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Stabilization of defect charge states in solids
Volume 3, number 2
CHEMICAL PHYSICS LETTERS
STABILIZATION
OF
DEFECT
G. K. WERTHEIM Bell
TeEephone Laboratories,
CHARGE
February 1969
STATES
IN
SOLIDS
and D. N. E. BUCHANAN
Incorporated,
Murray
Hill,
New Jersey,
USA
Received 13 January 1969
Electron and gamma radiation experiments in ferrous ammonium sulphate and oC.ler compounds show that the stabilization of Fe3+ following the decay of 57C02+ in hydrated salts is due to the radiolysis of water by Auger electrons.
For some years there has been interest in solid state effects of the Auger cascade following nuclear processes which create holes in the inner electronic shells, e.g., electron capture and internal conversion [l]. Attention has recently been focused on the mechanism which determines the final atomic state of a highly charged ion undergoing charge relaxation in solids. The body of experimental evidence available from Nii%sbauer effect studies is probably more extensive than any other. Moreover, there is generally good agreement among independent experiments on identical, well-characterized compounds. However, there is no agreement on the mechanism of stabilization which results in defect charge states One conclusion which appears to be well established by the experiments of TriftshXuser and Craig [2] is that there is no charge relaxation with characteristic time comparable to the 10B7 set nuclear half life of the first excited state of 57Fe. Experiments with cobalt compounds [2-4 in which the valence of the 57Co parent of the 5?Fe isomer was clearly established have shown that both Fe2+ and Fe8+ are produced by the decay of Co2+, ruling out the suggestion that Fe3+ is necessarily due to prior oxidation of the cobalt parent. The additional finding that both Fe2+ and Fe8+ are also observed following the decay of Co3+ compounds [4,5] clearly indicates that defect valence states are not the result of incomplete charge relaxation alone, since Eozoer valence states are not produced by the Auger cascade. These results require a mechanism producing a modification of the environment of the parent ion by the preceding nuclear and atomic events [3,4]. There are also a large number of well-established cases in which the daughter ion has the same valence as the parent ion [6-81. ‘.
The variety of experimentally observed beh;;vior suggests that there may not be a single mechanism responsible for the stabilization of defect charge states. In fact, a number of different mechanisms have been suggested in connection with such experiments in the past. For example, it was pointed out some years ago [9] that the size of daughter ion may determine its stability; e.g. , Fe8+ might be produced in a lattice that ~2s too small to accommodate the larger Fe2+ ion. Presumably charge compensation must be provided to make this possible. The significance of this parameter for the stabilization of defect charge states has not yet been systematically exzmined. However, data in the rutile structure fluorides MnF2 [lo], ZnF2 [7] and CoF2 [4] lend some support to this hypothesis. Another mechanism, which was postulated in the case of molecular crystals [5], considers-the electron-withdrawing properties of the highly charged ion at the end of the Auger cascade 2s the means for modifying the environment in such a way as to stabilize 2 huer charge state. It has been suggested that the presence of 2 highly charged ion may result in the complete disruption of the molecule [11,12]. It has been repeatedly recognized that the presence of water of hydration favors the stabilization of higher charge states. Friedt and Adloff [4] have suggested that this may result from ioazation by the K-Auger electron, which can readily convert a water molecule into an OK radical with strong oxidizing properties. The resulting OH’ ion tnen serves to stabilize a trivalent metal ion in the formerly divalent site. It is not clear, however, whether the radiolysis of the water is itself sufficient to change the valence of the metal ion or whether the Auger cascade is also required. 81
Volume 3, number 2
CHEMICAL PHYSICS LETTERS
This question is readily tested in a direct radiolysis experiment in which the ionization is due to an external source of radiation. We have carried out such experiments on a variety of iron compounds in which foreign charge states have been observed following electron capture. The radiations were carried out using 1 meV
Van de Graaff electrons or 66~0 gamma rays with the samples at room temperature in a dry nitrogen atmosphere. Results obtained on Fe(XI-I4)2(SO4)2 - 6H20 after an electron irradiation of 430 Mrad are shown in fig. 1. The Mbsshauer absorption spectra were taken with the samples immersed in liquid nitrogen. The data are very similar to those obtained by TriftsbXuser and Craig [2] from a 5ko2+ doped Fe(NH )2(SO4)2 - 6H2O source. In the data shown - 16s of the absorption area is in the Fe3+ spectrum. The radiation dose corresponds to an energy of 17.5 eV/molecule. On the average 110 eV are ‘&on required to convert one iron atom to the trivalent state. This is about
k3+
I
I
February 1969
10 times greater than the energy required for oxidation in aqueous solution [13]. Since only 5.1 eV are required to dissociate a water molecule (117.5 Kcal/mole), most of the radiation dose is dissipated in other processes. Only 4.6% of the ionization energy is utilized in the radiolysis of water. A simple calculation* of the average energy lost by the Auger electrons in leaving a molecule of Fe(NII4)2@04)2 - 6H2O yields - 50 eV, i.e., ten times the dissoctition energy of a water molecule. With a utilization of 4.6% we then conclude that 46% of the decaying iron will be converted to the trivalent state, a calculation which is in good agreement with the results of ref. [2]. _’ Results obtained by gamma irradiation are entirely consistent with those quoted, see table 1. Similar results have also been obtained in I Fe.504 0 ‘i’H20. On the other band the irradiation of anhydrous FeF2 or ferrocene up to 430 Mrad has produced no measurable change in the absorption spectrum ** (table 1).
The qualitative as well as quantitative results of these experiments indicate that the stabilization of Fe3+ following the decay of Fe2+ in hydrated salts is due to the radiolysis of water by Auger electrons and is not the result of incomplete charge relaxation. In anhydrous salts and molecular crystals other mechanisms must be considered.
We are indebted to J. P. Mitchell for the gamma irradiations, to R. Salovey for assistance with the electron bombardment and to H. J. Guggenheim for a supply of FeF2. * Based on the stopping power, -dE/dx, and the radius of the ferrous ammonium sulphate molecule. l * The fraction of atoms displaced by a direct collision is estimated to be no greater than 10-6. REFERENCES
t11 See H. Frauenfelder, in: Beta and Gamma Hay 88
80’
Fe~NH41202~
’ -0.3
I
-02
Spectroscopy, ed. K. Siegbahn (North-Holland, Amsterdam, 1955) for a discussion of such effects in perturbed angular correlations. The pertinence to
BHzO
I
--ON DOFPLER
I
I
0
01
VELOCITY
I
02
,
0.3
(cm/see)
Fig. 1. Mossbauer absorption spectra of Fe(NH4)2(SO )2 - 6H20, (a), before and, (b), after irradiation wit -if 436 Mrad of 1 MeV electrons. The source
was 57Co in palladium. The ,Fe3+ contribution in the line near zero velocity in (h) was determined by using
the known intensity and position of the Fe2+ line.
Miissbauer effect spectra was suggested by one of us (G.K. Wertheim) in Phys. Rev. 124 (1961) 764. and P. P. Craig, Phys. Rev. LetPI W. Triftshauser ters 16 (1966) 1161 and Phys. Rev. 162 (1967) 274. Phys. Letters 15 t31 R. Ingalfs and G. DePasquali. (1965) 262. R.Ing&ls et al.. J. Chem:Phys.44 (1966) 1057. 141 J. M. Friedt and J. P. Adloff. Comet. Rend. Acad. Sci. Paris 264 (1967) 1356.. PI G.K. Wertheim, W. R. Kingston and R. H. Herber. 3. Chem- Phys. 37 (1962) 687: G. K Wertheim atid R. H. Herber, J. Chem. Phys. 38 (1963) 2106.
Volume
3. number
2
Summary
CHEMICAL
of Mi%sbauer
PHYSICS
February
LETTERS
Table 1 data obtained on four compounds
before
and after
430 Mrad
Temp.
e-
unirradiated 50 Mrad gamma 430 Mrad eFeS04
radiolysis. Fe3+
Fez+ Compound and Bombardment
1969
Is(b)
QSta)
w
Is
(cm/set)
(cm/see)
0.090
0.045
(CK)
(cm/set)
(cm/set)
298 298
0.1739 0.1725
0.1251 0.1241
77.4 77.4 77.4
0.2698 0.2699 0.2691
0.1368 0.1368 0.1364
0.091
0.053
0.0 1.5 16-U
77.4 77.4
0.3519 0.349
0.1353 0.138
0.08
0.05
OX 14.6
298
0.279ic)
0.136(c)
0.0
298
0.2787
0.1354
0.0
77.4 77.4
0.2404 0.2401
0.0533 0.0531
%
0.0
- 7H20
unirradiated 430 Mrad eFeF2 unirradiated 430 Mrad
e-
Ferrocene unirradiated 430 Mrad e-
(a) Quadrupole splitting (b) Isomer shifts expressed relative to metallic (c) From Phys. Rev. 161 (1967) 478.
iron at 298?K
[6] J. G. Mullen, Phys. Rev. 131 (1963) 1410; H. Sano and F. HashimotL, Jull. Chem. Japan 38 (1965) 1565: A.Nath, P. P. Agarwal and P.K. Mathew, Inorg. Nucl. Chem. Letters 4 (1968) 161: A. Nath et al., Chem. Phys. Letters 2 (1968) 471. [7] G. K. Wertheim and H. J. Guggenheim, J. Chem. Phys. 42 (1965) 3873. IS] Further discussion and references may be found in Chemical Applications of Miissbauer Spectroscopy, Goldanskii and Herber, editors (Academic Press, New York, 1968) p. 6Odff. **
[9] H.Pollak, Phyc. Stat. Sol. 2 (1962) 720. (lo] G. K. Wertheim, H. J. Guggenheim and D. N. E. Buchanan, Phys. Rev. Letters 20 (1968) 1158. [ll] A. H. Snell, in: Alpha, Beta and Gamma-ray Spectroscopy, ed. K. Siegbahn (North-Holland, Amsterdam, 1964) p. 1545ff. [12] S. Wexler, Science 156 (1967) 901. [13] A-0. Allen, The Radiation Chemistry of Water and Aqueous ‘solutions (D. van Nostrand. Princeton, 1961) p. 32ff.
***
89
CHEMICAL PHYSICS LETTERS
STABILIZATION
OF
DEFECT
G. K. WERTHEIM Bell
TeEephone Laboratories,
CHARGE
February 1969
STATES
IN
SOLIDS
and D. N. E. BUCHANAN
Incorporated,
Murray
Hill,
New Jersey,
USA
Received 13 January 1969
Electron and gamma radiation experiments in ferrous ammonium sulphate and oC.ler compounds show that the stabilization of Fe3+ following the decay of 57C02+ in hydrated salts is due to the radiolysis of water by Auger electrons.
For some years there has been interest in solid state effects of the Auger cascade following nuclear processes which create holes in the inner electronic shells, e.g., electron capture and internal conversion [l]. Attention has recently been focused on the mechanism which determines the final atomic state of a highly charged ion undergoing charge relaxation in solids. The body of experimental evidence available from Nii%sbauer effect studies is probably more extensive than any other. Moreover, there is generally good agreement among independent experiments on identical, well-characterized compounds. However, there is no agreement on the mechanism of stabilization which results in defect charge states One conclusion which appears to be well established by the experiments of TriftshXuser and Craig [2] is that there is no charge relaxation with characteristic time comparable to the 10B7 set nuclear half life of the first excited state of 57Fe. Experiments with cobalt compounds [2-4 in which the valence of the 57Co parent of the 5?Fe isomer was clearly established have shown that both Fe2+ and Fe8+ are produced by the decay of Co2+, ruling out the suggestion that Fe3+ is necessarily due to prior oxidation of the cobalt parent. The additional finding that both Fe2+ and Fe8+ are also observed following the decay of Co3+ compounds [4,5] clearly indicates that defect valence states are not the result of incomplete charge relaxation alone, since Eozoer valence states are not produced by the Auger cascade. These results require a mechanism producing a modification of the environment of the parent ion by the preceding nuclear and atomic events [3,4]. There are also a large number of well-established cases in which the daughter ion has the same valence as the parent ion [6-81. ‘.
The variety of experimentally observed beh;;vior suggests that there may not be a single mechanism responsible for the stabilization of defect charge states. In fact, a number of different mechanisms have been suggested in connection with such experiments in the past. For example, it was pointed out some years ago [9] that the size of daughter ion may determine its stability; e.g. , Fe8+ might be produced in a lattice that ~2s too small to accommodate the larger Fe2+ ion. Presumably charge compensation must be provided to make this possible. The significance of this parameter for the stabilization of defect charge states has not yet been systematically exzmined. However, data in the rutile structure fluorides MnF2 [lo], ZnF2 [7] and CoF2 [4] lend some support to this hypothesis. Another mechanism, which was postulated in the case of molecular crystals [5], considers-the electron-withdrawing properties of the highly charged ion at the end of the Auger cascade 2s the means for modifying the environment in such a way as to stabilize 2 huer charge state. It has been suggested that the presence of 2 highly charged ion may result in the complete disruption of the molecule [11,12]. It has been repeatedly recognized that the presence of water of hydration favors the stabilization of higher charge states. Friedt and Adloff [4] have suggested that this may result from ioazation by the K-Auger electron, which can readily convert a water molecule into an OK radical with strong oxidizing properties. The resulting OH’ ion tnen serves to stabilize a trivalent metal ion in the formerly divalent site. It is not clear, however, whether the radiolysis of the water is itself sufficient to change the valence of the metal ion or whether the Auger cascade is also required. 81
Volume 3, number 2
CHEMICAL PHYSICS LETTERS
This question is readily tested in a direct radiolysis experiment in which the ionization is due to an external source of radiation. We have carried out such experiments on a variety of iron compounds in which foreign charge states have been observed following electron capture. The radiations were carried out using 1 meV
Van de Graaff electrons or 66~0 gamma rays with the samples at room temperature in a dry nitrogen atmosphere. Results obtained on Fe(XI-I4)2(SO4)2 - 6H20 after an electron irradiation of 430 Mrad are shown in fig. 1. The Mbsshauer absorption spectra were taken with the samples immersed in liquid nitrogen. The data are very similar to those obtained by TriftsbXuser and Craig [2] from a 5ko2+ doped Fe(NH )2(SO4)2 - 6H2O source. In the data shown - 16s of the absorption area is in the Fe3+ spectrum. The radiation dose corresponds to an energy of 17.5 eV/molecule. On the average 110 eV are ‘&on required to convert one iron atom to the trivalent state. This is about
k3+
I
I
February 1969
10 times greater than the energy required for oxidation in aqueous solution [13]. Since only 5.1 eV are required to dissociate a water molecule (117.5 Kcal/mole), most of the radiation dose is dissipated in other processes. Only 4.6% of the ionization energy is utilized in the radiolysis of water. A simple calculation* of the average energy lost by the Auger electrons in leaving a molecule of Fe(NII4)2@04)2 - 6H2O yields - 50 eV, i.e., ten times the dissoctition energy of a water molecule. With a utilization of 4.6% we then conclude that 46% of the decaying iron will be converted to the trivalent state, a calculation which is in good agreement with the results of ref. [2]. _’ Results obtained by gamma irradiation are entirely consistent with those quoted, see table 1. Similar results have also been obtained in I Fe.504 0 ‘i’H20. On the other band the irradiation of anhydrous FeF2 or ferrocene up to 430 Mrad has produced no measurable change in the absorption spectrum ** (table 1).
The qualitative as well as quantitative results of these experiments indicate that the stabilization of Fe3+ following the decay of Fe2+ in hydrated salts is due to the radiolysis of water by Auger electrons and is not the result of incomplete charge relaxation. In anhydrous salts and molecular crystals other mechanisms must be considered.
We are indebted to J. P. Mitchell for the gamma irradiations, to R. Salovey for assistance with the electron bombardment and to H. J. Guggenheim for a supply of FeF2. * Based on the stopping power, -dE/dx, and the radius of the ferrous ammonium sulphate molecule. l * The fraction of atoms displaced by a direct collision is estimated to be no greater than 10-6. REFERENCES
t11 See H. Frauenfelder, in: Beta and Gamma Hay 88
80’
Fe~NH41202~
’ -0.3
I
-02
Spectroscopy, ed. K. Siegbahn (North-Holland, Amsterdam, 1955) for a discussion of such effects in perturbed angular correlations. The pertinence to
BHzO
I
--ON DOFPLER
I
I
0
01
VELOCITY
I
02
,
0.3
(cm/see)
Fig. 1. Mossbauer absorption spectra of Fe(NH4)2(SO )2 - 6H20, (a), before and, (b), after irradiation wit -if 436 Mrad of 1 MeV electrons. The source
was 57Co in palladium. The ,Fe3+ contribution in the line near zero velocity in (h) was determined by using
the known intensity and position of the Fe2+ line.
Miissbauer effect spectra was suggested by one of us (G.K. Wertheim) in Phys. Rev. 124 (1961) 764. and P. P. Craig, Phys. Rev. LetPI W. Triftshauser ters 16 (1966) 1161 and Phys. Rev. 162 (1967) 274. Phys. Letters 15 t31 R. Ingalfs and G. DePasquali. (1965) 262. R.Ing&ls et al.. J. Chem:Phys.44 (1966) 1057. 141 J. M. Friedt and J. P. Adloff. Comet. Rend. Acad. Sci. Paris 264 (1967) 1356.. PI G.K. Wertheim, W. R. Kingston and R. H. Herber. 3. Chem- Phys. 37 (1962) 687: G. K Wertheim atid R. H. Herber, J. Chem. Phys. 38 (1963) 2106.
Volume
3. number
2
Summary
CHEMICAL
of Mi%sbauer
PHYSICS
February
LETTERS
Table 1 data obtained on four compounds
before
and after
430 Mrad
Temp.
e-
unirradiated 50 Mrad gamma 430 Mrad eFeS04
radiolysis. Fe3+
Fez+ Compound and Bombardment
1969
Is(b)
QSta)
w
Is
(cm/set)
(cm/see)
0.090
0.045
(CK)
(cm/set)
(cm/set)
298 298
0.1739 0.1725
0.1251 0.1241
77.4 77.4 77.4
0.2698 0.2699 0.2691
0.1368 0.1368 0.1364
0.091
0.053
0.0 1.5 16-U
77.4 77.4
0.3519 0.349
0.1353 0.138
0.08
0.05
OX 14.6
298
0.279ic)
0.136(c)
0.0
298
0.2787
0.1354
0.0
77.4 77.4
0.2404 0.2401
0.0533 0.0531
%
0.0
- 7H20
unirradiated 430 Mrad eFeF2 unirradiated 430 Mrad
e-
Ferrocene unirradiated 430 Mrad e-
(a) Quadrupole splitting (b) Isomer shifts expressed relative to metallic (c) From Phys. Rev. 161 (1967) 478.
iron at 298?K
[6] J. G. Mullen, Phys. Rev. 131 (1963) 1410; H. Sano and F. HashimotL, Jull. Chem. Japan 38 (1965) 1565: A.Nath, P. P. Agarwal and P.K. Mathew, Inorg. Nucl. Chem. Letters 4 (1968) 161: A. Nath et al., Chem. Phys. Letters 2 (1968) 471. [7] G. K. Wertheim and H. J. Guggenheim, J. Chem. Phys. 42 (1965) 3873. IS] Further discussion and references may be found in Chemical Applications of Miissbauer Spectroscopy, Goldanskii and Herber, editors (Academic Press, New York, 1968) p. 6Odff. **
[9] H.Pollak, Phyc. Stat. Sol. 2 (1962) 720. (lo] G. K. Wertheim, H. J. Guggenheim and D. N. E. Buchanan, Phys. Rev. Letters 20 (1968) 1158. [ll] A. H. Snell, in: Alpha, Beta and Gamma-ray Spectroscopy, ed. K. Siegbahn (North-Holland, Amsterdam, 1964) p. 1545ff. [12] S. Wexler, Science 156 (1967) 901. [13] A-0. Allen, The Radiation Chemistry of Water and Aqueous ‘solutions (D. van Nostrand. Princeton, 1961) p. 32ff.
***
89