components, but failed to describe the direction of the shift of the lowest frequency u1 multiplet. Since this formalism only calculates sulfate ion-sulfate ion correlations, it is clear in the case of the u1 multiplet that there are significant cation-anion interactions which must be included. (12) R. Frech, J . Chem. Phys., 61, 5344 (1974). (13) G. J. Wu and R. Frech, J . Chem. Phys., 66, 1352 (1977)
crystal
group
Li,SO, LiKSO, LiNaSO,
C;h Cb
NaK,(SO,),
Cju
K2SO4
D:d Di6h
Na2S0,
D:),
vi,
ref
cm-l
1017 1012 972, 998, 1026 996 985 996
14 15 this work
13 14 13
Of course some temperature dependence of the normal-mode frequencies in an internal optic multiplet is expected through the anharmonic interactions with other phonons as well as the thermal expansion of the lattice. However, it is expected that the total effect of the anharmonic interactions and the thermal expansion should be roughly similar for each member of a vibrational multiplet belonging to the same symmetry species. Therefore the variation in the amount of frequency shift in the various E components of u3 suggests that an explanation of the observed behavior in terms of anharmonic interactions plus lattice expansion is not sufficient. More compelling is the behavior of the lowest frequency A I component of u1, since the thermal expansion of the lattice would decrease the frequency and the anharmonic contributions would also, in general, decrease the frequency with increasing temperature. However, the frequency of this component increases with increasing temperature, requiring an alternative explanation such as the breakdown of factor group correlations within the unit cell. In a previous study of LiNaSOdSa low-frequency mode at 63 cm-I originating in the sulfate ion librational motion seemed to soften as the temperature increased. The rapidly increasing vibrational amplitude associated with this mode means that the orientations of the sulfate ions within each unit cell become increasingly disordered as the vibrational motion becomes more anharmonic. This increasing disorder in turn decreases the amount of correlated vibrational motion within a unit cell, consistent with the pattern observed here in the internal optic mode multplets. The frequency shifts that occur with increasing temperature in lithium potassium sulfate (LiKSO,) appear to be somewhat different in nature than those noted in LiNaSO,. The behavior of the El and components of u3 and u, is particularly noteworthy. In the P31c space group of LiKSO, the two sulfate ions are located on sites with C3site group symmetry. The triple degeneracy of the u3 and u, intramolecular vibrations of the free sulfate ions is lifted by the lower symmetry potential energy environment of the site, resulting in an A and an E component. Since there are two sulfate ions in the primitive cell, the two E site group components are correlated through intermolecular forces, yielding an E, component and an E2 component labeled according to the irreducible representations of the factor group. Therefore the frequency difference between these two factor group components is a direct measure of the correlation forces within a unit cell. On the basis of the data summarized in Table I, there seems to be very little correlation splitting between the E, and E2 components of both u j and u4. Furthermore this splitting does not seem to dramatically change with temperature as is observed in the case of the ul multiplet in LiNaSO,. This is not to say that there are weak sulfate ion-sulfate ion correlation forces in LiKSO,; indeed an earlier calculation shows that these are appreciable.13 However, the correlation forces between the sulfate ions result in an El and an E2 component of about the same frequency. Therefore from the temperature-dependent data the correlation coupling within the unit cell of LiKS04 does not seem to break down as it apparently does in LiNaSO,. This is consistent with the failure to observe a high-temperature plastic phase in LiKS04. The sulfate ions in the LiKS04 crystal retain their orientation and hence their factor group correlation coupling to a greater extent than in LiNaSO,. The vibrational frequencies of the totally symmetric components of u , in both LiNaSO, and LiKS0, can be compared to similar quantities in other sulfate crystals containing potassium, lithium,
J. Phys. Chem. 1984,88, 420-424
420
sodium, or binary combinations of the cations. A few of these are summarized in Table 11. It should be noted that only crystals containing lithium ions have v 1 mode frequencies above 1000 cm-l, suggesting that the interaction of the lithium ion with the oxygen atom of the sulfate ion stiffens the stretching force constant of the sulfate ion, leading to a somewhat higher v 1 frequency. In this regard the three frequencies of v 1 reported here for the LiN a S 0 4 crystal are particularly interesting. The six sulfate ions in a unit cell of LiNaS04 are distributed with two pairs on Wyckoff b sites and one pair on Wyckoff a sites. The resulting coordination of a lithium ion by the oxygen atoms of nearby sulfate ions is not a perfectly regular tetrahedron as in the case of LiKS04. In LiNaS04 the lithium ion-oxygen atom distances range from 1.872 to 2.087 A. The higher frequency v, component at 1026 cm-l may, to a greater extent, involve sulfate ions which are closer to lithium ions than the lower frequency v1 components. Of course (14) V. Ananthanorayanon, Ind. J . Pure Appl. Phys., 1, 58 (1963).
(15) J. Hiraishi, N. Taniguchi, and H. Takahashi, J . Chem. Phys., 65, 3821 (1976).
each v 1 component contains some contribution from the intramolecular motion of each sulfate ion since the Wyckoff a and b sites have the same point group symmetry (C,) and hence each contribute to the AI factor group vibrations. However, this view is consistent with the striking temperature dependence of the high frequency v 1 component and the previously mentioned study of the external optic modes.s As described earlier, an A, mode at 63 cm-' attributed to the librational motion of the sulfate ions appeared to soften with increasing temperature. This would require large sulfate ion vibrational amplitudes which primarily consist of oxygen atom motion. The lithium ion-oxygen atom interactions are strongly perturbed by the increasing oxygen atom motion, and this would be reflected most strongly in the internal mode most dependent on those interactions, namely, the highest frequency A, component of the v 1 vibrational multiplet.
Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. DMR8001666. Registry No. LiNaS04, 13568-34-8; LiKS04, 14520-76-4.
Particle Adhesion and Removal in Model Systems. 10. Effect of Chelating Species on the Hematite/Steel System C.-C.Lo, E. MatijeviL,* and N. Kallay Department of Chemistry and the Institute of Colloid and Surface Science, Clarkson College, Potsdam, New York 13676 (Received: June 22, 1983)
With small amounts of properly chosen additives such as EDTA (ethylenedinitrilotetraacetic acid), DFOB (desferrioxamine B, a trimeric hydroxamic acid derivative), and Co(bpy),(ClO& the deposition of monodispersed spherical hematite (a-Fe203) particles on stainless steel can be enhanced under the conditions which otherwise allow for little or no adhesion. In some cases, nearly maximum rates of deposition are secured at concentrations of the added chelating agents or metal chelates considerably smaller than the critical concentrations needed, if indifferent electrolytes are used. Detachment of adhered hematite particles from the same substrate is found not to be affected by the elution of the packed bed with EDTA solutions in strongly alkaline media.
Introduction Using the packed column technique, systematic studies on deposition of monodispersed colloidal particles (a-Fe203, flFeOOH, and Cr(OH),) onto solid surfaces (steel, glass) and on their detachment from these substrates have been carried out in this laboratory'-' over the past few years. It has been reported that the optimum uptake occurs when the particles and the adsorbent are of opposite charge. When both surfaces carry the charge of the same sign and of large magnitude the adhesion is prevented, as long as the ionic strength of the medium is low. In the latter case deposition can be induced either by adding an inert electrolyte in sufficiently high concentration or by adsorbing on one of the surfaces ionic species that can reverse the original charge. It has also been shown that adhered particles can be ~~~~~~~
~
~
~
(1) J. E. Kolakowski and E. MatijeviE, J . Chem. Soc., Faraday Trans. 1, 75, 65 (1979).
( 2 ) R.J. Kuo and E. MatijeviE, J . Chem. SOC.,Faraday Tram. I , 75,2014 (1979). (3) R. J. Kuo and E. MatijeviE, J . Colloid Interface Sci., 78,407 (1980). (4) N. Kallay and E. MatijeviE, J . Colloid Interface Sci., 83, 289 (1981). ( 5 ) J. D. Nelligan, N. Kallay, and E. MatijeviE, J . Colloid Interface Sci., 89, 9 (1982). ( 6 ) N. Kallay, J. D. Nelligan, and E. MatijeviE, J . Chem. SOC.,Faraday Trans. I , 19, 65 (1983). (7) G. Thompson, N. Kallay, and E. MatijeviE, Chem. Eng. Sci., 38, 1901 (1983).
0022-3654/84/2088-0420$01 S O / O
released without the application of appreciable hydrodynamic forces if the composition of the surrounding solution is properly altered. The kinetics of particle removal has been analyzed in terms of one-, two-, and three-population models by taking the topology of the substrate surface into consideration. Obviously, colloidal forces play an important role in the control of the deposition and detachment processes. So far, most of the work has concentrated on the effects of such parameters as pH, ionic strength, particle size, flow rate, and temperature. Less attention was placed on the adsorption of solute species which may have a profound influence on the surface chemical properties of the interacting materials. In the present work, three different chelating agents or metal chelates were added in very small amounts to modify surface characteristics of hematite particles at appropriate pH values (below and above the isoelectric p i n t s of hematite and steel). The conditions were chosen under which little or no uptake of dispersed solids was observed in the absence of such chelating agents (as long as the ionic strength was low) in order to evaluate the effects of these additives on particle adhesion. Attempts have also been made to elute the adhered particles in the packed bed by solutions of a chelating agent at various pH. The enhancement in deposition is relevant to a number of practical problems, especially to metal corrosion or deep bed filtering. In the latter case significant improvement can be achieved with media that fail to produce efficient filtration by 0 1984 American Chemical Society
