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19F NMR evidence for multiple fluoride ion sites in heavy metal fluoride glasses
Journal of Non-Crystalline Solids 108 (1989) 289-293 North-Holland, Amsterdam
289
tgF NMR EVIDENCE FOR MULTIPLE FLUORIDE ION SITES IN HEAVY M E T A L F L U O R I D E G L A S S E S
D.R. M a c F A R L A N E and J.O. B R O W N E Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia
T.J. BASTOW and G.W. WEST Division of Materials Science and Technology, C.S. L R.O. Clayton, Victoria 3168, Australia Received 2 September 1988 Revised manuscript received 11 November 1988
A 19F NMR study of a typical heavy metal fluoride glass has revealed the presence of three discrete fluoride ion sites in the glass structure. Line shape analysis suggests that one of these sites plays a role in the structural framework while the other types of fluoride ion become mobile with increasing temperature.
The study of the structure of heavy metal fluoride glasses has been limited largely to X-ray diffraction and molecular dynamics investigations. A number of workers have examined the site symmetry of cations in these glass systems using, for example, M~)ssbauer [1] and electron paramagnetic' resonance [2], however there are only a small number of authors [3-7] who have examined the properties of the fluoride atom within the systems. Most of these studies have focussed on the mobility of the fluoride ion in the glass rather than the structural role that the ion assumes. In 1983 Bray et al. [4] demonstrated from a 19F N M R relaxation study the existence of two different types of fluorine, the first being a bound fluoride ion type and the second being a mobile fluoride ion, where approximately 6% of the fluoride ions were mobile. Multiple fluoride ion sites were also suggested by Almeida and Mackenzie [3] on the basis of I R and R a m a n evidence. Bray's work was extended in 1986 with the suggestion [6] that the non-Gaussian line shape of the fluorine line in a Z r F 4 : B a F 2 : L a F 3 (ZBL) glass was not typical of N M R lines in glass samples. Also noted was a strong similarity between the fluorine line shape for crystalline Z r F 4 and ZBL glass. This 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
indicated the strong likelihood of short range structure in the fluoride glass rather than the more random structure of borate or silicate glasses. These conclusions were supported by Aujla et al. [5] in 1987 by the observation of two fluorine lines in the N M R spectra of Z r F 4 : B a F 2 : L a F 3 and Z r F 4 : BaF 2 : N a F : LaF 3 glasses. In this communication some preliminary results of a 19F N M R study of the ZBL glass system at a range of temperatures from 150 to 420 K are outlined. The results indicate the presence of at least three distinct fluoride ion sites in the glass. On the basis of the chemical shift and mobility differences of the fluoride ions in these sites certain conclusions can be drawn concerning their nature. The spectra were recorded on a Bruker MSL 400 N M R spectrometer operating at 376.5 MHz. The 19F N M R spectra were acquired using a solid echo pulse sequence, 90fl-r-90y°-~--acquisition. A spectral width of 2.5 M H z was used since the 19F linewidths were typically 60 M H z at 300 K. The 90 ° pulse width was 3 ~ts and the tau value was typically 15 p.s. Simulations of the 19F N M R lines were preformed using L I N S I M , one of the Bruker suite of programs. This program allows manual or automatic adjustment of linewidth, intensity,
290
.
D.R. MacFarlane et aL / Fluoride ion sites in heaoy metal fluoride glasses
~
|ill
X,~_~
~--
Bd0
~d0
d, .
.
3d0 .
.
~d, I'I'M
• 10B
zlu
_
4m.._ In
-1~0
-2~0
Fig. 1. Variation with temperature of the 19F NMR spectrum for a ZrF4 : BaF2 : LaF3 glass. Note the marked splitting of the peak at higher temperatures.
Gaussian to Lorentzian line shape ratio and number of lines in the simulation. Glass samples were prepared by accurate weighing and mixing of the heavy metal fluoride powders in a vitreous carbon crucible under a dry and oxygen free nitrogen atmosphere. The crucible was then placed in a muffle or tube furnace under nitrogen and heated slowly to 9 5 0 ° C for 2 - 3 hours. The molten material was then splat quenched between two brass blocks to produce a wafer of glass. This glass was ground to a fine powder with an agate pestle and mortar under dry and oxygen free nitrogen. The sample was packed tightly into a 4.5 X 0.5 cm N M R tube and the composition of the glass determined by electron microprobe. The composition was found to be 64.5ZrF 4 : 28.5BaF 2 : 7.0LaF 3. N M R spectra were also obtained for fl-ZrF4(s ) and a-ZrBaFr(s) obtained by cooling a molten mixture of ZrF 4 and BaF 2. The spectra shown in fig. I demonstrate the effect of variation of temperature on the spectra of the glass. The first spectrum at 250 K shows a broad peak at 233.4 ppm, with respect to N a F , and a full width a half m a x i m u m ( F W H M ) of 180 ppm. The second spectrum collected at 295 K has a linewidth of 164 p p m with a small minimum at the peak. T h e third spectrum was collected at 420 K and depicts an almost triangular line with
F W H M of 120 p p m and a sharp minimum at the peak. The apparent line narrowing exhibited by these spectra as a function of temperature is considerable. This narrowing is only partly explained by increased fluoride ion motion with increased temperature. In order to better understand this phenomenon, each spectrum was simulated by the superposition of several calculated lines. Such a procedure can never yield a unique fit to the data, hence the number of lines was kept to a minimum consistent with a satisfactory fit to all spectra. In fig. 2A the superposition is illustrated for the spectrum measured at 250 K. The plot shows the experimental spectrum as compared with the calculated line (the latter is offset downward for clarity). The component of the calculated lines are indicated in the lower part of the plot. The quality of the fit of the simulated spectra to the data was determined by a calculation of the residual sum of squares. The spectrum (fig. 2A) was not well fitted by a single Gaussian, Lorentzian or any combination of two lines. Lines were also fitted with component lines of a mixed nature, i.e. Gaussian and Lorentzian. In general, these too were unsuitable, however in the smallest peak a small percentage of Lorentzian function could be tolerated without adversely affecting the fit. It was felt that this tolerance was more a result of statistics than reflecting a physical phenomenon. When fitted with three Gaussian lines the agreement between simulated and experimental line shape was very good. Simulation of the spectrum using two Lorentzian functions and a Gaussian resulted in a considerably worse fit to the data. A distinction between mobile and immobile ions can generally be made on the basis of the N M R lineshape associated with each type of ion [7], a Lorentzian line arising from mobile fluoride ions and a Gaussian from immobile ions. The lines in this spectrum (fig. 2A, 250 K) were all Gaussian in shape, therefore indicating relatively immobile ions. Fig. 2B shows the superposition fit of the spectrum measured at 295 K and once again a minim u m of three lines was required to achieve a reasonable fit. However, in this case the best fit required two Gaussian lines and one Lorentzian. Thus, it appears that, at room temperature, one of
D.R. MacFarlane et al. / Fluoride ion sites in heaoy metal fluoride glasses
[IP'! |IN CCUP
,do
,~o
4c;o
,do
2do I'PN
la"
'0
-I00
-26o
IOI
___j
[OfT
J J do
s~o
ClU COUP
4do
~o
~do PP~
ICI
2.~J edo
s~o
'"r'
~ r" PPH
~
O01P ,
Fig. 2. Component fines and the resultant fits to the experimental data. Experimental indicates the actual 19F N M R spectrum. Simulation indicates the fit to the data as a sum of the component lines. (A) 250 K; (B) 295 K; (C) 420 K.
291
the immobile fluoride ion types acquires sufficient energy to become more mobile, resulting in the appearance of the Lorentzian line. The change in the chemical shift of this line probably occurs because the ion type giving rise to this signal encounters a different environment as a result of its mobility. The third spectrum, fig. 2C obtained at 420 K, indicates that the trends observed in the previous spectra are continued. Here both of the small Gaussian lines have become Lorentzian in shape, indicating mobile fluoride ions. The triangular shape of the line is due to the sum of the Gaussian line and two narrowly separated Lorentzian lines. Similar simulations of the spectra for /~-ZrF4 and a-ZrBaF 6 showed that the former is made up of a single line being a mixed Gaussian (80%) and Lorentzian (20%) line. The a - Z r B a F 6 spectrum was found to be composed of two pure gaussian lines and a third line which was 73% Gaussian and 27% Lorentzian; the fitting data for this spectrum is also given in table 1. Comparison of the spectra of the ZBL glass with the two crystal spectra indicates that the B-ZrF4 spectra has little in c o m m o n with the other two, both the chemical shift and lineshape being different. In /~-ZrF4 all fluoride ions are thought to reside in similar corner sharing sites [10]. In a-ZrBaF 6 the fluoride ions reside in a mixture of edge sharing and terminal positions with respect to (Zr2Flz) 4- polyhedra [11]. The gross chemical shift difference between /~-ZrF4 and a - Z r B a F 6 is thus principally the result of the presence of Ba 2+ ions in the environment of each fluorine. The splitting of the a-ZrBaF 6 resonance into 3 lines is therefore probably also due either to the different b r i d g i n g / t e r m i n a l positions of the fluoride ions a n d / o r their interaction with the Ba 2+ ions. A comparison of the glass spectra indicate two principal points of interest. First, the area and linewidth of the largest component line remains unchanged as a function of temperature (to within two standard deviations). This independence, along with the Gaussian nature of the line, indicates that the fluorine species responsible for this resonance are held in sites which limit their mobility. The lack of temperature dependence of the area of this line also indicates that the same type
D.R. MacFarlane et al. / Fluoride ion sites in heavy metal fluoride glasses
292
Table 1 19F N M R lineshape simulation data Temp. (K) +2
Lineshape Component
Chem. Shift (ppm) + 3
Area (%)
Width (Hz) + 500
Line Type
1 2 3
238 319 139
88.1+1.0 2.4 + 0.5 9.4 + 0.7
58910 36273 35338
Gaussian Gaussian Gaussian
295
1 2 3
241 215 144
91.1 + 1.0 1.6 +0.5 7.3 + 0.7
59227 10058 34579
Gaussian Lorentz, Gaussian
420
1 2 3
237 221 246
89.0 + 1.0 5.2 + 0.7 5.8 + 1.0
60498 6209 9740
Gaussian torentz, Lorentz.
a-ZrBaF 6
1 2 3
245 216 157
49.0 + 1.0 21.5 + 0.5 29.5 + 1.0
49054 21832 32134
73%G/27%L Gaussian Gaussian
ZBL glass 250
of fluoride ion is involved at each temperature. This supports Bray's [4] hypothesis of one immobile fluoride ion and short range structure. It is probable that these fluoride ions are associated strongly with one or m o r e Z r 4÷ cations. This conclusion is supported by two facts. First, the majority of fluoride ions in the glass are associated with Z r 4÷ cations since this is the predominant cation and each is coordinated to approximately eight fluoride ions [8]. Second, from the trends in chemical shift (table 2), fluoride ions coordinated to Z r 4÷ cation exhibit a more covalent interaction than those associated with Ba 2+ and La 3+ cations. The higher covalency of this interaction indicates that these fluoride ions should be less mobile. While the electronegativity is a good first order measure of covalency and chemical shift in a crystalline system, it is only one of several effects contributing to the chemical shift. In more complex systems such as ZrBaF6 and glasses other factors can influence the chemical
shift [9]. For example BaF 2 has a chemical shift of 230 ppm but a-ZrBaF6 has a chemical shift of 213 ppm. This difference is a result of the relative B a - F coordination distances; 2.68 A in BaF 2 [10], - 2.9 A in a-ZrBaF 6 [11], the fluoride ions in the latter being more shielded and therefore resonating further up-field than BaF 2. The second conclusion involves the two other types of fluoride ion observed. At low temperature both types are frozen into the structure and are separated by 180 ppm. Upon heating both types become more mobile. This mobility alters the spectrum, producing lorentzian line shapes and a reduction in the separation to 25 ppm. As expected with increased mobility, the fluoride ions involved experience a similar environment causing their chemical shifts to converge. At sufficiently high temperature the lines would eventually merge. The fluoride ions involved are probably, at low temperatures, situated in environments which are more ionic in nature and therefore more strongly
Table 2 19F N M R chemical shift data and electronegativity of some metal fluorides
Chemical Shift (ppm) Electronegativity [13]
NaF
ZrF 4
BaF2
LaF3
Z B L glass
a-ZrBaF 6
0.0 3.09
340 2.88
230 3.13
215 3.02
240 -
213 -
F 2 [12] 667 0
D.R. MacFarlane et al. / Fluoride ion sites in heavy metal fluoride glasses
influenced by the Ba 2+ a n d / o r La 3÷ cations. The fluoride ions in the glass appear not to be described by either a single broad distribution of sites or several unique symmetries. It appears, rather, that they are better described by three local maxima in the general distribution of sites and that these sites may have features in common with crystalline a-ZrBaF 6. For these reasons further work is in progress to clarify the problems of the particular geometries and the ions coordinated to the fluoride ions in these sites.
References [1] L.D. Bogomolova, E.G. Grechko, N.A. Krasil'nikova and V.V. Sakharov, J. Non-Cryst. Sol. 69 (1985) 299. [2] T. Nishida, T. Nonaka and Y. Takashima, Bull. Chem. Soc. Jpn. 58 (1985) 2255.
293
[3] R. Almeida and J.D. Mackenzie, J. Chem. Phys. 78 (1983) 6502. [4] P.J. Bray, D.E. Hintenlang, R.V. Mulkern, S.G. Greenbaum and D.C. Tran, J. Non-Cryst. Sol. 56 (1983) 27. [5] R.S. Aujla, R. Dupree, D. Holland and A.P. Kemp, Proc. 4th Intern. Symp. on Halide Glasses (1987) p. 234-42. [6] P.J. Bray and R.V. Mulkern, J. Non-Cryst. Sol. 80 (1986) 181. [7] A. Abragam, The Principles of Nuclear Magnetism (Oxford Press, 1961) p. 107, 433. [8] J. Lucas, C.A. Angell and S. Tamaddon, Mater. Res. Bull. 19 (1984) 945. [9] J.-P. Laval, R. Papiemik and F. Bernard, Acta Cryst. B34 (1978) 1070. [10] R.W.G. Wychoff, Crystal Structures, 2nd ed., vol 2 (John Wiley and Sons, New York, 1964) p. 127-8. [11] M. Mehring, Principles of High Resolution NMR in Solids, 2nd ed. (Springer-Verlag, Berlin, 1983) p. 233, 240-5. [12] V.M. Bouznik and L.M. Avkhutsky, J. Magn. Res. 5 (1971) 63. [13] R.S. Berry, S.A. Rice and J. Ross, Physical Chemistry (John Wiley and Sons, New York, 1980).
289
tgF NMR EVIDENCE FOR MULTIPLE FLUORIDE ION SITES IN HEAVY M E T A L F L U O R I D E G L A S S E S
D.R. M a c F A R L A N E and J.O. B R O W N E Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia
T.J. BASTOW and G.W. WEST Division of Materials Science and Technology, C.S. L R.O. Clayton, Victoria 3168, Australia Received 2 September 1988 Revised manuscript received 11 November 1988
A 19F NMR study of a typical heavy metal fluoride glass has revealed the presence of three discrete fluoride ion sites in the glass structure. Line shape analysis suggests that one of these sites plays a role in the structural framework while the other types of fluoride ion become mobile with increasing temperature.
The study of the structure of heavy metal fluoride glasses has been limited largely to X-ray diffraction and molecular dynamics investigations. A number of workers have examined the site symmetry of cations in these glass systems using, for example, M~)ssbauer [1] and electron paramagnetic' resonance [2], however there are only a small number of authors [3-7] who have examined the properties of the fluoride atom within the systems. Most of these studies have focussed on the mobility of the fluoride ion in the glass rather than the structural role that the ion assumes. In 1983 Bray et al. [4] demonstrated from a 19F N M R relaxation study the existence of two different types of fluorine, the first being a bound fluoride ion type and the second being a mobile fluoride ion, where approximately 6% of the fluoride ions were mobile. Multiple fluoride ion sites were also suggested by Almeida and Mackenzie [3] on the basis of I R and R a m a n evidence. Bray's work was extended in 1986 with the suggestion [6] that the non-Gaussian line shape of the fluorine line in a Z r F 4 : B a F 2 : L a F 3 (ZBL) glass was not typical of N M R lines in glass samples. Also noted was a strong similarity between the fluorine line shape for crystalline Z r F 4 and ZBL glass. This 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
indicated the strong likelihood of short range structure in the fluoride glass rather than the more random structure of borate or silicate glasses. These conclusions were supported by Aujla et al. [5] in 1987 by the observation of two fluorine lines in the N M R spectra of Z r F 4 : B a F 2 : L a F 3 and Z r F 4 : BaF 2 : N a F : LaF 3 glasses. In this communication some preliminary results of a 19F N M R study of the ZBL glass system at a range of temperatures from 150 to 420 K are outlined. The results indicate the presence of at least three distinct fluoride ion sites in the glass. On the basis of the chemical shift and mobility differences of the fluoride ions in these sites certain conclusions can be drawn concerning their nature. The spectra were recorded on a Bruker MSL 400 N M R spectrometer operating at 376.5 MHz. The 19F N M R spectra were acquired using a solid echo pulse sequence, 90fl-r-90y°-~--acquisition. A spectral width of 2.5 M H z was used since the 19F linewidths were typically 60 M H z at 300 K. The 90 ° pulse width was 3 ~ts and the tau value was typically 15 p.s. Simulations of the 19F N M R lines were preformed using L I N S I M , one of the Bruker suite of programs. This program allows manual or automatic adjustment of linewidth, intensity,
290
.
D.R. MacFarlane et aL / Fluoride ion sites in heaoy metal fluoride glasses
~
|ill
X,~_~
~--
Bd0
~d0
d, .
.
3d0 .
.
~d, I'I'M
• 10B
zlu
_
4m.._ In
-1~0
-2~0
Fig. 1. Variation with temperature of the 19F NMR spectrum for a ZrF4 : BaF2 : LaF3 glass. Note the marked splitting of the peak at higher temperatures.
Gaussian to Lorentzian line shape ratio and number of lines in the simulation. Glass samples were prepared by accurate weighing and mixing of the heavy metal fluoride powders in a vitreous carbon crucible under a dry and oxygen free nitrogen atmosphere. The crucible was then placed in a muffle or tube furnace under nitrogen and heated slowly to 9 5 0 ° C for 2 - 3 hours. The molten material was then splat quenched between two brass blocks to produce a wafer of glass. This glass was ground to a fine powder with an agate pestle and mortar under dry and oxygen free nitrogen. The sample was packed tightly into a 4.5 X 0.5 cm N M R tube and the composition of the glass determined by electron microprobe. The composition was found to be 64.5ZrF 4 : 28.5BaF 2 : 7.0LaF 3. N M R spectra were also obtained for fl-ZrF4(s ) and a-ZrBaFr(s) obtained by cooling a molten mixture of ZrF 4 and BaF 2. The spectra shown in fig. I demonstrate the effect of variation of temperature on the spectra of the glass. The first spectrum at 250 K shows a broad peak at 233.4 ppm, with respect to N a F , and a full width a half m a x i m u m ( F W H M ) of 180 ppm. The second spectrum collected at 295 K has a linewidth of 164 p p m with a small minimum at the peak. T h e third spectrum was collected at 420 K and depicts an almost triangular line with
F W H M of 120 p p m and a sharp minimum at the peak. The apparent line narrowing exhibited by these spectra as a function of temperature is considerable. This narrowing is only partly explained by increased fluoride ion motion with increased temperature. In order to better understand this phenomenon, each spectrum was simulated by the superposition of several calculated lines. Such a procedure can never yield a unique fit to the data, hence the number of lines was kept to a minimum consistent with a satisfactory fit to all spectra. In fig. 2A the superposition is illustrated for the spectrum measured at 250 K. The plot shows the experimental spectrum as compared with the calculated line (the latter is offset downward for clarity). The component of the calculated lines are indicated in the lower part of the plot. The quality of the fit of the simulated spectra to the data was determined by a calculation of the residual sum of squares. The spectrum (fig. 2A) was not well fitted by a single Gaussian, Lorentzian or any combination of two lines. Lines were also fitted with component lines of a mixed nature, i.e. Gaussian and Lorentzian. In general, these too were unsuitable, however in the smallest peak a small percentage of Lorentzian function could be tolerated without adversely affecting the fit. It was felt that this tolerance was more a result of statistics than reflecting a physical phenomenon. When fitted with three Gaussian lines the agreement between simulated and experimental line shape was very good. Simulation of the spectrum using two Lorentzian functions and a Gaussian resulted in a considerably worse fit to the data. A distinction between mobile and immobile ions can generally be made on the basis of the N M R lineshape associated with each type of ion [7], a Lorentzian line arising from mobile fluoride ions and a Gaussian from immobile ions. The lines in this spectrum (fig. 2A, 250 K) were all Gaussian in shape, therefore indicating relatively immobile ions. Fig. 2B shows the superposition fit of the spectrum measured at 295 K and once again a minim u m of three lines was required to achieve a reasonable fit. However, in this case the best fit required two Gaussian lines and one Lorentzian. Thus, it appears that, at room temperature, one of
D.R. MacFarlane et al. / Fluoride ion sites in heaoy metal fluoride glasses
[IP'! |IN CCUP
,do
,~o
4c;o
,do
2do I'PN
la"
'0
-I00
-26o
IOI
___j
[OfT
J J do
s~o
ClU COUP
4do
~o
~do PP~
ICI
2.~J edo
s~o
'"r'
~ r" PPH
~
O01P ,
Fig. 2. Component fines and the resultant fits to the experimental data. Experimental indicates the actual 19F N M R spectrum. Simulation indicates the fit to the data as a sum of the component lines. (A) 250 K; (B) 295 K; (C) 420 K.
291
the immobile fluoride ion types acquires sufficient energy to become more mobile, resulting in the appearance of the Lorentzian line. The change in the chemical shift of this line probably occurs because the ion type giving rise to this signal encounters a different environment as a result of its mobility. The third spectrum, fig. 2C obtained at 420 K, indicates that the trends observed in the previous spectra are continued. Here both of the small Gaussian lines have become Lorentzian in shape, indicating mobile fluoride ions. The triangular shape of the line is due to the sum of the Gaussian line and two narrowly separated Lorentzian lines. Similar simulations of the spectra for /~-ZrF4 and a-ZrBaF 6 showed that the former is made up of a single line being a mixed Gaussian (80%) and Lorentzian (20%) line. The a - Z r B a F 6 spectrum was found to be composed of two pure gaussian lines and a third line which was 73% Gaussian and 27% Lorentzian; the fitting data for this spectrum is also given in table 1. Comparison of the spectra of the ZBL glass with the two crystal spectra indicates that the B-ZrF4 spectra has little in c o m m o n with the other two, both the chemical shift and lineshape being different. In /~-ZrF4 all fluoride ions are thought to reside in similar corner sharing sites [10]. In a-ZrBaF 6 the fluoride ions reside in a mixture of edge sharing and terminal positions with respect to (Zr2Flz) 4- polyhedra [11]. The gross chemical shift difference between /~-ZrF4 and a - Z r B a F 6 is thus principally the result of the presence of Ba 2+ ions in the environment of each fluorine. The splitting of the a-ZrBaF 6 resonance into 3 lines is therefore probably also due either to the different b r i d g i n g / t e r m i n a l positions of the fluoride ions a n d / o r their interaction with the Ba 2+ ions. A comparison of the glass spectra indicate two principal points of interest. First, the area and linewidth of the largest component line remains unchanged as a function of temperature (to within two standard deviations). This independence, along with the Gaussian nature of the line, indicates that the fluorine species responsible for this resonance are held in sites which limit their mobility. The lack of temperature dependence of the area of this line also indicates that the same type
D.R. MacFarlane et al. / Fluoride ion sites in heavy metal fluoride glasses
292
Table 1 19F N M R lineshape simulation data Temp. (K) +2
Lineshape Component
Chem. Shift (ppm) + 3
Area (%)
Width (Hz) + 500
Line Type
1 2 3
238 319 139
88.1+1.0 2.4 + 0.5 9.4 + 0.7
58910 36273 35338
Gaussian Gaussian Gaussian
295
1 2 3
241 215 144
91.1 + 1.0 1.6 +0.5 7.3 + 0.7
59227 10058 34579
Gaussian Lorentz, Gaussian
420
1 2 3
237 221 246
89.0 + 1.0 5.2 + 0.7 5.8 + 1.0
60498 6209 9740
Gaussian torentz, Lorentz.
a-ZrBaF 6
1 2 3
245 216 157
49.0 + 1.0 21.5 + 0.5 29.5 + 1.0
49054 21832 32134
73%G/27%L Gaussian Gaussian
ZBL glass 250
of fluoride ion is involved at each temperature. This supports Bray's [4] hypothesis of one immobile fluoride ion and short range structure. It is probable that these fluoride ions are associated strongly with one or m o r e Z r 4÷ cations. This conclusion is supported by two facts. First, the majority of fluoride ions in the glass are associated with Z r 4÷ cations since this is the predominant cation and each is coordinated to approximately eight fluoride ions [8]. Second, from the trends in chemical shift (table 2), fluoride ions coordinated to Z r 4÷ cation exhibit a more covalent interaction than those associated with Ba 2+ and La 3+ cations. The higher covalency of this interaction indicates that these fluoride ions should be less mobile. While the electronegativity is a good first order measure of covalency and chemical shift in a crystalline system, it is only one of several effects contributing to the chemical shift. In more complex systems such as ZrBaF6 and glasses other factors can influence the chemical
shift [9]. For example BaF 2 has a chemical shift of 230 ppm but a-ZrBaF6 has a chemical shift of 213 ppm. This difference is a result of the relative B a - F coordination distances; 2.68 A in BaF 2 [10], - 2.9 A in a-ZrBaF 6 [11], the fluoride ions in the latter being more shielded and therefore resonating further up-field than BaF 2. The second conclusion involves the two other types of fluoride ion observed. At low temperature both types are frozen into the structure and are separated by 180 ppm. Upon heating both types become more mobile. This mobility alters the spectrum, producing lorentzian line shapes and a reduction in the separation to 25 ppm. As expected with increased mobility, the fluoride ions involved experience a similar environment causing their chemical shifts to converge. At sufficiently high temperature the lines would eventually merge. The fluoride ions involved are probably, at low temperatures, situated in environments which are more ionic in nature and therefore more strongly
Table 2 19F N M R chemical shift data and electronegativity of some metal fluorides
Chemical Shift (ppm) Electronegativity [13]
NaF
ZrF 4
BaF2
LaF3
Z B L glass
a-ZrBaF 6
0.0 3.09
340 2.88
230 3.13
215 3.02
240 -
213 -
F 2 [12] 667 0
D.R. MacFarlane et al. / Fluoride ion sites in heavy metal fluoride glasses
influenced by the Ba 2+ a n d / o r La 3÷ cations. The fluoride ions in the glass appear not to be described by either a single broad distribution of sites or several unique symmetries. It appears, rather, that they are better described by three local maxima in the general distribution of sites and that these sites may have features in common with crystalline a-ZrBaF 6. For these reasons further work is in progress to clarify the problems of the particular geometries and the ions coordinated to the fluoride ions in these sites.
References [1] L.D. Bogomolova, E.G. Grechko, N.A. Krasil'nikova and V.V. Sakharov, J. Non-Cryst. Sol. 69 (1985) 299. [2] T. Nishida, T. Nonaka and Y. Takashima, Bull. Chem. Soc. Jpn. 58 (1985) 2255.
293
[3] R. Almeida and J.D. Mackenzie, J. Chem. Phys. 78 (1983) 6502. [4] P.J. Bray, D.E. Hintenlang, R.V. Mulkern, S.G. Greenbaum and D.C. Tran, J. Non-Cryst. Sol. 56 (1983) 27. [5] R.S. Aujla, R. Dupree, D. Holland and A.P. Kemp, Proc. 4th Intern. Symp. on Halide Glasses (1987) p. 234-42. [6] P.J. Bray and R.V. Mulkern, J. Non-Cryst. Sol. 80 (1986) 181. [7] A. Abragam, The Principles of Nuclear Magnetism (Oxford Press, 1961) p. 107, 433. [8] J. Lucas, C.A. Angell and S. Tamaddon, Mater. Res. Bull. 19 (1984) 945. [9] J.-P. Laval, R. Papiemik and F. Bernard, Acta Cryst. B34 (1978) 1070. [10] R.W.G. Wychoff, Crystal Structures, 2nd ed., vol 2 (John Wiley and Sons, New York, 1964) p. 127-8. [11] M. Mehring, Principles of High Resolution NMR in Solids, 2nd ed. (Springer-Verlag, Berlin, 1983) p. 233, 240-5. [12] V.M. Bouznik and L.M. Avkhutsky, J. Magn. Res. 5 (1971) 63. [13] R.S. Berry, S.A. Rice and J. Ross, Physical Chemistry (John Wiley and Sons, New York, 1980).