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Mössbauer spectroscopy of europium hexammine, Eu(NH3)6
Volume 20, number
3
MijSSBAUER
CHEMICAL PHYSICS LETTERS
SPECTROSCOPY
1 June
1973
OF EUROPIUM HEXAMMINE, EufNH&
J.P. BROWN*, R.L. COHEN and K.W. WEST Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 19 March 1973
Results of hib’ssbauer measurements on Eu(NH& provide strong evidence that the material contains divaIent euxopium ions in the 4f’ configuration and orders magnetiwlly at low temperatures.
It has been established by vapor pressure-composition studies [ 11 and by X-ray studies [2] that when stoichiometric amounts of metallic europium are ctissolved in liquid ammonia, an isolable hexarnmine, ELI(NH~)~, is formed. The magnetic [2] and thermodynamic [3] properties of this golden-bronze solid have been examined and the metallic character of the compound suggested. In this paper we present the results of MGssbauer spectroscopy studies of lslEu in europium hexammine which provide further evidence of the metallic nature of this material and its magnetic properties. In all of the Mijssbauer experiments the source and the absorber were at the same temperature. Our source was 1.6 curie of 1s!Sm203, which P-decays to the 21.6 keV gamma-emitting level of 1srEu. The hexammine sample contained ~150 mg/cm* of natural Eu. Figs. 1 and 2 show the Mossbauer spectra of Eu(NH~)~ at various temperatures. The gamma ray absorption arises from three types of Eu site; namely,
ent europium (at ~0 mm/set). At iow temperatures the hexammine phase orders magnetically. The Eu ions produce an internal field which splits the lSIEu nuclear levels via the hyperfine interaction. This splitting is observed in the 4.2 and.l.6OK spectra of figs.
Samples were made by the direct reaction of Eu metal with liquid arrmonia at about -50°C for about 30 minutes and thereafter kept at 78°K or below. We were unable to prepare a pure sample of Eu(NH~)~, and the spectra show a weak line with the isomer shift expected for Eu3+ impurities and a strong line due to a phase consisting of Eu*+ dissolved in ammonia. A full article is in preparation describing the interesting properties of this second phase and further discussing the hexammine results. We did not expect the decomposition reactions leading to imide and amide formation to be significant, because recent careful experiments [4] have shown that the time constant for the reaction is a few days at -5O’C in contact with a catalyst. Confirmation of these expectations was provided by the fact that no significant amount of H2 gas, which results from amide or imide formation, was produced. This decomposition product would have been readily observed (because of its high pressure at 78°K) either in the preparation apparatus or in ,the sealed-off capsules used for the measurements. These considerations should assure that decomposition of the samples into Eu imides and amides should not have been a problem during these experirrents. It was possible to measure the isomer shift of both the hexammine and dissolved ELIphases, despite the overlap of the spectra, by means of the following ap-
land2
proaches:
europium europium
dissolved in ammonia (at -13.7 mm/set); hexammine (at -12.5 mm/set); and trival-
*Permanent address: Department of Chemistry, College, Baltimore, hiaryland 21239, USA.
Morgan State
At 1.6 and 4.2”K the hexanmrine spectrum is split and a least-squares fit computer program can easily determine separate isomer shiftvaIues for the split 271
Volume 20, number
g
3
CHEMICAL PHYSICS LETTERS
1 June 1973
0.99
2 ii?
u 0.98
z
DISSOLVED
Eu
?J 5 0.97 ii CL 0.96 10 -10 0 -20 SOURCE VELOCITY (mm/SeC)
Fig. 1. Mijssbauer spectrum of Eu(NHa), at 1.6’K and lastsquares fit to the data. The solid line, dashed line, and chain curve represent, respectively, the spectra of the Eu(NII~)~, Eu dissolved in ammonia, and the Eu3* impurity. The heavy line is the sum of the individual components. The hyperfine spectrum calculation assumed that the hexammine splitting was purely magnetic in origin, and the positions and intensities of the 18 components of the transition could be calculated from the hyperfine hamiltoninn and the ClebschGordon coefficients. Thus the isomer shift and Hint were the only free hyperfine parameters. Unsplit lorentzian lines were used for tne other spectral components.
and unsplit spectra. At 78”K, we know from other measurements that the recoil-free fraction of the dissolved Eu ions is negligible, so that the 78°K spectrum originates from the hexammine only. Unconstrained fits of three lorentzians to the 20°K spectra gave isomer shift values which were, within statistics, the same as those for the hexammine and dissolved Eu phases taken at other temperatures. The variation of line position due to second order Doppler shift is a.02 mm/set for the temperature range used here, and can be neglected. There are three results of the present measurements which can be related to previous research on Eu(NH,),. 1. The size of the isomer shift; -12.5 +- 0.2 mm/ set with respect to EuZ03 at 78°K. 2. The size of the hyperfine tield;Hint = 290 + 5 kOe (extrapolated to O”K). 3. The temperature independence of the isomer sNft, which varies by less than 0.2 mmisec from 1.6 to 78°K.
1
I
I
I
-20
-10
0
SOURCE
VELOCITY
I
IO (mm /see)
Fig. 2. hlijssbauer spectra and least-squares fits for a sample containing both hexammine and dissolved Eu phases. At 78”K, thef of the dissolved Eu is very small, and the spectrum can be adequately titted using oneline for the hcxammine phase (at - 12.5 mm/set) and a weak line (EJO mmlsec) for the trivalent impurity. The 20.4”K spectrum is fitted using two lorentzians for the hexammine and dissolved Eu phases, and a weak line for the Eu3+impurity. In the spectra taken at 4.2 and 1.6”K, the hesamminc phase is magnetically ordered; the model used to represent the spectrum is described in the fig. 1 caption.
The observed isomer shift lies well above the value (-14 mmlsec) typical of ionic Eu2+ compounds [5]. For lslEu more positive isomer shift values result from increased s-electron density at the nucleus. The observed value is in the region of isomer shifts measured for semiconducting chalcogenides of Euz+(t.he isomer shift of EuS is =12.7.mm/sec), and just below compounds (the isothe values for Et?+ intermetallic
Volume 20, number 3
1 June 1973
CHEMICAL PHYSICS LE’lTERS
mm/set). The observed mer shift of EuSn is -12.0 values are consi:tent with the picture of the hexammine as metallic, with a conduction band consisting primarily of electrons hybridized into 6s-5d orbit&. The obsenred value Of Hint is also consistent with this picture, being well away from the value of (-) 330 kOe expected [6] for ionic Eu2+ compounds. Both the value of the isomer shift and its temperature independence are inconsistent with the model of
ref. [2’l, in which the electrons freeze out from a 7s7p conduction band (above 47°K) to a 5d2 configuration below 47’K. Al’Lhough the charge density provided at the nucleus by the 7s electron is not easily calculable, a 7s electron on a free Eu ion would increase the charge density at the nucleus enough to increase the isomer shift by about 2 mm/set. The presence of two 5d electrons, on the other hard, would decrease the charge density at the nucleus enough to decrease the isomer shift by about 6 mm/set; even a small fraction of this change would have been observed by us. Minor deviations between the spectra and theoretical curves of figs. 1 and 2 suggest that there may not be a unique hyperfine field for all ELIions in the magnetically ordered he?ammine phase. This couid result from a ferrimagnetic or complex antiferromagnetic ordering, as is observed in the Eu chalcogenides. This would also explain the magnetic hardness reported in
ref. _ [21. - One would normally expect a cubic ferromagnet containing En 2+ to be &agnetically very soft. In the europium hexammine, we did not observe any anomalies of the type reported by Asch et.af. [7], though such phenomena may occur above 78% We would like to acknowledge helpfu1 discussions with A.P. Ginsberg, H.H. Wicknan, and P.H. Schmidt. References ill D.S. Thompson, M.J. Stone and J.S. Laugh, I. Phys. Chem. 70 (1966)
934.
121 H. Oesterreicher, N. hEammano and XI. Sienko, I. Solid StateChem. l(1969) 10; N. hfammano and M.J. Sienko. J. Solid State Chem. 1 (1970) 534. 131 R.H. Frisbee and N.h¶. Senozan. J. Chem. Phys. 54 (1972) 1248. [41 J.K. Howell Jr., Ph.D. Thesis, Drexel Institute ofTechnology (1970). [51 G. Gerth, P. Kienle and K. Luchner, Phys. Letters 27,2 (1968) 557. I61 GJ. Enholm, T.E. KatiIa, 0-V. Lounasmx~. P. Reivui. G.hf. Ksbiusand G.K. Shenoy, 2. Physik 235 (19’7Io) 289.
[71 L. Arch, J.P. Adloff, J.M. Friedt and J. Parron, Chcm. Phyr Letters5 (1970) 105; L. Asch and J.hl. Friedt, Proceedings of the Dresden Conference on Miissbauer Spectromctry, September 23, 1971 (Physikalische GeseIL DDR) p. 447.
273
3
MijSSBAUER
CHEMICAL PHYSICS LETTERS
SPECTROSCOPY
1 June
1973
OF EUROPIUM HEXAMMINE, EufNH&
J.P. BROWN*, R.L. COHEN and K.W. WEST Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 19 March 1973
Results of hib’ssbauer measurements on Eu(NH& provide strong evidence that the material contains divaIent euxopium ions in the 4f’ configuration and orders magnetiwlly at low temperatures.
It has been established by vapor pressure-composition studies [ 11 and by X-ray studies [2] that when stoichiometric amounts of metallic europium are ctissolved in liquid ammonia, an isolable hexarnmine, ELI(NH~)~, is formed. The magnetic [2] and thermodynamic [3] properties of this golden-bronze solid have been examined and the metallic character of the compound suggested. In this paper we present the results of MGssbauer spectroscopy studies of lslEu in europium hexammine which provide further evidence of the metallic nature of this material and its magnetic properties. In all of the Mijssbauer experiments the source and the absorber were at the same temperature. Our source was 1.6 curie of 1s!Sm203, which P-decays to the 21.6 keV gamma-emitting level of 1srEu. The hexammine sample contained ~150 mg/cm* of natural Eu. Figs. 1 and 2 show the Mossbauer spectra of Eu(NH~)~ at various temperatures. The gamma ray absorption arises from three types of Eu site; namely,
ent europium (at ~0 mm/set). At iow temperatures the hexammine phase orders magnetically. The Eu ions produce an internal field which splits the lSIEu nuclear levels via the hyperfine interaction. This splitting is observed in the 4.2 and.l.6OK spectra of figs.
Samples were made by the direct reaction of Eu metal with liquid arrmonia at about -50°C for about 30 minutes and thereafter kept at 78°K or below. We were unable to prepare a pure sample of Eu(NH~)~, and the spectra show a weak line with the isomer shift expected for Eu3+ impurities and a strong line due to a phase consisting of Eu*+ dissolved in ammonia. A full article is in preparation describing the interesting properties of this second phase and further discussing the hexammine results. We did not expect the decomposition reactions leading to imide and amide formation to be significant, because recent careful experiments [4] have shown that the time constant for the reaction is a few days at -5O’C in contact with a catalyst. Confirmation of these expectations was provided by the fact that no significant amount of H2 gas, which results from amide or imide formation, was produced. This decomposition product would have been readily observed (because of its high pressure at 78°K) either in the preparation apparatus or in ,the sealed-off capsules used for the measurements. These considerations should assure that decomposition of the samples into Eu imides and amides should not have been a problem during these experirrents. It was possible to measure the isomer shift of both the hexammine and dissolved ELIphases, despite the overlap of the spectra, by means of the following ap-
land2
proaches:
europium europium
dissolved in ammonia (at -13.7 mm/set); hexammine (at -12.5 mm/set); and trival-
*Permanent address: Department of Chemistry, College, Baltimore, hiaryland 21239, USA.
Morgan State
At 1.6 and 4.2”K the hexanmrine spectrum is split and a least-squares fit computer program can easily determine separate isomer shiftvaIues for the split 271
Volume 20, number
g
3
CHEMICAL PHYSICS LETTERS
1 June 1973
0.99
2 ii?
u 0.98
z
DISSOLVED
Eu
?J 5 0.97 ii CL 0.96 10 -10 0 -20 SOURCE VELOCITY (mm/SeC)
Fig. 1. Mijssbauer spectrum of Eu(NHa), at 1.6’K and lastsquares fit to the data. The solid line, dashed line, and chain curve represent, respectively, the spectra of the Eu(NII~)~, Eu dissolved in ammonia, and the Eu3* impurity. The heavy line is the sum of the individual components. The hyperfine spectrum calculation assumed that the hexammine splitting was purely magnetic in origin, and the positions and intensities of the 18 components of the transition could be calculated from the hyperfine hamiltoninn and the ClebschGordon coefficients. Thus the isomer shift and Hint were the only free hyperfine parameters. Unsplit lorentzian lines were used for tne other spectral components.
and unsplit spectra. At 78”K, we know from other measurements that the recoil-free fraction of the dissolved Eu ions is negligible, so that the 78°K spectrum originates from the hexammine only. Unconstrained fits of three lorentzians to the 20°K spectra gave isomer shift values which were, within statistics, the same as those for the hexammine and dissolved Eu phases taken at other temperatures. The variation of line position due to second order Doppler shift is a.02 mm/set for the temperature range used here, and can be neglected. There are three results of the present measurements which can be related to previous research on Eu(NH,),. 1. The size of the isomer shift; -12.5 +- 0.2 mm/ set with respect to EuZ03 at 78°K. 2. The size of the hyperfine tield;Hint = 290 + 5 kOe (extrapolated to O”K). 3. The temperature independence of the isomer sNft, which varies by less than 0.2 mmisec from 1.6 to 78°K.
1
I
I
I
-20
-10
0
SOURCE
VELOCITY
I
IO (mm /see)
Fig. 2. hlijssbauer spectra and least-squares fits for a sample containing both hexammine and dissolved Eu phases. At 78”K, thef of the dissolved Eu is very small, and the spectrum can be adequately titted using oneline for the hcxammine phase (at - 12.5 mm/set) and a weak line (EJO mmlsec) for the trivalent impurity. The 20.4”K spectrum is fitted using two lorentzians for the hexammine and dissolved Eu phases, and a weak line for the Eu3+impurity. In the spectra taken at 4.2 and 1.6”K, the hesamminc phase is magnetically ordered; the model used to represent the spectrum is described in the fig. 1 caption.
The observed isomer shift lies well above the value (-14 mmlsec) typical of ionic Eu2+ compounds [5]. For lslEu more positive isomer shift values result from increased s-electron density at the nucleus. The observed value is in the region of isomer shifts measured for semiconducting chalcogenides of Euz+(t.he isomer shift of EuS is =12.7.mm/sec), and just below compounds (the isothe values for Et?+ intermetallic
Volume 20, number 3
1 June 1973
CHEMICAL PHYSICS LE’lTERS
mm/set). The observed mer shift of EuSn is -12.0 values are consi:tent with the picture of the hexammine as metallic, with a conduction band consisting primarily of electrons hybridized into 6s-5d orbit&. The obsenred value Of Hint is also consistent with this picture, being well away from the value of (-) 330 kOe expected [6] for ionic Eu2+ compounds. Both the value of the isomer shift and its temperature independence are inconsistent with the model of
ref. [2’l, in which the electrons freeze out from a 7s7p conduction band (above 47°K) to a 5d2 configuration below 47’K. Al’Lhough the charge density provided at the nucleus by the 7s electron is not easily calculable, a 7s electron on a free Eu ion would increase the charge density at the nucleus enough to increase the isomer shift by about 2 mm/set. The presence of two 5d electrons, on the other hard, would decrease the charge density at the nucleus enough to decrease the isomer shift by about 6 mm/set; even a small fraction of this change would have been observed by us. Minor deviations between the spectra and theoretical curves of figs. 1 and 2 suggest that there may not be a unique hyperfine field for all ELIions in the magnetically ordered he?ammine phase. This couid result from a ferrimagnetic or complex antiferromagnetic ordering, as is observed in the Eu chalcogenides. This would also explain the magnetic hardness reported in
ref. _ [21. - One would normally expect a cubic ferromagnet containing En 2+ to be &agnetically very soft. In the europium hexammine, we did not observe any anomalies of the type reported by Asch et.af. [7], though such phenomena may occur above 78% We would like to acknowledge helpfu1 discussions with A.P. Ginsberg, H.H. Wicknan, and P.H. Schmidt. References ill D.S. Thompson, M.J. Stone and J.S. Laugh, I. Phys. Chem. 70 (1966)
934.
121 H. Oesterreicher, N. hEammano and XI. Sienko, I. Solid StateChem. l(1969) 10; N. hfammano and M.J. Sienko. J. Solid State Chem. 1 (1970) 534. 131 R.H. Frisbee and N.h¶. Senozan. J. Chem. Phys. 54 (1972) 1248. [41 J.K. Howell Jr., Ph.D. Thesis, Drexel Institute ofTechnology (1970). [51 G. Gerth, P. Kienle and K. Luchner, Phys. Letters 27,2 (1968) 557. I61 GJ. Enholm, T.E. KatiIa, 0-V. Lounasmx~. P. Reivui. G.hf. Ksbiusand G.K. Shenoy, 2. Physik 235 (19’7Io) 289.
[71 L. Arch, J.P. Adloff, J.M. Friedt and J. Parron, Chcm. Phyr Letters5 (1970) 105; L. Asch and J.hl. Friedt, Proceedings of the Dresden Conference on Miissbauer Spectromctry, September 23, 1971 (Physikalische GeseIL DDR) p. 447.
273