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Raman spectral studies of molten lithium nitrate-lithium perchlorate mixtures
Volume 9, number 3
CHEMICAL PHYSICSLETTERS
RAMAN MOLTEN
LITHIUM
SPECTRAL,STUDIES
NITRATE-LITHIUM
M H. BROOKER**,
1 May 1971
OF
PERCHLORATE
MIXTURES
*
A. S. QLJIsT and G. E. BOYB
Oak Ridge National Laborutorq,, Oak Ridge, Tennessee 37830, USA
Received 12 March 1971 The Ratnan spectra of molten LiN03, LiClO.+ and mixtures of the two salts are reported. All bands appear tc arise from highly localized vibrational modes. The opectral features of molten LiNO3 can be accounted
for by the hppothesis
that two non-equivalent
1. INTRODUCTION One of the fundamental goals in the interpretation of data from molten salt systems is to determine the nature and extent of the ordering forces. It is known from neutron and X-ray diffraction studies that the long range perioclicity of crystals is lost when they meit [l, 21. Atom pair correlation functions derived from diffraction data show peaks characteristic of nearneighbor interactions and indicate charge alternation in successive shells about any ion. Significantly, sitional correlation appears to be lost about 8 lr from any given starting point. Although these facts lend partial support to an irregular or distorted lattice model for molten salts, they also set a practical upper limit on possible long range correlation of vibrational modes. Within the confines of this limited local order, the extent to which motions of molecules are correlated ln the liquid phase has remained a matter of interest and speculation. The vibrational spectra of ionic nitrate melts have been studied by many workers in an effort to resolve this problem, but the results are still inconclusive [3-71. Dynamical coupling between identical vibrations of nitrate molecules has been proposed [6]. Low frequency infrared and Raman bands have been observed for melts in the same regions where external lattice mode bands are * Research sponsored by the U.S. Atomic Energy Commissfon under contract with the Union Carhide, Corporation. u National Research Council of Canada Postdoctoral Fellow. -‘. 242. -.
; :..,
;
_, .’
..
environments
ara available
to the nitrate
ion.
found for solids. Presence of these bands has prompted speculation that the vibrational spectrum of melts can be interpreted in terms of phonons much like that of the solid [S, 8-101. However, there are many reasons for doubting the existence of non-localized modes in liquids. In a previous publication [7], we proposed that the vibrational spectrum from molten NaN03 could be generated without invoking a species consisting of more than one nitrate ion. Bands in the low frequency region have been observed for many polar liquids and they are not unusual [ll-131. Although their origin is uncertain, dilution studies [ll-131 have clearly shown that the band structure does not coilapse and disappear as might be expected if long-range phase related motions were dominant factors in their origin Similar dilution studies with ionic nitrate melts should give considerable information about the nature of interactions in fused salts. The term “quasi-lattice” has been employed by many authors to suggest some type of order in the liquid phase, The meaning of the word is not consistent and a clarification seems to be in order. In vibrational spectroscopy, the term has been used in conjunction with simple electrolyte melts to convey the idea that the long-range order of the crystal was still present. In the thermodynamic sense, the “quasi-lattice” model implies short-range order and is applicable only to mixtures. We prefer to restrict our use of this name to the. “quasi-lattice* modei described by Braunstein [14,15]$ and use other terms when ‘, ‘. $ Footncl.e see ne$ page. :
: .’
1 May 1971
CHEMICALPHYSICSLETTERS
Volume 9, number 3
referring to possible long-range order in simple melts. Raman spectra from molten LiNO3, LiC104, and mixtures of the two molten salts have been obtained An almost complete insensitivity of the LiN03 spectrum to increased Cl04 concentration strongly suggests that formulations requiring long-range order are not required to explain the vibrational spectra of fused nitrates. Molten LiClO4 proved to be an excellent dilutant because it has no Raman bands which overlap those of the NO; ion in either the internal or external regions. In addition, the low melting point of LiC104‘meant that spectra of the molten mixtures could be recorded at the same temperature as with pure, molten LiNO3. Complications to the spectra due to hot bands, temperature broadening effects, and thermal emission were thus minimized. 2. EXPERIMENTAL High purity reagent grade LiNO3 and LiCiO4 were recrystallized from saturated solutions which had been treated with Norit A decolorizing carbon. The cr stals were dried for more than 48 hours at 180g C before use. The anhydrous sGts were melted under vacuum and filtered Qrcugh a sintered-glass frit (porosity ‘DO into a quartz sample cell which was placed into a furnace pre-set above the melting point of the sample. Mixtures of melts containing 5.3, 25, 67, 90 mcle percent LMO3 in LiClO4 were also prepared. A complete description of the furnace has been reported elsewhere [16]. Samples were heated to 35O’C under a vacuum for about one hour before the spectra were recorded. The sample temperature, as determined with chromel-alumel thermocouples, was known to d”C. Raman spectra were obtained for each of the above samples at several temperatures between their melting points and 380°C, the decomposition temperature of LiC104. Spectra were recorded with a Jarrell-Ash 25-300 model Raman spectrometer after sample excitation with the $ In the model of Blander and Braunstein. cat&s and anions occupy positions on cation and anion sublattices with different kinds of ions of the same charge mixed non-randomly. in accordance .with short range interaction energies with different kinds of ions of opposite charge. In’ ref. 1151 the ?quasi-lattic+2” model basbeen applied in the thermodynamic sense because of the competition between water molecules and aniqns fo? positions in the cation coordination sphere.
4880 A line from a Spectra Physics model 141 argon ion laser. Curve resolving was achieved with a Dupont 313 curve resolver. 3. RESULTS The observed Raman frequencies obtained from molten LiNO3, LiC104, and a mixture of 5.3% LiN03 and LiClO4 at temperatures near the melting points are collected in table 1 together with recently reported infrared frequencies of LiNO3 61. Portions of the Raman spectra from the 5.3 d LiNO3 + LXX04 melt are shown in fig. 1. Similar results were obtained for the other mlxtures. Temperature changes of as much as 130°C caused only small variations in the measured spectra. The results for the pure salts as well as the mixtures are similar to those obtained recently for pure, molten LiNO3 [5, GJ and LiCLO4 [ 171; however, several additional observations should be emphasized. 3.1. Nilrate baxds (a) At least two bands are present in the u3 +. 1400 cm-l) and u4 (ca 720 cm-l) nitrate regions, respectively, for pure LiNO3 and each of the LiNO3, LiC104 mixtures (table 1, fig. 1). Frequencies of tke band maxima irk these regions were essentially independent of temperature but did exhibit a small sensitivity to the nitrate ion concentration In the u3 region the band maxima occur at 1360 and 1465 cm’ 1 for pure LiNC3, but these were shifted to 1370 and 1475 cm-1 for the 5.3% LiN03 in LiClG4 mixtures (table 1). It is noteworthy that the separation between the band maxima in both the v3 and u4 regions was constant over the concentration and temperature range studied The relative intensities of the band pairs in each region change slightly with changes in temperature and nitrate concentration. This aspect of the work is currently under investigation In the u3 region the component at 1360 cm-l may be weakly polarized (b) The v1 (ca. 1050) symmetric stretching frequency was sensitive to temperature (1064 cm-l at 258OC; 1060 cm-1 at 35OOC) but not to nitrate ion concentration A slight asymmetry was apparerit on the low frequency side of this band (c) The.Raman compopent of the r~2out-ofplane deformation (forbidden by D3h selection rules) wasobzerved at 823 cm-1 for pure LiN03 and cou!d still be detected in LiNO3 diluted to 50 mole percent by LicIO4. -The frequency of the F2aman’active overtone df ~2 was slightly temper-
:
.-, ‘.
,243
..
‘.
-,
.’ .,
-.
Volume 9, number 3
CHEMICALPHYSICSLETTERS
LiC104 (fig. 1). This fact should make LiC104 an excellent solvent for future Raman studies of other anions in the low frequency region 4. DISCIJ~SION The significant observation of this work is the virtual insensitivity of the frequencies in the Raman spectrum of the molten mixtures,to anion concentration (table 1). Most dramatic is the presence of the low frequency band and the two components in the u3 region in a melt containing only 5.33,mole % nitrate ions (fig. 1). LEthe low frequency mode and the doublet structure of the degenerate u3 and v4 modes were the result of order that required specific phase related motions of nitrate ions (dynamical coupling) similar to that found in a lattice, it would be expected that as dilution proceeded the band at 145 cm-1 would disappear and the doublet structure of the ~3 and u4 modes would collapse to glve single components [ll, 12,181. It would appear that dynamical coupling does not effect the vibrational spectrum of the molten nitrates. Bands of the nitrate ion apparently arise from “specie(s)” which contain only a single nitrate group. This is not to suggest that the nitrate ion dees not feel the repulsive forces of other ions of like charge, but rather that one nitrate does not know if the next nearest neighbor is a nitrate or a perchlorate ion The presence of low frequency infrared and Raman bands in’the spectra of ionic salts dlssolved in polar solvents [13] and from most pure liquids [ll-121 suggests that the occurrence of similar bands in molten salts need not be construed as evidence for a lattice-like order. It is true that these bands are only observed for llquids which have strongly active lattice bands in the solid phase, and while this may be interpreted as evidence for vibrational, modes in the liquid much like those of the so!id, very little can be said concerning the possible non-localized nature of these modes. This is because the lattice modes in solids may aIso be of a localiied nature. Although the crystal has long-range periodicity, the intermolecular potentials tli&t determine the field at an? ion site are known to be essentially determined by near neighbor inter-, actions [19,20].. Since near nel&bor aistances’in melt remain similar_!0 thdse oc the.solid [1,2]’ (ebsentially only orientational correlatians_are lost on melting), it is not surprising that the’ gross features of the vibration@ spectra are similti for the solid tid melt. “:’ ‘. ‘, ,‘. :
.,
‘:’
.
1 May 1971
The semi-quantitative dexcription of ions vibrating in a cavity proposed by Edgell et al. [13] appears to present an extremely attractive pictorial representation of the motions involved in the generation of low frequency modes. Translational motions of an ion vibrating in a cage formed by its counter ions can give rise to a large dipole moment change and an infrared band. These infrared bands should be strongly dependent on the mzss of the caged particle [13]. Hindered rotational motions will give rise to a polarizability change and a Raman active band if the rotator has anisotropic symmetry (i.e., NOi but not C!lO,, Li+, etc.). Raman band frequencies will depend on the interaction between the cage and the rotating ion su& that the greater the interaction (smaller cavity size) the higher the rotational barrier and the greater the force constant and frequency. The insensitivity of the LiN% spectrum to the amount of added LiC104 would indicate that the NO; ion tibrates in a cavity created by Li+ ions. The stie of the cavity would be of the order of the Li+- Lit distances in the fused salts, ca 4 A [1,2]. For the molten NaNC$, the cavitv would be slightly larger, the barrier to rotation smaller; consequently, the corresponding band should be at lower frequencies. This band has been reported at 115 cm-1 in molten NaK03 [S-S]. The internal modes of the nitrate ion also can be rationalized on the hypothesis of a cage formed by the nearest neighbor ions. Selection rules for the nitrate group will be determined by the symmetry of the field and not by the free ion selection rules. Asymmetry of the cage may be responsible for the infrared activity of ~1 and the Raman activity of ~2. Field asymmetry also may have lifted the degeneracies of the ~3 and v4(E) modes causing the doublet structure in these regions. E’urthrr, as will be discussed below, the possibility that more than one type of cage can exist about a nitrate ion must be considered. A second type of cage would have the effect of doubling the number of vibrational bands. In addition, lf we allow for the possibility of hindered rotation to explain low frequency modes, it might be expected that this robtionai motion would also complicate,the region of the internal modes. It has beRn shown that even in liquids, damped rotational motions of moIecu:ar groups can couple with internal.vibratioti motions to calrse spectral features similar to those found for the nitrate’ lon in aqueous solution and certain melts [213. This possibility will ,be discussed more fully el$wherd,[22]. Recently 1151 it has been shown that in very .’ : ,‘, -.
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9, number 3
CHEMICAL PHYSICS LETTERS
concentrated aqueous nitrate solutions the bands in the internal regions could be assigned on the basis of two types of nitrate environment. It was suggested that two differing Qpes of nitrate en-’ vironment could exist in the melts. Several results from our work on molten LiNO3 lend credence to this suggestion. Slight asymmetry in the q and 213 Raman bands (table 1) could be due to overlap of bands originating from nitrate ions in two different cages. The two bands ca 720 and 740 cm-l in molten LiNO3 and the LiNO3, LiC104 mixtures are reminiscent of similar bands in aqueous nitrate systems which have been used successfully to quantitatively measure the free and bound nitrate concentrations, respectively [23 1. In a dilute aqueous solution of LiNO3 oniy one band is present in the v4 region ca. ‘720 cm-l, but at concentrations above 6 M the band characteristic of a nitrate ton in the coordination sphere of a cation appears at ca. 740 cm-l. The intensity of the band ca, 740 increases slowly at the expense of the band ca. 720 cm-1 as the water content is decreased [23]. In the limit of zero water (i.e., the melt) the bands have almost equal intensity. Slight changes in the relative intensities of the band pair in the melts with changes in temperature and concentration are consistent with a model which assumes two iqpes of nitrate ion. Analogous arguments can be applied to the 2~3region except that an additional explanation is required to account for the doublet structure in this region which is observed even in very dilute aqueous solutions 122,231. This doublet structure could be caused by lifting of degeneracy of the E modes by the local field created by water dipole8 or it could be that rotational motion of the nitrate ion in Zts solvation cage ctiuples with the vibrational motion to give unresolved rotational structure to this band. The magnitude of the separation for bands in the v3 region of LiNO2 increases gradually from 56 cn-1 in dilute solution (0.1 M) to 120 cm-1 in the molten salt. In very concentrated aqueous solutions of Li.NCs (above 10 I@, the ~3 envelope can be resolved into four bands made up of two pairs which can be assigned to free (1352, 1429 cm-l) and bound (1389, 2458 cm-l)-nitrate ion [15]. The envelope of the u3 region of molten LiNO3 can be similarly resolved (table 1). In conclusion, we feel that our observations with molten LiNO3:LiC104 mixtures greatly weaken the case for dynamical coupling between
1 .May 1971
identical vibrational modes of neighboring nitrate groups in alkali-metal nitrate melts. An adequate description of the vibrational spectrum of these melts appears to come from consideration of each of the,anion and cation in potential cages created by the near neighbor counter ions. All the bands appear to arise from highly localized vibrational modes. For molten LiN03, it appears possible that some of the spectral features can be accounted for if two non-equivalent environments are available to the nitrate ion. REFERENCES [l] 11.A. Levy and M. D. Danford, in: Molten sait chemistry, ed. M. Blander (Wiley, New York, 1964) p. 109. [Z] K. Furukawa, Discussions Faraday Sot. 32 (1961) [3] ?K. Wiimshurst
and S. Senderoff,
J. Chem. Phys.
35 (1961) 1078. [4] S.C. Wait, A. T. Ward and G. J. Jauz, J.Chem.
Phys. 45,(1966) 133. [5] D. W. James and W.H. Leong, J. Chem. Phys. 51 (1969) 640. [S] J. P.Devlin, P.C. Li and G. Pollard, J. Chem. Phys.
52 (1970)
2267;
J.P.Devlin, D. W. James and R. Frech, J.Chem. Fhys. 53 (1970) 4394. [7] M. H. Brooker, A.S. Quist and G. E. Boyd, Chem. Phys. Letters 5 (1970) 357. [S] J. H. R. Clarke, Chem. Phys. Letters 4 (1969) 39. [9] G. H. Wegdam. R. Bonn and J. van der Elsken, Chem.Phys. Letters 2 (1968) 182. [lo] C.A. Angell. J. Woag and W. F. Edgull, J. Chem. Phys. 51 (1969) 4519. [ll] B. J. Bulkin, Helv. Chim. _4cte 52 (1969) 1348. [12] N. T. McDevitt and W. G. Fately, J. Mol. Structure 6 (1970) 477. [13] W. F.Edgell, J.Lyford N, R.Wright, W.Rieen Jr. and A. Watts, J. Am.Chem. Sot. 92 (1970) 2240. i141 J. Braunstein. Inorn. Chim. Acta. Rev. 2 (1968) 19. [‘15j D. E. Irish, d. L.N&scn and M. H. Brook&, J: Chem. Phys., to be published. [lS] A. S. Quist, Appl. Spectry.25 (1971) 82. [17] W. H. Leong and D. W. James, Australian J. Chem.
2% (1969) 499. [lS] A. A. Maradudin arid J. Oitmaa,
Solid State Commun. 7 11969) 1143. [I91 iy J. Cocktug: Advan. Phys. 16 (1967) 185. .1201 J. C. Laufer and R. Kooelman. J. Chem. Phvs. 53 I
(1970) 3674. * ’ [Zl] P. V. Huong, M.Couzi and M. Perrot,
Chem. Phys. Letters 7 (1970) 189. [22] M. H. Brooker, A. R. Davis, G.Chang and D. E. Irish, to be published. (231 D. E. Irish, A. R, Davis aod,R. A. Plane. J. Chem. Ptys. 50 (1969) 2262. DI E. Irish and A. R. Davis,’ Ca’n.,J. Chem. 46’ (1968) ,943.
CHEMICAL PHYSICSLETTERS
RAMAN MOLTEN
LITHIUM
SPECTRAL,STUDIES
NITRATE-LITHIUM
M H. BROOKER**,
1 May 1971
OF
PERCHLORATE
MIXTURES
*
A. S. QLJIsT and G. E. BOYB
Oak Ridge National Laborutorq,, Oak Ridge, Tennessee 37830, USA
Received 12 March 1971 The Ratnan spectra of molten LiN03, LiClO.+ and mixtures of the two salts are reported. All bands appear tc arise from highly localized vibrational modes. The opectral features of molten LiNO3 can be accounted
for by the hppothesis
that two non-equivalent
1. INTRODUCTION One of the fundamental goals in the interpretation of data from molten salt systems is to determine the nature and extent of the ordering forces. It is known from neutron and X-ray diffraction studies that the long range perioclicity of crystals is lost when they meit [l, 21. Atom pair correlation functions derived from diffraction data show peaks characteristic of nearneighbor interactions and indicate charge alternation in successive shells about any ion. Significantly, sitional correlation appears to be lost about 8 lr from any given starting point. Although these facts lend partial support to an irregular or distorted lattice model for molten salts, they also set a practical upper limit on possible long range correlation of vibrational modes. Within the confines of this limited local order, the extent to which motions of molecules are correlated ln the liquid phase has remained a matter of interest and speculation. The vibrational spectra of ionic nitrate melts have been studied by many workers in an effort to resolve this problem, but the results are still inconclusive [3-71. Dynamical coupling between identical vibrations of nitrate molecules has been proposed [6]. Low frequency infrared and Raman bands have been observed for melts in the same regions where external lattice mode bands are * Research sponsored by the U.S. Atomic Energy Commissfon under contract with the Union Carhide, Corporation. u National Research Council of Canada Postdoctoral Fellow. -‘. 242. -.
; :..,
;
_, .’
..
environments
ara available
to the nitrate
ion.
found for solids. Presence of these bands has prompted speculation that the vibrational spectrum of melts can be interpreted in terms of phonons much like that of the solid [S, 8-101. However, there are many reasons for doubting the existence of non-localized modes in liquids. In a previous publication [7], we proposed that the vibrational spectrum from molten NaN03 could be generated without invoking a species consisting of more than one nitrate ion. Bands in the low frequency region have been observed for many polar liquids and they are not unusual [ll-131. Although their origin is uncertain, dilution studies [ll-131 have clearly shown that the band structure does not coilapse and disappear as might be expected if long-range phase related motions were dominant factors in their origin Similar dilution studies with ionic nitrate melts should give considerable information about the nature of interactions in fused salts. The term “quasi-lattice” has been employed by many authors to suggest some type of order in the liquid phase, The meaning of the word is not consistent and a clarification seems to be in order. In vibrational spectroscopy, the term has been used in conjunction with simple electrolyte melts to convey the idea that the long-range order of the crystal was still present. In the thermodynamic sense, the “quasi-lattice” model implies short-range order and is applicable only to mixtures. We prefer to restrict our use of this name to the. “quasi-lattice* modei described by Braunstein [14,15]$ and use other terms when ‘, ‘. $ Footncl.e see ne$ page. :
: .’
1 May 1971
CHEMICALPHYSICSLETTERS
Volume 9, number 3
referring to possible long-range order in simple melts. Raman spectra from molten LiNO3, LiC104, and mixtures of the two molten salts have been obtained An almost complete insensitivity of the LiN03 spectrum to increased Cl04 concentration strongly suggests that formulations requiring long-range order are not required to explain the vibrational spectra of fused nitrates. Molten LiClO4 proved to be an excellent dilutant because it has no Raman bands which overlap those of the NO; ion in either the internal or external regions. In addition, the low melting point of LiC104‘meant that spectra of the molten mixtures could be recorded at the same temperature as with pure, molten LiNO3. Complications to the spectra due to hot bands, temperature broadening effects, and thermal emission were thus minimized. 2. EXPERIMENTAL High purity reagent grade LiNO3 and LiCiO4 were recrystallized from saturated solutions which had been treated with Norit A decolorizing carbon. The cr stals were dried for more than 48 hours at 180g C before use. The anhydrous sGts were melted under vacuum and filtered Qrcugh a sintered-glass frit (porosity ‘DO into a quartz sample cell which was placed into a furnace pre-set above the melting point of the sample. Mixtures of melts containing 5.3, 25, 67, 90 mcle percent LMO3 in LiClO4 were also prepared. A complete description of the furnace has been reported elsewhere [16]. Samples were heated to 35O’C under a vacuum for about one hour before the spectra were recorded. The sample temperature, as determined with chromel-alumel thermocouples, was known to d”C. Raman spectra were obtained for each of the above samples at several temperatures between their melting points and 380°C, the decomposition temperature of LiC104. Spectra were recorded with a Jarrell-Ash 25-300 model Raman spectrometer after sample excitation with the $ In the model of Blander and Braunstein. cat&s and anions occupy positions on cation and anion sublattices with different kinds of ions of the same charge mixed non-randomly. in accordance .with short range interaction energies with different kinds of ions of opposite charge. In’ ref. 1151 the ?quasi-lattic+2” model basbeen applied in the thermodynamic sense because of the competition between water molecules and aniqns fo? positions in the cation coordination sphere.
4880 A line from a Spectra Physics model 141 argon ion laser. Curve resolving was achieved with a Dupont 313 curve resolver. 3. RESULTS The observed Raman frequencies obtained from molten LiNO3, LiC104, and a mixture of 5.3% LiN03 and LiClO4 at temperatures near the melting points are collected in table 1 together with recently reported infrared frequencies of LiNO3 61. Portions of the Raman spectra from the 5.3 d LiNO3 + LXX04 melt are shown in fig. 1. Similar results were obtained for the other mlxtures. Temperature changes of as much as 130°C caused only small variations in the measured spectra. The results for the pure salts as well as the mixtures are similar to those obtained recently for pure, molten LiNO3 [5, GJ and LiCLO4 [ 171; however, several additional observations should be emphasized. 3.1. Nilrate baxds (a) At least two bands are present in the u3 +. 1400 cm-l) and u4 (ca 720 cm-l) nitrate regions, respectively, for pure LiNO3 and each of the LiNO3, LiC104 mixtures (table 1, fig. 1). Frequencies of tke band maxima irk these regions were essentially independent of temperature but did exhibit a small sensitivity to the nitrate ion concentration In the u3 region the band maxima occur at 1360 and 1465 cm’ 1 for pure LiNC3, but these were shifted to 1370 and 1475 cm-1 for the 5.3% LiN03 in LiClG4 mixtures (table 1). It is noteworthy that the separation between the band maxima in both the v3 and u4 regions was constant over the concentration and temperature range studied The relative intensities of the band pairs in each region change slightly with changes in temperature and nitrate concentration. This aspect of the work is currently under investigation In the u3 region the component at 1360 cm-l may be weakly polarized (b) The v1 (ca. 1050) symmetric stretching frequency was sensitive to temperature (1064 cm-l at 258OC; 1060 cm-1 at 35OOC) but not to nitrate ion concentration A slight asymmetry was apparerit on the low frequency side of this band (c) The.Raman compopent of the r~2out-ofplane deformation (forbidden by D3h selection rules) wasobzerved at 823 cm-1 for pure LiN03 and cou!d still be detected in LiNO3 diluted to 50 mole percent by LicIO4. -The frequency of the F2aman’active overtone df ~2 was slightly temper-
:
.-, ‘.
,243
..
‘.
-,
.’ .,
-.
Volume 9, number 3
CHEMICALPHYSICSLETTERS
LiC104 (fig. 1). This fact should make LiC104 an excellent solvent for future Raman studies of other anions in the low frequency region 4. DISCIJ~SION The significant observation of this work is the virtual insensitivity of the frequencies in the Raman spectrum of the molten mixtures,to anion concentration (table 1). Most dramatic is the presence of the low frequency band and the two components in the u3 region in a melt containing only 5.33,mole % nitrate ions (fig. 1). LEthe low frequency mode and the doublet structure of the degenerate u3 and v4 modes were the result of order that required specific phase related motions of nitrate ions (dynamical coupling) similar to that found in a lattice, it would be expected that as dilution proceeded the band at 145 cm-1 would disappear and the doublet structure of the ~3 and u4 modes would collapse to glve single components [ll, 12,181. It would appear that dynamical coupling does not effect the vibrational spectrum of the molten nitrates. Bands of the nitrate ion apparently arise from “specie(s)” which contain only a single nitrate group. This is not to suggest that the nitrate ion dees not feel the repulsive forces of other ions of like charge, but rather that one nitrate does not know if the next nearest neighbor is a nitrate or a perchlorate ion The presence of low frequency infrared and Raman bands in’the spectra of ionic salts dlssolved in polar solvents [13] and from most pure liquids [ll-121 suggests that the occurrence of similar bands in molten salts need not be construed as evidence for a lattice-like order. It is true that these bands are only observed for llquids which have strongly active lattice bands in the solid phase, and while this may be interpreted as evidence for vibrational, modes in the liquid much like those of the so!id, very little can be said concerning the possible non-localized nature of these modes. This is because the lattice modes in solids may aIso be of a localiied nature. Although the crystal has long-range periodicity, the intermolecular potentials tli&t determine the field at an? ion site are known to be essentially determined by near neighbor inter-, actions [19,20].. Since near nel&bor aistances’in melt remain similar_!0 thdse oc the.solid [1,2]’ (ebsentially only orientational correlatians_are lost on melting), it is not surprising that the’ gross features of the vibration@ spectra are similti for the solid tid melt. “:’ ‘. ‘, ,‘. :
.,
‘:’
.
1 May 1971
The semi-quantitative dexcription of ions vibrating in a cavity proposed by Edgell et al. [13] appears to present an extremely attractive pictorial representation of the motions involved in the generation of low frequency modes. Translational motions of an ion vibrating in a cage formed by its counter ions can give rise to a large dipole moment change and an infrared band. These infrared bands should be strongly dependent on the mzss of the caged particle [13]. Hindered rotational motions will give rise to a polarizability change and a Raman active band if the rotator has anisotropic symmetry (i.e., NOi but not C!lO,, Li+, etc.). Raman band frequencies will depend on the interaction between the cage and the rotating ion su& that the greater the interaction (smaller cavity size) the higher the rotational barrier and the greater the force constant and frequency. The insensitivity of the LiN% spectrum to the amount of added LiC104 would indicate that the NO; ion tibrates in a cavity created by Li+ ions. The stie of the cavity would be of the order of the Li+- Lit distances in the fused salts, ca 4 A [1,2]. For the molten NaNC$, the cavitv would be slightly larger, the barrier to rotation smaller; consequently, the corresponding band should be at lower frequencies. This band has been reported at 115 cm-1 in molten NaK03 [S-S]. The internal modes of the nitrate ion also can be rationalized on the hypothesis of a cage formed by the nearest neighbor ions. Selection rules for the nitrate group will be determined by the symmetry of the field and not by the free ion selection rules. Asymmetry of the cage may be responsible for the infrared activity of ~1 and the Raman activity of ~2. Field asymmetry also may have lifted the degeneracies of the ~3 and v4(E) modes causing the doublet structure in these regions. E’urthrr, as will be discussed below, the possibility that more than one type of cage can exist about a nitrate ion must be considered. A second type of cage would have the effect of doubling the number of vibrational bands. In addition, lf we allow for the possibility of hindered rotation to explain low frequency modes, it might be expected that this robtionai motion would also complicate,the region of the internal modes. It has beRn shown that even in liquids, damped rotational motions of moIecu:ar groups can couple with internal.vibratioti motions to calrse spectral features similar to those found for the nitrate’ lon in aqueous solution and certain melts [213. This possibility will ,be discussed more fully el$wherd,[22]. Recently 1151 it has been shown that in very .’ : ,‘, -.
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9, number 3
CHEMICAL PHYSICS LETTERS
concentrated aqueous nitrate solutions the bands in the internal regions could be assigned on the basis of two types of nitrate environment. It was suggested that two differing Qpes of nitrate en-’ vironment could exist in the melts. Several results from our work on molten LiNO3 lend credence to this suggestion. Slight asymmetry in the q and 213 Raman bands (table 1) could be due to overlap of bands originating from nitrate ions in two different cages. The two bands ca 720 and 740 cm-l in molten LiNO3 and the LiNO3, LiC104 mixtures are reminiscent of similar bands in aqueous nitrate systems which have been used successfully to quantitatively measure the free and bound nitrate concentrations, respectively [23 1. In a dilute aqueous solution of LiNO3 oniy one band is present in the v4 region ca. ‘720 cm-l, but at concentrations above 6 M the band characteristic of a nitrate ton in the coordination sphere of a cation appears at ca. 740 cm-l. The intensity of the band ca, 740 increases slowly at the expense of the band ca. 720 cm-1 as the water content is decreased [23]. In the limit of zero water (i.e., the melt) the bands have almost equal intensity. Slight changes in the relative intensities of the band pair in the melts with changes in temperature and concentration are consistent with a model which assumes two iqpes of nitrate ion. Analogous arguments can be applied to the 2~3region except that an additional explanation is required to account for the doublet structure in this region which is observed even in very dilute aqueous solutions 122,231. This doublet structure could be caused by lifting of degeneracy of the E modes by the local field created by water dipole8 or it could be that rotational motion of the nitrate ion in Zts solvation cage ctiuples with the vibrational motion to give unresolved rotational structure to this band. The magnitude of the separation for bands in the v3 region of LiNO2 increases gradually from 56 cn-1 in dilute solution (0.1 M) to 120 cm-1 in the molten salt. In very concentrated aqueous solutions of Li.NCs (above 10 I@, the ~3 envelope can be resolved into four bands made up of two pairs which can be assigned to free (1352, 1429 cm-l) and bound (1389, 2458 cm-l)-nitrate ion [15]. The envelope of the u3 region of molten LiNO3 can be similarly resolved (table 1). In conclusion, we feel that our observations with molten LiNO3:LiC104 mixtures greatly weaken the case for dynamical coupling between
1 .May 1971
identical vibrational modes of neighboring nitrate groups in alkali-metal nitrate melts. An adequate description of the vibrational spectrum of these melts appears to come from consideration of each of the,anion and cation in potential cages created by the near neighbor counter ions. All the bands appear to arise from highly localized vibrational modes. For molten LiN03, it appears possible that some of the spectral features can be accounted for if two non-equivalent environments are available to the nitrate ion. REFERENCES [l] 11.A. Levy and M. D. Danford, in: Molten sait chemistry, ed. M. Blander (Wiley, New York, 1964) p. 109. [Z] K. Furukawa, Discussions Faraday Sot. 32 (1961) [3] ?K. Wiimshurst
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J.P.Devlin, D. W. James and R. Frech, J.Chem. Fhys. 53 (1970) 4394. [7] M. H. Brooker, A.S. Quist and G. E. Boyd, Chem. Phys. Letters 5 (1970) 357. [S] J. H. R. Clarke, Chem. Phys. Letters 4 (1969) 39. [9] G. H. Wegdam. R. Bonn and J. van der Elsken, Chem.Phys. Letters 2 (1968) 182. [lo] C.A. Angell. J. Woag and W. F. Edgull, J. Chem. Phys. 51 (1969) 4519. [ll] B. J. Bulkin, Helv. Chim. _4cte 52 (1969) 1348. [12] N. T. McDevitt and W. G. Fately, J. Mol. Structure 6 (1970) 477. [13] W. F.Edgell, J.Lyford N, R.Wright, W.Rieen Jr. and A. Watts, J. Am.Chem. Sot. 92 (1970) 2240. i141 J. Braunstein. Inorn. Chim. Acta. Rev. 2 (1968) 19. [‘15j D. E. Irish, d. L.N&scn and M. H. Brook&, J: Chem. Phys., to be published. [lS] A. S. Quist, Appl. Spectry.25 (1971) 82. [17] W. H. Leong and D. W. James, Australian J. Chem.
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