ates, and G . E. Boyd
21
Raman Spectra of B ~ Z F and ~ ~Higher Polymers of eryllium Fluorides in the Crystalline and Molten State'
. 'both, J. 5. Bates, and G. E. Boyd* Oak flidge National Laboratory, Oak Ridge, Tennessee 37830 (Received June 19, 1972) Publication costs assisted by the Oak Ridge National Laboratory
Raman spectroscopy was employed to obtain direct evidence for some beryllium fluoride complexes present in molten mixtures of alkali metal fluorides with BeFZ. The dimer anion, BezF73-, was identified in KiF-NaF-BeFz mixtures by comparing the melt spectrum with infrared and Raman spectra of BezF73in the crystalline compound, NazLiBezF7. The presence of this dimer anion in molten LiF-BeFz mixtures with comparable alkali metal fluoride to beryllium fluoride ratios also was indicated. Evidence for progressively more complex beryllium fluoride species was found when the MF: eF2 ratio was decreased,
Introduction The molten fluoride svstem. LiF-BeF2. has been a subject of practical interest in the molten salt breeder reactor (MSBFL) Accordingly, much attention has been given to determining the properties of this system as a high-temperature solvent for transition metal ions and how these properties change with composition and temperature variations. One of the components of the solvent, beryllium fluoride, tends to form three-dimensional networks of +BeF-Bec chains in its crystalline and glassy state. It was' shown recently8 that X-ray scattering data from noncrystalline BeF2 could be interpreted by assuming a model for the glass based on the structure of /3 quartz. The vibrational spectrum of vitreous BeFz also was found to be consistent with the @-quartz structure. & In particular, the fact that the Ramsan spectrum of noncrystalline BeFz4b exhibits only one polarized band at ea. 282 cm-1 in addition to weaker depolarized bands at higher frequencies is accounted for by the model. The Raman spectrum of molten BeFz is nearly indistinguishable from the correspondspectrum of the glass,4b andl it was concluded that 'molten BeFz exhibits a network structure very similar to that of the glass When a basic. fluoride such as LiF is added to molten IEleFz, the viscosity of the resulting mixture drops presumably because the bridging fluoride linkages are broken and the degree of ~ o ~ y m ~ r i ~ aistdecreased ion *Be-F-Be
c
4-
F-
-5 [
+Eie-F]1/2-
+ [F- Be-
11'2 -
(1)
If the amount of LiF added equals or exceeds the amount in 2LiF-BeF2, a complete disruption of the network should result with the formation of BeF42-. Indeed, Raman spectra have been reported5 for solutions such as NaF-LiF-Bel72 (53-30-17 mol %) which show the expected four normal vibrational modes of the tetrahedral BeF42- ion. The formation of other complex ion species such 4s b21'73-, Be3Flo4-, Be2Fs2-, etc. might be expected, and these have been assumed in Baes' polymer model6 for molten alkali metal fluoride-beryllium fluoride mixturee. Basic assumptions in the model are that all beryllium ions are tetrahedrally coordinated to four fluoride ions which may in turn be shared with other beryllium The Journal of Physical Chemistry, Vol. 77,No. 2, 7973
ions, and that polymeric species in the melt must include beryllium fluoride tetrahedra with shared edges in addition to the corner sharing indicated in eq 1. The work reported here is an extension of earlier Raman experiments on the LiF-BeF2 system in this laboratory5 and includes an examination of the polymer model by experimentally observing those ions involved in the polymerization process. The dimer ion, B ~ z F , ~ -represents , the first step in polymerization where two beryllium fluoride tetrahedra share a corner. It is anticipated that a measurement of the spectrum of the crystalline system where it was known to be present would facilitate its identification in the melts where a variety of network fragments may coexist. Further steps in the polymerization process were also of interest; but since no crystalline compounds containing identifiable species larger than the dimer are known, the detailed process beyond the first step could not be characterized. Experimental Section NazLiBe2F~ was prepared by melting stoichiometric quantities of NaF, LiF, and BeFz in a graphite crucible and sparging the melt with HF to remove oxide inipurities. The NaF and LiF starting materials were crystal fragments from the Harshaw Chemical @osand the BeFz (Brush Beryllium e o , ) was distilled before use to yield a transparent, colorless mass. The congruently melting mixture was slowly cooled to 25" to give a solid which was shown by differential thermal analysis and by examination with a polarizing microscope to be approximately 98% NazLiBezF7, the major impurity being NaF. A single crystal suitable for X-ray structure determination was separated from the solid and identified as NazLiBezF~.~ The compound, which is isostructural with t'he minerals (1) The research was sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp. (2) (a) M. W. Rosenthal, P, M. Haubenreich, H. E. McCoy, and L. E. McNeese, At. Energy Rev., 9 [3], 601 (197'1); (b) Ini. R. Grimes. Nucl. Appl. Techno/., 8, 137 (1970). (3) A. H. Narten,J. Chem. Phys., 56, 1905 (1972). (4) (a) J. 3. Bates, J. Chem. Phys., 56, 1910 (1972); (b) A. S. Quist, J. B. Bates, and G. E. Boyd, Spectrochim. Acta, Part A. 28, 1103 (1972). (5) A. S . Quist, J. 3. Bates, and G , E. Boyd, J. Phys. Chem., 7 6 , 78 (1972). ( 6 ) C. F. Raes, Jr., J. Solid State Chem., 1, 159 (.IS70). ( 7 ) G. D. Brunton, Mater. Res. Bull., 7 , 641 (1972).
Raman Spectra of Be2F73LE I: Calculated
217
Frequencies and Force Constants for Be2FT3- Fundamental Modes
Frequencies, cm-' Obsd
A i 890
779 520 443 370 237 206
9a
Ira S
rn
rn rn S
n.0. 11.0.
A2
420 270
n.0, 11.0.
B1 910
S
505 362 195 B2 935 835 490 41 5 298 281
rn m rn S
rn s
m m W
Calcd
954 767 529 437 349 249 94 91 8 449 253 61 909 496 355 134 923 827 514 408 333 244
Force constants, mdyn/Ab
Assignment
Be-F (2-4and 2-6) Be-F (1-2and 2-4) Be-F (2-4and 2-6),Be-F-Be F-Be-F (4-2-6),Be-F-Be F-Be-F (6-2-8) F-Be-F (1-2-4) F-Be-F (1-2-4 and 6-2-8) Be-F (2-6) A7, F-Be-F (4-2-6) F-Be-F (1-2-6),AT F-Be-F (4-2-6),A7 Be-F (2-6) A7, F-Be-F (1-2-6) F-Be-F (4-2-6) F-Be-F (I-2-6),A7 Be-F (2-4 and 2-6) Be-F (1-2) Be-F (2-6 and 2-4) F-Be-F (1-2-4and 1-2-6). A7 F-Be-F (6-2-8 and 1-2-4),A7 F-Be-F (1-2-4and 1-2-6)
Be-F (1-2) Be-F (2-4and 2-6) Be-F- Be F-Be-F (1-2-4 and 1-2-6) F-Be-F (4-2-6 and 6-2-8)
A7 Be-F (1-2),Be-F (1-3) Be-F (2-4),Be-F (2-6) Be-F (1-2),Be-F-Be Be-F (1-2),F-Be-F (1-2-4) Be-F (2-4),F-Be-F (1-2-4) Be-F (2-4),F-Be-F (4-2-6) A T , Ar
3.415 4.068 0.307 0.452 0.400 0.153 0.267 -0.698 -0.168 0.598 0.914 0.792 0.066
R = Raman, ir = infrared, s = strong, m = medium, w = weak, v = very, n.0. = band not observed OThe number of significant figures reported for the force constants does not imply a commensurate accuracy in the observed frequencies.
melilite and hardystonite, contains the B ~ z F ion ~ ~which exhibits approximate 6 2 , symmetry and consists of two BeF4 tetrahedra that share a corner and have a coplanar base. The four Be-F distances vary from 1.507 to 1.591 A, and the tetrahedral angles from 104.4 to 114.7".? LiF-BeF:a solutions of predetermined stoichiometries were prepared in the same manner as was the Na2LiBe2F7, but they were quenched from 700" to prevent phase separation. Each polycrystalline solid was loaded into a windowless eel18 which was then aligned in a Raman furnace. The loading procedure and furnace have been previously described in detail.8,9 The temperatures at which the melt srpectra were measured were generally kept as low as possible to limit the tendency of the melt to leak through the siit in the windowless cell. Raman spectra were measured with a Jarrell-Ash Model 25-300 spectrometer (Jarrell-Ash Co., Waltham, Mass.) using approximately 900 mW of 4880-A radiation from an argon ion laser lM,odel 52, Coherent Radiation Laboratories, Palo Alto, Calif.) for excitation. Spectra of polycrystalline Naz1LiDezF-i were measured at 77 and 298°K with a transparent piece of polished salt measuring approximately 4 x 4 x 2 m1n. Infrared spectra of crystalline Na2LiBezF.r at 298 and 77°K were obtained in the region, Y >300 cm-1, from a sample ground onto the surface of a AgBr plate with a Perkin-Elmer Model 621 spectrometer. Far-infrared spectra (50-400 c i n - a ) were obtained from a sample mulled with Nujol on B polyethylene window with a Digilab Model 20 infrared Fourier transform spectrometer. esulta NaF-LiF--BcFz Mixtures. The Raman spectra of crystalline NazLiBe2F-i at 298 and 77°K are shown in Figure 1, frequencies of the infrared and Raman bands (at e collected ~n Table 1. Because the Raman bands
listed in Table I at 236, 362, and 835 cm-I were very weak, they appeared only in high-gain scans and consequently are not apparent in Figure 1. The Raman spectra of molten NazLiBeZF? at 478" and of molten NaF-LiF-BeFz (28.6-42.9-28.5 mol 7%) at 617" are presented in Figure 2. The observed band frequencies for these melt systems are given in Table 11. The melt spectra of all the BeFz mixtures are characterized by an intense polarized band in the 500-cm-1 region and by weaker depolarized bands at ca. 200, 350, and 800 cm-l (Table 0). As shown in Figure 2 and Table II, two polarized bands were observed with molten Na2LilBe2F7 at 522 and 550 cm-1. When the fluoride ion concentration i s increased by the addition of LiF (trace B), the band at 522 cm-I disappears and the 550-cm-1 band remains. Temperature changes from 350 (melting point of EazLiBezF7) to 700" produced no measurable shift in the position OT intensity of any of the Raman bands. In particular, no change in the relative intensities of the 522- and 5 5 0 - ~ m - ~ bands was observed. Glasses formed from the molten salts were of interest not only because their Raman spectra are easier to obtain than those for the fluoride melts, but also because better resolution of overlapping bands could be achieved at lower temperatures. Glasses of NazLiBezF7 were produced by quenching the encapsulated melt in water. k typical spectrum is given in Figure 3 and the band frequencies are included in Table II. No glasses could be produced for melt compositions of greater alkali metal fluoride content than in NazLiBezFT. As with molten NazLiBezFT (Figure 2A), two polarized bands were observed with the glass, but they are better resolved than in the melt spectrum. LiF-BeFZ Mixtures. The Raman spectrum of molten (8) A S Quist, Appl. Spectfosc, 25, 80 (1971) (9) A S Quist, J B Bates, and G E Boyd J Chern P h y s , 54, 4896 (1971) The Journal of Physical Chemistry, Vor. 77, No. 2, 1973
21 8
L. M. Toth, J, B. Bates, and 6.E.
@
A@
n
298°K S l i t 3.3 cm-'
443
--
u'L@
779
900 Figure 1.
77°K S l i t 2cm-' 1 io3
700
800
600 FREQUENCY
Raman spectra of polycrystalline Na2LiBe2F7: (A)
I
I
I
I
A
40
28 6
(
500 cm-' 1
400
77'K; (B) 298°K. Slit width in
I
'
MELT SPEC-RA NaF LiF BeF2
B
Boyd
i
'
i
'
200
300
cm-I and gain in counts per second ( c i s ) . !
'
I
t
/
/
,
'
l
,
I
"
'
mole % ) NaF- L i F - BeF;. (40-20-40 G L A S S SPECTRUM AT 77°K QUENCHED FROM 8iO" MELT
2C 40 mole % 478°C 4.29 28.5 mole % 617°C
, 3
I
900
I
800
700
,
,
600
!
500
/
400
l 300
200
IC
FREQUENCY ( c m - ' )
Figure 3. Raman spectrum of Na2LiBe2F7glass at 77°K quenched from 810" melt: slit = 6 cm-I, 2 X lo4 cis
900
7co
500
FREQUENCY
300
3
icm-')
Raman spectrd of the NaF-LiF-BeFz melt system: (A) slit = 9 cm-', 1 x i o 4 c i s ; (B) slit = 10 cm-I, 5 X IO3 c/s. Figure 2.
LiF-BeF2 (60-40 mol %) at 495" is shown in Figure 4. The band frequencies observed from spectra of melt compositions of LiF-BeF2 from 75-25 to 48-52 mol 7'0 are given in Table III. As with thle 66-34 mol % composition,5 bands at ca. 220, 383, 525, and 800 cm-1 were observed. However, the 525-cm-" band was shifted to lower frequencies than reported for the 66-34 mol % composition (cf. 550 cm-1 in ref 5 ) . This apparent discrepancy will be discussed in the following paragraphs. The sharp band at 352 cm-1, the shoulder a t b50 cin-l, and possibly some contribution to the broad 220-cm- 1 band arise from laser emission lines.10 The 220..~rn--~ band was still present when an optical transmission filter was used to eliminate the emission lines; but because F I E . 50% of the 4880-A intensity was lost The Journal of Physical Chemistry, Vol. 77, No. 2, 7973
by transmission through the filter, the location of this Raman band assigned to BezF$- was difficult to fix to within 3220 crn-l. Three weak depolarized bands at 1140, 1340, and 1590 crr-l, not previously reported for LiFBeFz melts, also were found. As the melt composition i s changed, the features revealed in Figure 4 remain unaltered except for the strong polarized band at 525 cm which shifts progressively to lower frequencies and decreases in intensity as the BeF2 content of the melt is increased. The data of Table' ID and Figure 5 depict this trend with the latter showing a band envelope for melt compositions from 75-25 to 48-52 mol % LiF-BeF2 in which the frequency shifts from 550 to 479 cm-1, respectively. It is anticipated that this band continues to shift as the BeF2 content of the melt is increased until, in the limiting case of pure molten BeF2, it converges to the previously measured 282-cm-l band (cf. ref 4b). Experimental evidence for this was not obtained because the spectra melts of very high BeF2 content are weak and of extremely poor quality. The effect of temperature on the LiF-BeF2 spectra was negligible except for the expected thermal. line broaden(IO) Emission lines for the argon ion laser are found at 221 6, 351 7 , 529.7,560 8.and 737 5 cm-'
Raman Spectra of B ~ z F ~ ~ I
219
I
I
I
I
TABLE 11: Vibrational Frequencies (in c m - I ) Observed in the Raman Spectra of the NaF-LiF-BeF2 System _.-_I_-
I
1
I
= 0 optical modes of crystalline Na2LiBezF.1 which are derived from the 21 internal modes of the BezF73- ion i s given by
Ra~o of NsiF LIF BeF2 r0P(D2d)
2:I:Z ___I
Molten (4780)
Cilassu (77°K)
2:3:2 Molten (617')
Assignmentb
175 200 223 27a 375 522 (P) 550 (PI 795 =(p) indicates polarized bands. bAssignments based on the results in Table I. TABLE 111: Vibrational Frequencies Observed in the Raman Spectra of LBF-BeF2 Meltsa
75-25 (550')
70-30 (550')
66-34 (540")
60-40 (525")
54-46 (495")
48-52 (479")
490 (PI 800
480 (P)
_I____-
235 375 550 (P) 795
230 367 365 550 (P) 540 (P) 790 -190 1190 I140 1340 1350 1590 I 585
220 383 525 (P) 800 1180 1345 1590
820 1210 1360 1605
= 7A1+ 4Az -k 4B1 C 7B2 + 10E
Correlation field splitting of the A1 modes of B ~ Z F pre~ ~ dicted in Table IV was not observed in either Raman (Figure 1) or infrared spectra of the polycrystalline material. This result is consistent with earlier spectroscopic studies with Li2BeFl in which correlation field effects were not apparent in the observed spectra.5 Therefore, since the point group and site group of Be2F73-- in Na2LiBe2F7 are isomorphic (Table IV), crystal spectra may be used to characterize the normal modes o f this anion. Normal Modes of the BezF73- Anion. Frequencies of the bands assigned to B ~ ~- Ffundamentals T ~ observed in the Raman and infrared spectra of crystalline NatLiBe2F.l at 77°K are given in Table I. The assignments were based partly on the results from a calculation of the normal modes of the BezF73- ion. The symmetry species for the 21 normal modes of this ion are determined by
r ( C z U )= 7A1 f 4A2 -+ 4B1+ 6B2 All modes are Raman active, and the AI, B1, and Bz modes are infrared active (Table IV), The normal mode frequencies were calculated using GF matrix methods.IlJ2 Bond angles and bond lengths employed in calculating elements of G were taken from the crystallographic data for NazLiBezF7.7 The molecular geometry of the Be2F73- ion may be represented by I. The 6 2 axis as shown passes through the
aCompositions given in mol per cent LiF-BeFZ, respectively, and frequencies in cni-I. (p) indicates polarized bands.
ing. Therefore, the spectra in Figure 5 were measured at various teniperatures depending upon their melting characteristics.
Discussion Crystalliiae Na~LiBe2F7.The crystal structure of NazLiBe2F7 has been shown to be tetragonal, space group D32d(P%21rr~),with two formula units in the primitive cell.7 The Be2F73- ions occupy CzU sites and exhibit a structure analogous to that of dichromate ion, C r ~ 0 7 ~ - . The correlation diagram presented in Table IV gives the mapping of the CzU symmetry species onto those of the D2d factor group. The irreducible representation for the k
I
central F atom. Atoms 4, 2, 1, 3, and 5 lie in the mirror plane. Six types of internal Goordinates were employed: two types of Be-F stretching modes, 1-2 and 2-4; a central bend, Be-F-Be, two types of F-Be-F bends, 1-2-4 and 4-2-6; and two torsional coordinates, Ad4-2-1-3) and (11) E. B. Wilson, J. C. Decius, and P. C. Cross, "Molecular Vibrations," McGraw-Hill, New York, N. Y., 1955, Chapter 4. (12) J. H. Schachtschneider, Technical Report No 5765, Shell Development Go., 1964 The Journal of Physical Chemistry, Vol. 77, No. 2, 7973
L. M .lo:h, J . B. Bates, and G. E. Boyd
220
TABLE IY: Correlation Diagram for the
rr540 550
Molecular group
CP"
c D
75 25 66 34 60 40 54 46
3 Figure 5. Raman spectra of LiF-BeF2 system as the melt composition is changed showing a shift in the symmetric vibrational mode: (A) 690"; (B) 588"; (C) 452'; (D) 480'; (E) 434". A7(5-3-1-2). Tae total number of each type of internal coordinate employed was determined by the requirements needed to satisfy each symmetrically equivalent set. An initial set of valence and interaction force constants for Be-F (2-4) and F-Be-F coordinates and their interactions were chosen from the results of the earlier calculations on BeF42- (ref 5). The central Be-F (1-2) stretching force constant was initially set at ca. 10% less than the terminal Be-F (2-4) force constant. Values for the central bend and torsional coordinates. were initially assumed to be equal to the F-Be-F (4-2-69 constant. The initial force field for BezF73- was modified on successive calculations by addition and/or removal of interaction terms. No attempt was made to achieve exiact agreement between calculated and observed frequencies because only a few of the observed barids can be assigned with confidence. The calculated frequencies and force constants are presented in Table 1. Tho assignments were determined from the major contributors to the eiglenvectors of each mode. f particular interest to discussions presented below, the u3(&) mode at 520 em-l (observed) i s derived from the stretching of the 2-4 and 2-6 terminal Be-F bonds and from the Be- F-Be bending coordinate. Stretching of the central Be-F bonds con ibutes primarily to the 779-em-1 (observed) u2 ~ltodt?an to a lesser degree, to the 443cm- uq mode. Because of the small mass of the Be atom, the torsional coordinates contributed significantly to the us(&) and ~ 4 3 1 modes ) (Table I). The poor fit of the low-frequency modes, Y ~ J A ull(Az), ~), and U15(B1), may reflect the sensttivity of these vibrations to interaction terms. Because of uncea.8ainty in the assignments for these modes, an exhaustive search for the best set of interaction terms (i~e., that providing the best fit) was not made. Molten and Glusuy NaF-LiF-BeF4 Mixtures. The spectrum of crystalline NazLiBezFT was sought primarily to The Journai of Physical Chernistry, Yo/. 77, No. 2, 1973
R,ira
Ai
R
A2
R,ir
B1
R,ir
B2
Site group CZu
Factor group 92d
a Rarnan active, ir = infrared active, and N A = not active in Raman or infrared.
-A B
Be2FT3-ion in
Crystalline NazLiBe2F7
aid in identifying the Be2F73- species in the melts. The band at 520 cm-I in the crystalline spectrum assigned to the symmetric stretching mode of Be2F73- corresponds to the 522-cm polarized band in molten NazLiBe2F.1 (Figure 2A); therefore, the species in the melt giving rise to this band is identified as BezF73-. The additional polarized band at 550 cm-I and the depolarized band at 375 em-l are not caused by Be2FT3- since they remain when alkali metal fluoride is added to the melt while the 522cm band disappears (cf. Figure 2B). Therefore, the 550and 375-cm-1 bands are assigned to the VI(&) symmetric stretching mode and the uq(F2) bending mode, respectively, of BeF42 - ion produced by an association-dissociation process 2 B e z F ~ ~ BeFa2-
+ Be3Flo4-
(2)
when Na2LiBezF.r is melted or when the equilibrium is shifted to the right in eq 1, causing dissociation of Be2F.13- and formation of BeF42-. This contention is supported by the previous results5 in which u1 and u4 of BeF42- were observed at 550 and 385 emi1, respectively, in molten NazBeF4. In accordance with eq 2, a band due to Be3Flo4- is expected in the 5 0 0 - ~ m -region ~ (see below) but was not observed. The disappearance of the moderately strong 443-em-1 band of crystalline BezF73(Figure 1) when NazLiBezF~is melted can only be explained as being lost in the trough between the 520- and 375-cm-f bands shown in Figure 2. The possibility that it shifts downward by 68 cm-1 to contribute to the 375cm-l band is unlikely since we see no comparable frequency shifts with the other B ~ ~ F TFurthermore, ~-. assuming that our assignment of the 443-em-1 band i s correct, it should be polarized whereas the 375-em-f band is depolarized. The reverse of the process in eq 2 should occur when molten Na2LiBezF.1 is crystallized. The BeF42- and Be3Flo4- ions must recombine to form 2Be2Far3- ions for crystal growth to proceed. A weak band at ea. 568 cm-1 in the Raman spectrum of Na2LiBezF.r also was assigned to u1 of BeF4*- because its frequency agrees ciosely with the previous measurement of this mode in crystalline LizBeF4.5 The appearance of BeF42-- demonstrates that even under conditions of crystallization, the equilibrium in eq 2 is not completely shifted to the left. The quenched samples which formed glasses did not have sufficient time to undergo such a process and therefore represent a situation like that of the melt. They offer promise in studying melts under more favorable conditions than the high-tem-
Haman Spectre ef
Be2F73-
perature liquid state and merit further consideration in molten salt studies. Some limitations are incurred in the LiF-MaF-BeF2 system; however, in that only a few compositions will actually form glasses. Thermal shifts in the equilibrium of eq 2 were not observed; the Raman gpectra of NazLiBe2F.r melts at various temperatures showed no variations in the relative intensities of the 525- and 550-cm-1 bands. Because this observation was impaired by band overlap at higher temperatures, glasses of M812LiBe2F7 quenched from various temperatures between 350 and 800" were measured at 77°K to support the melt data. It was anticipated that it might be possible to quench the equilibrium configuration present at elevated temperatures into a glass and to examine the Raman spectrum of this configuration at lower temperatures. No change m the relative intensities of the two bands could be observed in the glass spectra either. The results from the glass experiments are ambiguous, because the equilibrium sf cq 2 may be subject to a rapid relaxation process which could shift it to an equilibrium position just abate the liquid-glass transition temperature. As a result, all melt quenches from different temperatures might in fact only represent the same condition just above the glass-forming temperature. If the equilibririrn constant for eq 2 does not change with temperature, tihen AH" = 0 would be required for the equilibrium rcwAion. The change in enthalpy can be estimated by considering the changes in bonding for the reaction. On the left-hand side of eq 2 there are 6 terminal and 2 bridging fluoride bonds for each B ~ z F group ~ ~ - or 12 terminal and 4 bridging bonds for the two. On the right-hand srtle, BeF42- has 4 terminal bonds and terminal and 4 bridging bonds; hence, there is a total of 12 terminal and 4 bridging fluoride bonds. Thus, them 11sno change in the number of terminal and bridging fluoride bonds and so there is very likely no change in enthalpy for the reaction. It can also be shown that ~ o ~ i g u r ~ other ~ t ~ othan n ~ the Be3F104- group, such as B rirrg structure of fluoride bridged beryllium ions, would lead to the same conclusion. There is merely a rearrangement of the terminal and bridging fluoride units upon melting the sample to form a number of different complexes in equilibrium with each other. LiF-BeFz ~ ~ It has ~ not ~been ~possible s to. resolve the individual intense Raman bands for the various beryllium fluoride species present in the spectra of melts where LiF is the sole alkali mebtal fluoride constituent because of the greater band widths involved. Therefore, the interpreta2 system is made by comparison with system. The same phenomena are observed in both cases33 as the alkali metal fluoride content is decreased from 7 5 to 60 mol %, there is a shift in the strong polasizad band from 550 to 525 cm-1 (Figure 5 and Table Ill). 'This arms from a shift in the equilibrium of eq 1 to the left from terminal to bridging species where eF42- monomers combine to form BezF?- dimers. As the alkali metal fluoride content is further decreased, the additional hequeney shift is interpreted as caused by increasing polymer formation. In the limit of pure BeF2, the polarized band appears at 282 cm-1.4b We believe that this frequency shift is caused by a change in the form of the symme1,ri~mode from stretching (BeF42-) to bending
221
(BeF2) as the extent of polymerization increases. Some evidence for this process is indicated by the mixing of the Be-F-Be bend and BeF (2-4 and 3-41 stretches in the v3 mode of BezF73-. The three depolarized bands observed in the LiF-BeF2 system at 1140, 1340, and 1590 cm -1 possibly arise either from combinations and overtones of BezF73- fundamentals, from fluorescing impurities, or from impurities producing Raman bands themselves. For overtones and combinations, three bands could plausibly arise from 550- and 8 0 0 - ~ m -bands, ~ e.g., 2 X 550 = 1100 cm--l, 2 X 800 = 1600 cm-l, and 550 + 800 = 1350 cm--1. However, the overtone of the 550-cm-1 v s ( A 1 ) mode also has AI symmetry and would be polarized. Furthermore, as the symmetric mode shifts to a lower frequency (Figure 5) the overtone and combination arising from it should also show a shift. They do not. If these were impurity fluorescence bands, they should shift or disappear when other exciting radiation is used. These bands showed no shift in frequency when 5145-A exciting line of the argon ion laser was used. They did, however, fade away over a period of time as the samples remained in the windowless cells, supporting the interpretation that they are Raman bands from a dissolved impurity in the melt.
Conclusions The primary purpose of this work was to extend our investigations of the polymerization mechanism of BeFz in LiF-BeFz melts. The results of the present and earlier work4 also provide a test of the Baes' polymer model. In general, the model as represented by the polymerization process described in eq 1 is consistent with the results of these studies. However, the hypothesis of edge sharing6 is not supported in view of the following evidence: (a) identification of the dimer, BezF73-, in LiF-NaF-BeFz melts which represents corner sharing of two tetrahedra, (b) studies with vitreous and molten beryllium flu0ride3.~in which, except for lattice defects, the BeFz network was determined to be constructed from corner sharing of tetrahedra, and (c) only corner sharing is found in analogous silicate systems. On the other hand, it is recognized that the experimental evidence is subject to the possibility that some small concentration of edge-shared dimers, BezFsZ-, could exist and not be identified because of extreme band overlap found in melt spectra. Finally, it must be noted that the technique employed here for identifying a complex species in solution by the position and intensity of a symmetric vibrational mode is often subject to uncertainties because several competing processes, such as changes in the coordination number and network formation by cross linking, may be occurring simultaneously and cannot be separately identified by this process alone. In the BeF2 system where only tetrahedral coordination around the beryllium ion occurss,this procedure is valid because there appear to be no other probable processes competing with the network formation mechanism. Acknowledgment. The authors wish to thank Max Bredig, Chemistry Division, QRNL, for his many helpful suggestions and interest in this work.
The Journal of Physical Chemistry, VoI. 77, No. 2, 1973
