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A Mössbauer spectroscopic study of the Eu-Hg system
Journal
of the Less-Common
A MijSSBAUER
SAMUEL
Metals,
SPECTROSCOPIC
J. LYLE and WILLIAM
The Chemical (Gt. Britain)
(Received
October
99 (1984)
Laboratories,
265
265
- 272
STUDY OF THE Eu-Hg SYSTEM
A. WESTALL
University
of
Kent
at
Canterbury,
Kent
CT2
7NH
17,1983)
Summary Samples of EuHg, (x = 1, 2, 3 and 3.6) have been prepared and characterized by chemical analysis and X-ray powder diffractomet~. Evidence is presented for narrow ranges of solid solubility and together with other data it is concluded that these substances are better described as being intermetallic rather than metallic alloy in character. The Mijssbauer spectroscopic study confirms that the europium exhibits a divalent core in each case. The isomer shift is dependent on the number density of europium atoms in the unit cell. This latter quantity correlates in a similar way with the computed conduction electron density at the europium nucleus. The decrease in electron density at the europium nucleus in these substances compared with that for europium metal is indicative of some degree of normal chemical bonding in these compounds.
1. Introduction With a few exceptions, the lanthanides have long presented difficulties in the separation of one element from another. Europium represents one of the exceptions in that it can be concentrated and separated by electrolytic methods, particularly at a mercury cathode. Electrolytic reduction to E&O4 at a mercury cathode was first reported by Yntema [ 11; later the electrolysis was modified by McCoy [2] so that a europium amalgam formed at the cathode. McCoy observed that removal of surplus mercury by distillation resulted in the formation of solid Eu-Hg products. More recently, the EuHg system has formed part of a broader study [3 - 61 of lanthanide intermetallic systems with B metals of groups I - III of the periodic table. These studies used X-ray diffraction data in a search for correlations between lanthanide ionic size and lattice parameters; hence structures were determined. From lattice parameters and magnetic data europium behaved in an anomalous way in that in the ionic cores it appeared to be divalent as @ Elsevier
Sequoia/Printed
in The Netherlands
266
opposed to the trivalent cores exhibited by typical lanthanide elements. However, information in general is sparse on the chemical and physical properties of the Eu-Hg system. In the study reported here the Miissbauer effect in europium is used as a tool to gain insight into the nature of the species produced and the effect the interaction with mercury has on the electronic environment of the europium atoms. Two methods of preparation were examined. The first was that of McCoy and the second, which was preferred, followed the procedure adapted by later workers in which predetermined amounts of the elements were heated together. The products were characterized by chemical analysis and X-ray diffractometry.
2. Experimental
details
2.1. Reagents Europium Products Ltd., grade chemicals
metal was obtained Widnes, Gt. Britain. were used.
in sealed ampoules from Rare Earth Triply distilled mercury and reagent
2.2. Preparation and analysis of Eu-Hgproducts A europium amalgam was prepared by the method given by McCoy [ 21. Excess mercury was removed by distillation at low pressures. In the alternative, preferred, method required atomic proportions of europium and mercury were heated together in sealed silica ampoules. The europium metal (0.3 - 0.4 g) was divided up into small pieces under dry argon in a glove-box and mixed with the mercury in the ampoule which was next sealed under vacuum and heated at 600 “C for between 12 and 24 h. All products were handled under dry argon in the glove-box. The composition of the products was checked by titrimetric analysis using ethylenediaminetetraacetic acid as titrant. The weighed sample (about 0.2 g) was dissolved in dilute nitric acid and the volume made up to 50 ml with water. Mercury and europium were determined [7] in one aliquot (10 ml) and europium was determined in another after masking mercury(I1) with cyanide [ 81. 2.3. Instrumental
measurements
The X-ray powder diffraction measurements were performed using a Philips PW lOOS/SO X-ray unit. The Mijssbauer spectra were recorded in the transmission mode using a spectrometer and a liquid nitrogen flow cryostat both previously described [9]. A 50 mCi “‘Srn source incorporated in an anhydrous SmF, matrix and supplied by the Radiochemical Centre, Amersham, Gt. Britain, was used. Both the sample which was diluted with dry Al,O, and the source were held at 93 K in recording the spectra. Isomer shifts are reported relative to EuFs and the errors quoted on them are standard deviations of mean values.
261
3. Results Removal of excess mercury by distillation from the europium amalgam produced electrolytically resulted in poor control over the composition of the residual material. Consequently almost all the work described below was based on products derived from the second method of preparation. The samples were air and possibly moisture sensitive necessitating the use of an argon-filled glove-box for handling purposes. The products prepared in the course of the work are listed in Table 1 together with previously published crystallographic data. The compounds (Table 1) all had a silver-grey appearance and were hard and brittle; difficulty in grinding increased with decreasing mercury content. In addition to these products of known composition, mixtures of europium and mercury in atomic ratios 2:3 and 3:2 were heated together, the former as a test of range of solid solubility and the latter because McCoy claimed to have isolated Eu,Hg, from europium amalgam. The X-ray powder diffraction data were used to check the authenticity of the samples by comparison with reported crystal parameters. TABLE Structural Compound
1 data for known Structure
compounds type
Space
group
Lattice
parameter
Reference
(8) a
EuHg EuHgz EuHgs EuHkh.6
CsCl (b.c.c.) CeCdz (hexagonal) Mg,Cd (hexagonal) GdAgs.6 (hexagonal)
P43m PG2m P63/mmc P6/m
C
3.88 4.98 13.57
3.11 9.74
131 [41 [51 [6,101
The formulae assigned to the compounds were determined from this structural data supported by the mole ratios of reactants used in the preparation and subsequent chemical analysis. However, small ranges of solid solubility may occur. The Mijssbauer spectra are generally of somewhat poorer quality than is usual because of the high degree of scattering of the 22 keV y rays by the mercury; dilution of the sample by grinding with AlzO, to give finely divided sample source material helped to alleviate this problem. A typical spectrum is presented in Fig. 1. The isomer shifts lie between -10 and -11 mm sl, showing the europium ionic core to be divalent. The Mossbauer spectral parameters and X-ray structural data are presented in Table 2. The X-ray diffraction patterns confirmed the authenticity of the samples. Except for Eu,Hg, only those lines corresponding to the required published structure were obtained so that each sample consisted of a single phase. The Eu,Hg, was found by chemical analysis to have an Eu:Hg atomic ratio of 1:1.47 but
268
VELOCITY Fig. 1. A typical
TABLE
Mijssbauer
for the Eu-Hg
system,
in this case for EuHgz.
2
Compositional data, found for compounds
Compound
EuHg EuHgz EuHg, EuHgs.6
SCALE (mm s-l)
spectrum
isomer shifts 6, peak widths at half-height prepared in the present work
Eu:Hg ratio
1:0.98 1:1.98 1:3.15 1:4.07
r and X-ray
parameters
Lattice parameter
6 (mm s-l)
fmm s-l)
a
-10.37 -10.1 -10.85 -10.7
3.1 2.6 2.7 2.0
3.16 4.9’7 6.30 13.24
f 0.06 f 0.4 _+0.02 f 0.1
f + f f
0.1 0.1 0.1 0.2
(A)
C
+ + * f
0.02 0.06 0.08 0.01
3.56 5.22 9.27
f 0.05 + 0.05 + 0.05
the X-ray diffraction pattern consisted of lines indexable on the CeCd, and CsCl structures. The Mossbauer spectra could be fitted to two overlapping peaks corresponding to EuHg, and EuHg with intensities in the ratio 1:5 assuming equal recoil-free fractions. The chemical analysis implies that the correct ratio is 1:1.13. The disparity in the ratios can be attributed to differences in the recoil-free fractions of EuHg, and EuHg and to the quality of the Mijssbauer spectra subjected to the resolution treatment. The attempt to produce Eu,Hg, led to a compound with the composition EuSHg,. The X-ray diffraction pattern was indexable on the CsCl-type structure with only one phase apparent. The existence of only one phase was supported by the Mossbauer spectrum.
269
TABLE 3 Attempted preparations of products relating to the actual products Compound
In tended
Eu:Hg Actual
EuzW3
EuHgt
Eu&k,
EuHg,I EusHh
1:1
.
($7
1:0.79
ratio
of compositions
EutHgs and Eu,Hgz and data
6
r
(mm s-l)
(mm s-- ‘)
-10.5 -10.2
-10.14
k 0.2 r 0.3
rt 0.05
Lattice
parameter
a
(A)
C
4.6 f 0.5 2.1 _+0.8
3.80 4.82
t 0.05 i 0.06
3.6 rt 0.1
3.80 f 0.02
3.63
+ 0.04
-
Relevant numerical data are presented in Table 3 for the last two mentioned preparations.
4. Discussion Evidence for metal-like character in the substances listed in Table 1 comes from their visual appearance and re~ectivity. It is supported by electrical conductivity measurements on EuHg,,, for which the resistivity was found to increase with temperature. The crystal systems are common with those of metals. However, the compositional studies outlined above point to a limited non-stoichiometric range of solid solubihty for the Eu-Hg system as is perhaps to be expected since europium and EuHg are isostructural. In contrast, the attempt to make Eu2Hg, highlights the narrow compositional range for these compounds in general. This restricted range of solid solubility is not characteristic of metallic alloys and implies some chemical bonding between europium and mercury. The electronegativities, 1.1 and 1.9 for europium and mercury on the Pauling scale, together with differences in the atomic radii, 1.98 A and 1.56 .& respectively, point to the formation of normal valence compounds. The Miissbauer spectra confirm the divalent ionic core deduced for europium from magnetic susceptibility measurements. In general, the compounds are metallic in appearance but with the evidence for the presence of well-defined chemical bonds they are probably best described as intermetallic substances rather than metallic alloys [ll, 12]. Simple correlations between the isomer shifts derived from Mtissbauer spectra and chemical composition of the substances listed in Table 1 were not found. Neither the amount of mercury present nor the Eu-Hg distances (Table 4) appear to be directly related to the isomer shift. A quantity independent of structure type is required. Such a parameter, in the form of the cube root of the unit cell volume, has been used by Parthe [13] to eliminate structural differences between compounds. However, this parameter does not correlate directly with the isomer shift, but when the number rz of europium atoms contained in the unit cell is divided by the cell volume u the cube root (n/u) 1’3 of this parameter correlates with the isomer
270 TABLE 4 Observed isomer shifts, intermetallic distances and (n/~)“~ values for the compounds Compound
EusHg4 EuHg EuHgz EuHg, EuHg3.6
s (mm s--l)
Eu-Hg distance
(n/uy
(8)
(8-l)
-10.14 f 0.05 -10.37 F 0.06 -10.1 f 0.4 -10.85 + 0.02 -10.7 + 0.2
3.29 3.26 4.00 5.02 4.00a
0.287 0.266 0.283 0.169 0.205
aA weighted average.
0.0.
6
0.5
0.15
0.20
0.25
0.30
in/v)‘” K1) Fig. 2. Plot of isomer shift 6 us. (n/u)1’3 (see Table 4).
shift. This new parameter reflects the number density of europium “atoms” in the unit cell. From Fig. 2, the isomer shift is seen to decrease with decreasing values of (n/~)“~. Following the treatment of Brix et al. [ 141, the conduction electron density at the europium nucleus has been estimated for each of the intermetallic systems with the results summarized in Table 5. The ratio E of conduction electron density at the europium nucleus to that of the isolated europium atom has also been calculated; from it the “actual” 6s electron confi~ration recorded in the table is derived. However, in absolute terms, the errors in these computed values are likely to be large (perhaps as much as 30%) and too much reliance must not be put on them. Nevertheless, relative errors are probably of the same order of magnitude as those for isomer shifts. When the conduction electron density is plotted against (n,/~)“~ the curve obtained is linear and similar in form to that presented in Fig. 2. The isomer shift is primarily a measure of the s electron density at the europium nucleus; hence as less europium is contained in the unit cell this electron density on each atom decreases. For systems in which the europium always
271 TABLE
5
Computed etal.
conduction
electron
densities
E and 6s electron
configurations
following
Brix
[14]
Compound
Conduction electron (X1O26 cmd3)
EusHth
0.62 0.59 0.62 0.53 0.55
EuHg EuHg, EuHg, EuHg3.6
density
E
Electron
0.26 0.24 0.26 0.22 0.21
[Xe] [Xe] [Xe] [Xe] [Xe]
configuration
(4Q7 (4f)7 (4f)7 (4f)7 (4f)7
(6~)‘.~’ (6~)O.~s (~s)O.“~ (~s)O.~~ (~s)O.~”
exhibits a divalent core only 6s electrons are available for conduction and bonding. The electron density at the europium nuc!eus is dependent on the number of europium atoms in the unit cell rather than on stoichiometric proportions of europium and mercury. This is consistent with the results of the survey of intermetallic systems conducted by van Steenwijk and Buschow [ 151. Further qualitative evidence for “chemical” bonding may be deduced from a comparison of the electron densities at the europium nucleus in these compounds with that of europium in the metal. Europium metal has an isomer shift of -7.8 mm s- ’ from which an electron density of 0.9 X 1O-26 cmm3 may be derived by calculation. This is far greater than those values found for europium in the compounds studied here (Table 5). The reduction in conduction electron density at the europium nucleus due to the presence of mercury in these samples is seen to be a consequence of chemical bonding.
Acknowledgment One of us (W.A.W.) thanks the Science and Engineering Council for the award of a postgraduate studentship.
Research
References L. F. Yntema, J. Am. Chem. Sot., 52 (1930) 2782. H. N. McCoy, Znorg. Synth., 2 (1946) 65. A. Iandelli and A. Palenzona, J. Less-Common Met., 9 (1965) 1. A. Iandelli and A. Palenzona, J. Less-Common Met., 15 (1968) 273. A. Palenzona, J. Less-Common Met., 10 (1966) 290. F. Merlo and M. L. Fornasini, J. Less-Common Met., 64 (1979) 221. S. J. Lyle and M. Rahman, Talanta, 10 (1963) 1177. A. I. Vogel, Text Book of Quantitative Inorganic Analysis, Longman, 4th edn., 1978. 9 C. M. Jenden and S. J. Lyle, J. Chem. Sot., Dalton Trans., (1982) 2409. 10 D. M. Bailey and G. R. Kline, Acta Crystallogr., Sect. B, 27 (1971) 650. 1 2 3 4 5 6 7 8
New
York,
272 11 W. Hume-Rothery, 12 W. Hume-Rothery Monogr. Rep. Ser. 13 E. Parthb, Colloq. 14 P. Brix, S. Hiifner, 15 F. J. van Steenwijk
J. Inst. Met., 35 (1926) 295. and G. V. Raynor, The Structure of Metals and Alloys, Inst. Met., 1 (1954). Int. CNRS, 157 (1965) 195. P. Kienle and D. Quitmann, Phys. Lett., 13 (1964) 140. and K. H. J. Buschow, Physica B, 85 (1977) 327.
of the Less-Common
A MijSSBAUER
SAMUEL
Metals,
SPECTROSCOPIC
J. LYLE and WILLIAM
The Chemical (Gt. Britain)
(Received
October
99 (1984)
Laboratories,
265
265
- 272
STUDY OF THE Eu-Hg SYSTEM
A. WESTALL
University
of
Kent
at
Canterbury,
Kent
CT2
7NH
17,1983)
Summary Samples of EuHg, (x = 1, 2, 3 and 3.6) have been prepared and characterized by chemical analysis and X-ray powder diffractomet~. Evidence is presented for narrow ranges of solid solubility and together with other data it is concluded that these substances are better described as being intermetallic rather than metallic alloy in character. The Mijssbauer spectroscopic study confirms that the europium exhibits a divalent core in each case. The isomer shift is dependent on the number density of europium atoms in the unit cell. This latter quantity correlates in a similar way with the computed conduction electron density at the europium nucleus. The decrease in electron density at the europium nucleus in these substances compared with that for europium metal is indicative of some degree of normal chemical bonding in these compounds.
1. Introduction With a few exceptions, the lanthanides have long presented difficulties in the separation of one element from another. Europium represents one of the exceptions in that it can be concentrated and separated by electrolytic methods, particularly at a mercury cathode. Electrolytic reduction to E&O4 at a mercury cathode was first reported by Yntema [ 11; later the electrolysis was modified by McCoy [2] so that a europium amalgam formed at the cathode. McCoy observed that removal of surplus mercury by distillation resulted in the formation of solid Eu-Hg products. More recently, the EuHg system has formed part of a broader study [3 - 61 of lanthanide intermetallic systems with B metals of groups I - III of the periodic table. These studies used X-ray diffraction data in a search for correlations between lanthanide ionic size and lattice parameters; hence structures were determined. From lattice parameters and magnetic data europium behaved in an anomalous way in that in the ionic cores it appeared to be divalent as @ Elsevier
Sequoia/Printed
in The Netherlands
266
opposed to the trivalent cores exhibited by typical lanthanide elements. However, information in general is sparse on the chemical and physical properties of the Eu-Hg system. In the study reported here the Miissbauer effect in europium is used as a tool to gain insight into the nature of the species produced and the effect the interaction with mercury has on the electronic environment of the europium atoms. Two methods of preparation were examined. The first was that of McCoy and the second, which was preferred, followed the procedure adapted by later workers in which predetermined amounts of the elements were heated together. The products were characterized by chemical analysis and X-ray diffractometry.
2. Experimental
details
2.1. Reagents Europium Products Ltd., grade chemicals
metal was obtained Widnes, Gt. Britain. were used.
in sealed ampoules from Rare Earth Triply distilled mercury and reagent
2.2. Preparation and analysis of Eu-Hgproducts A europium amalgam was prepared by the method given by McCoy [ 21. Excess mercury was removed by distillation at low pressures. In the alternative, preferred, method required atomic proportions of europium and mercury were heated together in sealed silica ampoules. The europium metal (0.3 - 0.4 g) was divided up into small pieces under dry argon in a glove-box and mixed with the mercury in the ampoule which was next sealed under vacuum and heated at 600 “C for between 12 and 24 h. All products were handled under dry argon in the glove-box. The composition of the products was checked by titrimetric analysis using ethylenediaminetetraacetic acid as titrant. The weighed sample (about 0.2 g) was dissolved in dilute nitric acid and the volume made up to 50 ml with water. Mercury and europium were determined [7] in one aliquot (10 ml) and europium was determined in another after masking mercury(I1) with cyanide [ 81. 2.3. Instrumental
measurements
The X-ray powder diffraction measurements were performed using a Philips PW lOOS/SO X-ray unit. The Mijssbauer spectra were recorded in the transmission mode using a spectrometer and a liquid nitrogen flow cryostat both previously described [9]. A 50 mCi “‘Srn source incorporated in an anhydrous SmF, matrix and supplied by the Radiochemical Centre, Amersham, Gt. Britain, was used. Both the sample which was diluted with dry Al,O, and the source were held at 93 K in recording the spectra. Isomer shifts are reported relative to EuFs and the errors quoted on them are standard deviations of mean values.
261
3. Results Removal of excess mercury by distillation from the europium amalgam produced electrolytically resulted in poor control over the composition of the residual material. Consequently almost all the work described below was based on products derived from the second method of preparation. The samples were air and possibly moisture sensitive necessitating the use of an argon-filled glove-box for handling purposes. The products prepared in the course of the work are listed in Table 1 together with previously published crystallographic data. The compounds (Table 1) all had a silver-grey appearance and were hard and brittle; difficulty in grinding increased with decreasing mercury content. In addition to these products of known composition, mixtures of europium and mercury in atomic ratios 2:3 and 3:2 were heated together, the former as a test of range of solid solubility and the latter because McCoy claimed to have isolated Eu,Hg, from europium amalgam. The X-ray powder diffraction data were used to check the authenticity of the samples by comparison with reported crystal parameters. TABLE Structural Compound
1 data for known Structure
compounds type
Space
group
Lattice
parameter
Reference
(8) a
EuHg EuHgz EuHgs EuHkh.6
CsCl (b.c.c.) CeCdz (hexagonal) Mg,Cd (hexagonal) GdAgs.6 (hexagonal)
P43m PG2m P63/mmc P6/m
C
3.88 4.98 13.57
3.11 9.74
131 [41 [51 [6,101
The formulae assigned to the compounds were determined from this structural data supported by the mole ratios of reactants used in the preparation and subsequent chemical analysis. However, small ranges of solid solubility may occur. The Mijssbauer spectra are generally of somewhat poorer quality than is usual because of the high degree of scattering of the 22 keV y rays by the mercury; dilution of the sample by grinding with AlzO, to give finely divided sample source material helped to alleviate this problem. A typical spectrum is presented in Fig. 1. The isomer shifts lie between -10 and -11 mm sl, showing the europium ionic core to be divalent. The Mossbauer spectral parameters and X-ray structural data are presented in Table 2. The X-ray diffraction patterns confirmed the authenticity of the samples. Except for Eu,Hg, only those lines corresponding to the required published structure were obtained so that each sample consisted of a single phase. The Eu,Hg, was found by chemical analysis to have an Eu:Hg atomic ratio of 1:1.47 but
268
VELOCITY Fig. 1. A typical
TABLE
Mijssbauer
for the Eu-Hg
system,
in this case for EuHgz.
2
Compositional data, found for compounds
Compound
EuHg EuHgz EuHg, EuHgs.6
SCALE (mm s-l)
spectrum
isomer shifts 6, peak widths at half-height prepared in the present work
Eu:Hg ratio
1:0.98 1:1.98 1:3.15 1:4.07
r and X-ray
parameters
Lattice parameter
6 (mm s-l)
fmm s-l)
a
-10.37 -10.1 -10.85 -10.7
3.1 2.6 2.7 2.0
3.16 4.9’7 6.30 13.24
f 0.06 f 0.4 _+0.02 f 0.1
f + f f
0.1 0.1 0.1 0.2
(A)
C
+ + * f
0.02 0.06 0.08 0.01
3.56 5.22 9.27
f 0.05 + 0.05 + 0.05
the X-ray diffraction pattern consisted of lines indexable on the CeCd, and CsCl structures. The Mossbauer spectra could be fitted to two overlapping peaks corresponding to EuHg, and EuHg with intensities in the ratio 1:5 assuming equal recoil-free fractions. The chemical analysis implies that the correct ratio is 1:1.13. The disparity in the ratios can be attributed to differences in the recoil-free fractions of EuHg, and EuHg and to the quality of the Mijssbauer spectra subjected to the resolution treatment. The attempt to produce Eu,Hg, led to a compound with the composition EuSHg,. The X-ray diffraction pattern was indexable on the CsCl-type structure with only one phase apparent. The existence of only one phase was supported by the Mossbauer spectrum.
269
TABLE 3 Attempted preparations of products relating to the actual products Compound
In tended
Eu:Hg Actual
EuzW3
EuHgt
Eu&k,
EuHg,I EusHh
1:1
.
($7
1:0.79
ratio
of compositions
EutHgs and Eu,Hgz and data
6
r
(mm s-l)
(mm s-- ‘)
-10.5 -10.2
-10.14
k 0.2 r 0.3
rt 0.05
Lattice
parameter
a
(A)
C
4.6 f 0.5 2.1 _+0.8
3.80 4.82
t 0.05 i 0.06
3.6 rt 0.1
3.80 f 0.02
3.63
+ 0.04
-
Relevant numerical data are presented in Table 3 for the last two mentioned preparations.
4. Discussion Evidence for metal-like character in the substances listed in Table 1 comes from their visual appearance and re~ectivity. It is supported by electrical conductivity measurements on EuHg,,, for which the resistivity was found to increase with temperature. The crystal systems are common with those of metals. However, the compositional studies outlined above point to a limited non-stoichiometric range of solid solubihty for the Eu-Hg system as is perhaps to be expected since europium and EuHg are isostructural. In contrast, the attempt to make Eu2Hg, highlights the narrow compositional range for these compounds in general. This restricted range of solid solubility is not characteristic of metallic alloys and implies some chemical bonding between europium and mercury. The electronegativities, 1.1 and 1.9 for europium and mercury on the Pauling scale, together with differences in the atomic radii, 1.98 A and 1.56 .& respectively, point to the formation of normal valence compounds. The Miissbauer spectra confirm the divalent ionic core deduced for europium from magnetic susceptibility measurements. In general, the compounds are metallic in appearance but with the evidence for the presence of well-defined chemical bonds they are probably best described as intermetallic substances rather than metallic alloys [ll, 12]. Simple correlations between the isomer shifts derived from Mtissbauer spectra and chemical composition of the substances listed in Table 1 were not found. Neither the amount of mercury present nor the Eu-Hg distances (Table 4) appear to be directly related to the isomer shift. A quantity independent of structure type is required. Such a parameter, in the form of the cube root of the unit cell volume, has been used by Parthe [13] to eliminate structural differences between compounds. However, this parameter does not correlate directly with the isomer shift, but when the number rz of europium atoms contained in the unit cell is divided by the cell volume u the cube root (n/u) 1’3 of this parameter correlates with the isomer
270 TABLE 4 Observed isomer shifts, intermetallic distances and (n/~)“~ values for the compounds Compound
EusHg4 EuHg EuHgz EuHg, EuHg3.6
s (mm s--l)
Eu-Hg distance
(n/uy
(8)
(8-l)
-10.14 f 0.05 -10.37 F 0.06 -10.1 f 0.4 -10.85 + 0.02 -10.7 + 0.2
3.29 3.26 4.00 5.02 4.00a
0.287 0.266 0.283 0.169 0.205
aA weighted average.
0.0.
6
0.5
0.15
0.20
0.25
0.30
in/v)‘” K1) Fig. 2. Plot of isomer shift 6 us. (n/u)1’3 (see Table 4).
shift. This new parameter reflects the number density of europium “atoms” in the unit cell. From Fig. 2, the isomer shift is seen to decrease with decreasing values of (n/~)“~. Following the treatment of Brix et al. [ 141, the conduction electron density at the europium nucleus has been estimated for each of the intermetallic systems with the results summarized in Table 5. The ratio E of conduction electron density at the europium nucleus to that of the isolated europium atom has also been calculated; from it the “actual” 6s electron confi~ration recorded in the table is derived. However, in absolute terms, the errors in these computed values are likely to be large (perhaps as much as 30%) and too much reliance must not be put on them. Nevertheless, relative errors are probably of the same order of magnitude as those for isomer shifts. When the conduction electron density is plotted against (n,/~)“~ the curve obtained is linear and similar in form to that presented in Fig. 2. The isomer shift is primarily a measure of the s electron density at the europium nucleus; hence as less europium is contained in the unit cell this electron density on each atom decreases. For systems in which the europium always
271 TABLE
5
Computed etal.
conduction
electron
densities
E and 6s electron
configurations
following
Brix
[14]
Compound
Conduction electron (X1O26 cmd3)
EusHth
0.62 0.59 0.62 0.53 0.55
EuHg EuHg, EuHg, EuHg3.6
density
E
Electron
0.26 0.24 0.26 0.22 0.21
[Xe] [Xe] [Xe] [Xe] [Xe]
configuration
(4Q7 (4f)7 (4f)7 (4f)7 (4f)7
(6~)‘.~’ (6~)O.~s (~s)O.“~ (~s)O.~~ (~s)O.~”
exhibits a divalent core only 6s electrons are available for conduction and bonding. The electron density at the europium nuc!eus is dependent on the number of europium atoms in the unit cell rather than on stoichiometric proportions of europium and mercury. This is consistent with the results of the survey of intermetallic systems conducted by van Steenwijk and Buschow [ 151. Further qualitative evidence for “chemical” bonding may be deduced from a comparison of the electron densities at the europium nucleus in these compounds with that of europium in the metal. Europium metal has an isomer shift of -7.8 mm s- ’ from which an electron density of 0.9 X 1O-26 cmm3 may be derived by calculation. This is far greater than those values found for europium in the compounds studied here (Table 5). The reduction in conduction electron density at the europium nucleus due to the presence of mercury in these samples is seen to be a consequence of chemical bonding.
Acknowledgment One of us (W.A.W.) thanks the Science and Engineering Council for the award of a postgraduate studentship.
Research
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