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A Mössbauer spectroscopic study of the europiu-mammonia system
Journal
of the Less-Common
Metals,
118
A MijSSBAUER SPECTROSCOPIC AMMONIA SYSTEM
(1986)
135
STUDY
135
- 140
OF THE EUROPIUM-
0. BIRGtiL Department
of Chemistry,
Middle
East Technical
University,
Ankara
(Turkey)
S. J. LYLE The Chemical (Received
Laboratory,
May
17,
University
of Kent
at Canterbury,
Kent
(Ct. Britain)
1985)
Summary The solids isolated from solutions of europium metal in liquid ammonia in the absence of air or moisture were found to contain increasing proportions of a europium(II1) product with increasing reaction times (0 to 1.5 h). Long (several days) reaction times lead to formation of Eu(NH,)~ and it is suggested that the europium(II1) is derived from unstable Eu(NH*)~ present in the reaction mixture, the latter acting as a reaction intermediate in the formation of the former.
1. Introduction Europium metal dissolves in liquid ammonia [l] to give a solution containing solvated electrons and from it an isolatable hexammine Eu(NH~)~. Mossbauer studies on this hexammine [ 2 - 41 have provided evidence that the metal in it is divalent and has a spherically symmetrical 4f7 electron configuration. It has also been well established that the europium-ammonia solution decomposes slowly in a sealed tube at room temperature (rapidly at higher temperatures or in the presence of a catalyst) to form europium diamide Eu(NH,)~ [5 - 71. In the present work an attempt has been made to obtain information about the intermediates produced during the diamide formation from the europium-ammonia solution. For this purpuse, following the dissolution of the europium metal in ammonia, the reaction was stopped at different times by cooling the reaction tube in liquid nitrogen, and Mossbauer and IR spectroscopic measurements and chemical analysis were performed on the solid products obtained after the ammonia had been completely removed by evaporation. The results of these studies are reported here. 0022-5088/86/$3.50
@ Elsevier
Sequoia/Printed
in The
Netherlands
136
2. Experimental
details
Europium metal was obtained in sealed glass ampoules from Rare Earth Products Limited, Gt. Britain, and had a purity of 99.99%. Ammonia was doubly distilled over sodium metal before use and the reaction tube was washed several times with dried ammonia before addition of the europium metal. When europium metal (0.3 - 0.4 g) and ammonia (10 - 15 ml) were introduced into the reaction tube (approximate volume, 50 ml), it was sealed while the contents were chilled in liquid nitrogen. It was then left at room temperature to warm up and timing was started when a blue colour appeared. The reaction was stopped 0 to 1.5 h afterwards by immersing the tube in liquid nitrogen again. No catalyst was used. In the reaction to obtain europium diamide, FezOX (0.5 mg) was used as catalyst and the sealed reaction tube was kept at room temperature for one day and then at 50 “C for three days. All samples were handled in a glove box in an atmosphere of dry argon. The Mossbauer spectrometer and the “‘Eu source in SmF3 are described elsewhere [ 81. Whenever metal was present the Mossbauer spectrum was taken at 100 K, that is, above the first-order t~nsition below which the magnetic hyperfine spectrum is observed. However, the spectra for Eu(NH~)~ were obtained at 78 K. The absorber and the europium-151 source were held at the same temperature during the recording of each spectrum. Isomer shifts were measured relative to EuF3. IR spectra in Nujol were obtained using a Perkin-Elmer 683 spectrometer. For chemical analysis, the sample (about 30 mg) was weighed out, added to 10 wt.% NaOH (20 ml) and the ammonia collected by distillation in 20 ml of standard 0.05 M H,S04. The excess acid was titrated with standard NaOH. Europium metal was determined in the distillation flask by EDTA titration. 3. Results and discussion Typical Mossbauer spectra of the samples corresponding to different reaction times are given in Figs. 1 - 4. A typical spectrum for Eu(NH2)* is presented in Fig. 5. Figures 1 - 4 indicate, to varying extents, the presence of metal, di- and trivalent europium compounds. The presence of metal was confirmed by running some spectra at 78 K for the samples referred to in these figures. The ranges of isomer shift, measured relative to EuF3, for europium(I1) and europium(III) compounds are well established [ 91. The most interesting feature associated with the spectra (Figs. 1 - 4) is the increasing size of the europium( III) peak with time. If it is assumed that the recoil-free fractions for the europium(II1) component of the mixture are the same for each sample then the ratio R, of the
137
Fig. 1. Mksbauer spectrum at 100 diately after dissolution of europium.
K of a sample
obtained
by removal
of NH3 imme-
I
-28
-16
-12
-R
-+
6
4
6
I2
16 ‘/LLOCITY
Fig. 2. Mijssbauer of europium.
spectrum
at 100
K of a sample
2(
2i mm/5
obtained
about
0.5 h after
dissolution
channel count at the peak maximum to the baseline count gives a rough measure of the relative fraction of each sample in this valence state. Values are presented in Table. 1. Keeping in mind that the concentrations of the europium metal in the liquid ammonia could only be controlled crudely
138 r
42068
3 8 41400
. 40800
40200
39668
39600
38408
37868
37200 VELOCITY
Fig. 3. MGssbauer of em-opium. m %
mm/i
spectrum
at 100
K of a sample obtained
spectrum
at 100
K of a sample
about
0.75
h after dissolution
48125
6 47500
46250
45625
45000
43125 VELOCITY
Fig. 4. Mijssbauer of europium.
mm15
obtained
about
1.5 h after dissolution
(perhaps varying by 25% to 30% from experiment to experiment), the data suggest a roughly linear increase in europium(II1) with time for these short reaction times in the absence of a catalyst. Treatment of the peak heights corresponding to europium metal in a similar way and taking ratios R2 of
139
6’ep” -7
t
’ 8
’ 12
’ 16 vmm
Fig. 5. Mijssbauer TABLE
spectrum
at 77 K of Eu(NH2
?‘B, w5
)2.
1
The quantities R1 and R2, defined in the text, derived from the spectra Isomer shifts are also recorded for the europium( III) components Time
RI
R2
0 0.5 0.75 1.5
0.01 0.02 0.08 0.10
6 3 0.5 0.4
Errors
quoted
are standard
deviations
in Figs.
1
- 4.
Isomer shift europium(III) (mm 5-l ) 1.52 1.49 1.51 1.46
f f f f
0.06 0.07 0.04 0.03
of the mean values.
peak heights for the metal to those for the europium(II1) species gives the data recorded in Table 1. It is seen that the europium(II1) increases at the expense of the metal. Low relative concentrations and the poorly resolved spectral components for europium(I1) prevent a similar analysis being carried out for this component of the mixture. The spectrum presented in Fig. 5 coupled with data from the chemical analysis indicate that the Eu(NH,)~ obtained had a good state of purity. In view of the relatively long reaction time to obtain the amide from the same apparatus as the samples referred to in Figs. 1 - 4, it can be concluded that europium(II1) is not produced in the latter as a consequence of entry of air or water vapour. This conclusion was qualitatively supported by the data from chemical analysis and IR spectra of the samples used to obtain the
140
Mijssbauer spectra. These data gave nitrogen contents increasing with increasing reaction times (0 to 1.5 h) and evidence for N-H bonds from the vibration spectra. The isomer shift for the europium(II1) component has values from 1.46 to 1.52 mm s- ’ (Table 1). This is close to the value 1.327 reported for EuN [lo]. However, in view of the thermodynamic stability of EuN and the observation that after long reaction times Eu(NH*), is produced from the reaction mixtures discussed here, it seems reasonable to conclude that the europium(II1) is not present in the ammonia solution as the nitride. Failure to produce Eu(NH2)* from EuN in liquid ammonia provides further supportive evidence. Salot [ll] obtained a mixture of Yb(NH2)2 and Yb(NH,)3 from the metal in liquid ammonia in the presence of a catalyst at 20 “C. Juza and coworkers [ 7, 121 found that similar products were produced at 20 “C without a catalyst, but the relative yields of the amides were different. It seems reasonable to suggest from the observations presented here that either Eu( NH2 )a or europium( III) amide-imide mixture by analogy with ytterbium [ 131, are produced as unstable intermediates which decompose to a more stable europium(II1) compound during the isolation of the solid from the excess ammonia. Of these two possible substances the amide would necessarily be present since it precedes imide formation. However, under the conditions for production of Eu(NH~)~, this intermediate would appear to undergo a valence change from III to II perhaps under the influence of solvated electrons in the system. Further experimental work is required to establish the optimum conditions for production and isolation of Eu(NH*)~. Acknowledgment One of us (O.B.) gratefully acknowledges part financial the British Council during the time this work was carried out.
support
from
References 1 2 3 4
5 6 7 8 9 10 11 12
13
J. C. Warf and W. L. Korst, J. Phys. Chem., 60 (1956) 1590. J. P. Brown, R. L. Cohen and K. W. West, Chem. Phys. Lett., 20 (1973) 271. F. T. Parker and M. Kaplan, J. Chem. Phys., 60 (1974) 1328. R. F. Marzke and W. S.’ Glaunsinger, J. Phys. Chem., 79 (1975) 2976. R. Juza and C. Hadenfeldt, Naturwissenschaften, 55 (1968) 229. K. Howell and L. L. Pytlewski, J. Less-Common Met., 19 (1969) 399. C. Hadenfeldt, H. Jacobs and R. Juza, 2. Anorg. Allg. Chem., 379 (1970) 144. C. M. Jenden and S. J. Lyle, J. Chem. Sot., Dalton Trans., (1982) 2409. G. K. Shenoy and F. E. Wagner, Mkbauer Isomer Shifts, North-Holland, Amsterdam, 1978. I. Colquhoun, N. N. Greenwood, I. J. McCoIm and G. E Turner, J. Chem. Sot., Dalton Trans., (1972) 1337. S. Salot, Ph.D. Thesis, University of Southern California, 1969 (NTIS 70-366). R. Juza and C. Hadenfeldt, Naturwissenschaften, 56 (1969) 282. S. Salot and J. C. Warf, J. Am. Chem. Sot., 90 (7) (1968) 1932).
of the Less-Common
Metals,
118
A MijSSBAUER SPECTROSCOPIC AMMONIA SYSTEM
(1986)
135
STUDY
135
- 140
OF THE EUROPIUM-
0. BIRGtiL Department
of Chemistry,
Middle
East Technical
University,
Ankara
(Turkey)
S. J. LYLE The Chemical (Received
Laboratory,
May
17,
University
of Kent
at Canterbury,
Kent
(Ct. Britain)
1985)
Summary The solids isolated from solutions of europium metal in liquid ammonia in the absence of air or moisture were found to contain increasing proportions of a europium(II1) product with increasing reaction times (0 to 1.5 h). Long (several days) reaction times lead to formation of Eu(NH,)~ and it is suggested that the europium(II1) is derived from unstable Eu(NH*)~ present in the reaction mixture, the latter acting as a reaction intermediate in the formation of the former.
1. Introduction Europium metal dissolves in liquid ammonia [l] to give a solution containing solvated electrons and from it an isolatable hexammine Eu(NH~)~. Mossbauer studies on this hexammine [ 2 - 41 have provided evidence that the metal in it is divalent and has a spherically symmetrical 4f7 electron configuration. It has also been well established that the europium-ammonia solution decomposes slowly in a sealed tube at room temperature (rapidly at higher temperatures or in the presence of a catalyst) to form europium diamide Eu(NH,)~ [5 - 71. In the present work an attempt has been made to obtain information about the intermediates produced during the diamide formation from the europium-ammonia solution. For this purpuse, following the dissolution of the europium metal in ammonia, the reaction was stopped at different times by cooling the reaction tube in liquid nitrogen, and Mossbauer and IR spectroscopic measurements and chemical analysis were performed on the solid products obtained after the ammonia had been completely removed by evaporation. The results of these studies are reported here. 0022-5088/86/$3.50
@ Elsevier
Sequoia/Printed
in The
Netherlands
136
2. Experimental
details
Europium metal was obtained in sealed glass ampoules from Rare Earth Products Limited, Gt. Britain, and had a purity of 99.99%. Ammonia was doubly distilled over sodium metal before use and the reaction tube was washed several times with dried ammonia before addition of the europium metal. When europium metal (0.3 - 0.4 g) and ammonia (10 - 15 ml) were introduced into the reaction tube (approximate volume, 50 ml), it was sealed while the contents were chilled in liquid nitrogen. It was then left at room temperature to warm up and timing was started when a blue colour appeared. The reaction was stopped 0 to 1.5 h afterwards by immersing the tube in liquid nitrogen again. No catalyst was used. In the reaction to obtain europium diamide, FezOX (0.5 mg) was used as catalyst and the sealed reaction tube was kept at room temperature for one day and then at 50 “C for three days. All samples were handled in a glove box in an atmosphere of dry argon. The Mossbauer spectrometer and the “‘Eu source in SmF3 are described elsewhere [ 81. Whenever metal was present the Mossbauer spectrum was taken at 100 K, that is, above the first-order t~nsition below which the magnetic hyperfine spectrum is observed. However, the spectra for Eu(NH~)~ were obtained at 78 K. The absorber and the europium-151 source were held at the same temperature during the recording of each spectrum. Isomer shifts were measured relative to EuF3. IR spectra in Nujol were obtained using a Perkin-Elmer 683 spectrometer. For chemical analysis, the sample (about 30 mg) was weighed out, added to 10 wt.% NaOH (20 ml) and the ammonia collected by distillation in 20 ml of standard 0.05 M H,S04. The excess acid was titrated with standard NaOH. Europium metal was determined in the distillation flask by EDTA titration. 3. Results and discussion Typical Mossbauer spectra of the samples corresponding to different reaction times are given in Figs. 1 - 4. A typical spectrum for Eu(NH2)* is presented in Fig. 5. Figures 1 - 4 indicate, to varying extents, the presence of metal, di- and trivalent europium compounds. The presence of metal was confirmed by running some spectra at 78 K for the samples referred to in these figures. The ranges of isomer shift, measured relative to EuF3, for europium(I1) and europium(III) compounds are well established [ 91. The most interesting feature associated with the spectra (Figs. 1 - 4) is the increasing size of the europium( III) peak with time. If it is assumed that the recoil-free fractions for the europium(II1) component of the mixture are the same for each sample then the ratio R, of the
137
Fig. 1. Mksbauer spectrum at 100 diately after dissolution of europium.
K of a sample
obtained
by removal
of NH3 imme-
I
-28
-16
-12
-R
-+
6
4
6
I2
16 ‘/LLOCITY
Fig. 2. Mijssbauer of europium.
spectrum
at 100
K of a sample
2(
2i mm/5
obtained
about
0.5 h after
dissolution
channel count at the peak maximum to the baseline count gives a rough measure of the relative fraction of each sample in this valence state. Values are presented in Table. 1. Keeping in mind that the concentrations of the europium metal in the liquid ammonia could only be controlled crudely
138 r
42068
3 8 41400
. 40800
40200
39668
39600
38408
37868
37200 VELOCITY
Fig. 3. MGssbauer of em-opium. m %
mm/i
spectrum
at 100
K of a sample obtained
spectrum
at 100
K of a sample
about
0.75
h after dissolution
48125
6 47500
46250
45625
45000
43125 VELOCITY
Fig. 4. Mijssbauer of europium.
mm15
obtained
about
1.5 h after dissolution
(perhaps varying by 25% to 30% from experiment to experiment), the data suggest a roughly linear increase in europium(II1) with time for these short reaction times in the absence of a catalyst. Treatment of the peak heights corresponding to europium metal in a similar way and taking ratios R2 of
139
6’ep” -7
t
’ 8
’ 12
’ 16 vmm
Fig. 5. Mijssbauer TABLE
spectrum
at 77 K of Eu(NH2
?‘B, w5
)2.
1
The quantities R1 and R2, defined in the text, derived from the spectra Isomer shifts are also recorded for the europium( III) components Time
RI
R2
0 0.5 0.75 1.5
0.01 0.02 0.08 0.10
6 3 0.5 0.4
Errors
quoted
are standard
deviations
in Figs.
1
- 4.
Isomer shift europium(III) (mm 5-l ) 1.52 1.49 1.51 1.46
f f f f
0.06 0.07 0.04 0.03
of the mean values.
peak heights for the metal to those for the europium(II1) species gives the data recorded in Table 1. It is seen that the europium(II1) increases at the expense of the metal. Low relative concentrations and the poorly resolved spectral components for europium(I1) prevent a similar analysis being carried out for this component of the mixture. The spectrum presented in Fig. 5 coupled with data from the chemical analysis indicate that the Eu(NH,)~ obtained had a good state of purity. In view of the relatively long reaction time to obtain the amide from the same apparatus as the samples referred to in Figs. 1 - 4, it can be concluded that europium(II1) is not produced in the latter as a consequence of entry of air or water vapour. This conclusion was qualitatively supported by the data from chemical analysis and IR spectra of the samples used to obtain the
140
Mijssbauer spectra. These data gave nitrogen contents increasing with increasing reaction times (0 to 1.5 h) and evidence for N-H bonds from the vibration spectra. The isomer shift for the europium(II1) component has values from 1.46 to 1.52 mm s- ’ (Table 1). This is close to the value 1.327 reported for EuN [lo]. However, in view of the thermodynamic stability of EuN and the observation that after long reaction times Eu(NH*), is produced from the reaction mixtures discussed here, it seems reasonable to conclude that the europium(II1) is not present in the ammonia solution as the nitride. Failure to produce Eu(NH2)* from EuN in liquid ammonia provides further supportive evidence. Salot [ll] obtained a mixture of Yb(NH2)2 and Yb(NH,)3 from the metal in liquid ammonia in the presence of a catalyst at 20 “C. Juza and coworkers [ 7, 121 found that similar products were produced at 20 “C without a catalyst, but the relative yields of the amides were different. It seems reasonable to suggest from the observations presented here that either Eu( NH2 )a or europium( III) amide-imide mixture by analogy with ytterbium [ 131, are produced as unstable intermediates which decompose to a more stable europium(II1) compound during the isolation of the solid from the excess ammonia. Of these two possible substances the amide would necessarily be present since it precedes imide formation. However, under the conditions for production of Eu(NH~)~, this intermediate would appear to undergo a valence change from III to II perhaps under the influence of solvated electrons in the system. Further experimental work is required to establish the optimum conditions for production and isolation of Eu(NH*)~. Acknowledgment One of us (O.B.) gratefully acknowledges part financial the British Council during the time this work was carried out.
support
from
References 1 2 3 4
5 6 7 8 9 10 11 12
13
J. C. Warf and W. L. Korst, J. Phys. Chem., 60 (1956) 1590. J. P. Brown, R. L. Cohen and K. W. West, Chem. Phys. Lett., 20 (1973) 271. F. T. Parker and M. Kaplan, J. Chem. Phys., 60 (1974) 1328. R. F. Marzke and W. S.’ Glaunsinger, J. Phys. Chem., 79 (1975) 2976. R. Juza and C. Hadenfeldt, Naturwissenschaften, 55 (1968) 229. K. Howell and L. L. Pytlewski, J. Less-Common Met., 19 (1969) 399. C. Hadenfeldt, H. Jacobs and R. Juza, 2. Anorg. Allg. Chem., 379 (1970) 144. C. M. Jenden and S. J. Lyle, J. Chem. Sot., Dalton Trans., (1982) 2409. G. K. Shenoy and F. E. Wagner, Mkbauer Isomer Shifts, North-Holland, Amsterdam, 1978. I. Colquhoun, N. N. Greenwood, I. J. McCoIm and G. E Turner, J. Chem. Sot., Dalton Trans., (1972) 1337. S. Salot, Ph.D. Thesis, University of Southern California, 1969 (NTIS 70-366). R. Juza and C. Hadenfeldt, Naturwissenschaften, 56 (1969) 282. S. Salot and J. C. Warf, J. Am. Chem. Sot., 90 (7) (1968) 1932).