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121Sb Mössbauer spectroscopy in alkali antimony (V) hexafluorides
Va!umc 35, number 3
‘*%I
CHEh’LICAL PHYSLCS
MijSSBAUER
J.P. DEVORT Lahratoire
SPECTROSCOPY
IN ALKALI
LETTERS
15 September 1975
ANIIMONY(V)
HEXAFLUORJDES
and J.M. FRIEDT
de Cilirrlie Nuclkzire, Cenrre de Reckrches
Nucl&ircs.
Fence
6 703 7 Smxbmrg,
Received 6 June 1975
The dependence of the isomer shift of 12’Sb on the alkali cation in the hISoF (X! = Li, Na, K, Rb, Cs and NH4) series is investigated and dbcussed in terms of a vuying electron transfer from the cation :o the SbF, molecuh ion. :
1. Introduction A number of chemical applications of Mtissbauer spectrosco y using the 37.15 keV (7/2’5/2+) transition in ’ 2p’ Sb have been published recently [ l] . One of the characteristic features of the resonance is
provided by the large change in the nuclear mean squared radius between the ground and excited levels (A$> = -32 X low3 frn2 [2,3]) which involves a high sensitivity of the isomer shift to small changes in electron densities at the nucleus. We report here the variation of the isomer shift (IS) of Sb(V) along the series of alkali antimony(V)
hexafluorides
MSbF6 (M = Ii, Na, K, Rb, C-s,NH4).
In addition, WE have measured the Raman spectra of the complexes since for 2 given coordination, the vibrational frequencies reflect the bond strength which is releted to the bond covalency. The crystal structures of the MSbFs compounds have been described in the literature [4]. The Sb(V9 ion is always octahedrally coordinated io six Flig+nds. In the Li, Na, Rb, Cs and NH4 salts, rbe SbFg molecular ion is surrounded by an octahedron of six metal cations, whereas in the K salt, it is surrounded by eight cations.
2. Expetienhl The Mikbauer spectra have been measured with the Ca121mSn03 (0.66 mCi) source and the absorbers
cooled at 4.2 K. The absorbers were carefuily ground
I
-6
I
I
-6
-4
I
-2
jv ,, I
0
2
4
6
8
vELom-Y(mcrrh)
Fig_ 1. !.i.iiissbauer spectra of (a) KSbF6 and (b) NzSbFc at 4.2 .K. Both spec:ra se fitted as single lines (solid curves). with boron
carbide
powder and the thickness was The IS data refer to tFe
typically 9 mg Sb/cm2.
CaSnO3 source (IS of inSb versus this source: -8.53 ? 0.03 mm/s). The MSSF6 compounds were prepared by reaction of the alkali fluorides MF with SbFs dissolved in HF [5], and checked from X-ray powder diffraction. All spectra were fitted 2s single hnes of lorentzian shape (fig. 1). Although the Sb site does not present a local cubic symmetry in some of the irtvestigated compounds [4], there was no evidence for any 423
: Volume
35, number
3
CHEMICAL PHYSICS LETTERS
Table 1 : IL1 Sb isomer shi.% at 4.2 K, Raman frequerxies NaSbF6 2.89 674
and Sb-F
bond lengths
LiSbFs (3) (1)
1.74
2.73.
(3) (1)
1.877
2.59 658.5
1975
[4] (the error on the last figure is given in brxkets)
KSbF6
668
1.5 September
RbSbFB (3) (1)
1.77
CsSbF6
2.58 (3) 654.5
2.63
(1)
1.97
NH4SbF6
652.5
-
(3) (2)
1.97
2.53 654.5
(3) (I)
1.97
asymmetry in the resonance shape even for the thinnest absorbers. As an example, for KSbF, the experimental linewidth is 2.8 mm/s for a thickness of 2.3 mg Sb/cm’ and the zero-thickness extrapolated Knewidth is 2.5 mm/s (fiJr cubic InSb the extrapolated linewidth Is 2.4 mm/s). It has been checked ‘&at the fitted IS do not change with absorber thic.kness, i.e., that no fortuitous error due to the neglect of a. small quadrupole interaction i; introduced in the data analysis.
Furthermore,
spectral
shape
for small
values
has been simulated on computer and fitted as a single line; it appear; that coupling constants smaller thao 4.5 mm/s al-e detected only as a sLi@t of e2@
tine broadening [6] and cannot be meaningfully extracted from experimenta! data. The FLsnan spectra have been recorded at room temperature using a He-Ne laser. The speed was 5 cm-‘/mm, anclihe error on the A!, peak positiofi is less t!-IzIl 1 cm .
- 2.65
T &Kll
0.8
+a
0.9
2.55
1.0
1.1
ELECTRONEGATIVITY
3. Results
Fig. 2. (a) Change of the isomer shift of l*lSb in MSbFe znd (b) of Raman frequency y1 verw the electronegativity of the alkali cation.
and discus.sion
The experL?ental
dat;l are surnanarized in table 1. The IS of ‘IISb depends slightly on the nature of the cation (fig. 2). The IS of _KSbF, is in agreement with one of the previously reForted data [2,7], whereas the value for NaSbF6 mination 181.
disagrees
with an earlier deter-
The increase of IS from the Rb to the Na salt signifies a decrease of electron density at the nucleus in that order; this trend arises essentially from a de-
crease in the population of the 5s valer,ce orbital. It is understood fr.om simple chemical arguments: as the eiectronegatitity of the alktali cation increases [9], the aLkaIi-fluorine bond cocalency increases at the expenses of the Sb-F bc’nd covalency. Thus, the pcpuiation OF the Sb hybridized. orbitals diminishes; 424.
..
‘.
.,
the direct ef%ct of the decrease of the 5s orbital
population is decisive in determining the change of IS [IO]. The frequency of the symmetrical stretching vibration increases tith the electronegativity of the alkali cation (fig. 2), in agreement.with the corresponding shortening of the Sb-F bond [4]. A similar correlation between bond ionicity and bond length has already been reported in other systems [ 1 I 1. it may be noticed that the diffkrences in ZS as well as in frequency ~1 for the Cs, Rb and K salts barely exceed the experimental error; this may be accounted for bj, the small difference bf electronegativity of these cations.
Vohnne
35, number 3
CHEMICAL P~SICS
References [l]
SH. Bowen, in: MG:sbauer Effect Data Index, cds. J.C. Stevens and V.E. Ste.vens (1972) p. 71. [2] S.L. Ruby, G.M. Kalvius, G.B. Bardand R.E. Snyder, Phys. Rev. 159 (1967) 239. [3] D.I. Baltrunas, S.P. Ionov. A. Aleksandrov and E.F. &lakarov. Chem. Phys. Letters 20 (1973) 55. [4] V. Gutmxm, Haagen chcrnistry, VoL 2 (Academic Press, New York, 1967) p. 120; N. Schrewetius, Z. Anorg. AU& Chcm. 238 (1938) 246; If. Bode and E. Voss, Z. Anor& Al!%. Chem. 264 (1951) 144; J.H. Burns, Acta Cryst. 15 (1962) 1058; R.W.G. Wyckoff, Crystal structures, Vol. 3 (Interscience. New York, 1965) p. 324.
LETTERS
15 September
1975
Z. *org. A!_lg.Chem. [51 V.W. Lznge and K. Askitopoulos, 223 (1935) 379. I61 G.K. Shenoy and SM. Friedt, Nucl. Instr. hfethods 116 (1974) 573. I.71 T. Birchnll and B. Dellavalle, Can. J. Chem. 49 (1971) 2808. 181 V.A. Brukhanov, B-Z. Iofa, V. Kotkhckar. %I. Semenov snd V.S. Shpinel, Soviet Phys. JBTP 26 (1968) 912. 191 A.L. Allrcd and E.G. Rochow, J. inorg. Nucl. Chem. 5 (1958) 2644. [!Ol J.hf. Friedt, G.K. Shenoy and hf. Burgard, J. Chem. Phys. 59 (1973) 4468. illI C.N.R. Rae and J.R. Ferraro, in: Spectroscopy in inorganic chemistry (Academic Press, New York, 1970) p. 85; R.D. Shannon and H. Vincent, Structure Bonding 19 (i374! 32.
425
‘*%I
CHEh’LICAL PHYSLCS
MijSSBAUER
J.P. DEVORT Lahratoire
SPECTROSCOPY
IN ALKALI
LETTERS
15 September 1975
ANIIMONY(V)
HEXAFLUORJDES
and J.M. FRIEDT
de Cilirrlie Nuclkzire, Cenrre de Reckrches
Nucl&ircs.
Fence
6 703 7 Smxbmrg,
Received 6 June 1975
The dependence of the isomer shift of 12’Sb on the alkali cation in the hISoF (X! = Li, Na, K, Rb, Cs and NH4) series is investigated and dbcussed in terms of a vuying electron transfer from the cation :o the SbF, molecuh ion. :
1. Introduction A number of chemical applications of Mtissbauer spectrosco y using the 37.15 keV (7/2’5/2+) transition in ’ 2p’ Sb have been published recently [ l] . One of the characteristic features of the resonance is
provided by the large change in the nuclear mean squared radius between the ground and excited levels (A$> = -32 X low3 frn2 [2,3]) which involves a high sensitivity of the isomer shift to small changes in electron densities at the nucleus. We report here the variation of the isomer shift (IS) of Sb(V) along the series of alkali antimony(V)
hexafluorides
MSbF6 (M = Ii, Na, K, Rb, C-s,NH4).
In addition, WE have measured the Raman spectra of the complexes since for 2 given coordination, the vibrational frequencies reflect the bond strength which is releted to the bond covalency. The crystal structures of the MSbFs compounds have been described in the literature [4]. The Sb(V9 ion is always octahedrally coordinated io six Flig+nds. In the Li, Na, Rb, Cs and NH4 salts, rbe SbFg molecular ion is surrounded by an octahedron of six metal cations, whereas in the K salt, it is surrounded by eight cations.
2. Expetienhl The Mikbauer spectra have been measured with the Ca121mSn03 (0.66 mCi) source and the absorbers
cooled at 4.2 K. The absorbers were carefuily ground
I
-6
I
I
-6
-4
I
-2
jv ,, I
0
2
4
6
8
vELom-Y(mcrrh)
Fig_ 1. !.i.iiissbauer spectra of (a) KSbF6 and (b) NzSbFc at 4.2 .K. Both spec:ra se fitted as single lines (solid curves). with boron
carbide
powder and the thickness was The IS data refer to tFe
typically 9 mg Sb/cm2.
CaSnO3 source (IS of inSb versus this source: -8.53 ? 0.03 mm/s). The MSSF6 compounds were prepared by reaction of the alkali fluorides MF with SbFs dissolved in HF [5], and checked from X-ray powder diffraction. All spectra were fitted 2s single hnes of lorentzian shape (fig. 1). Although the Sb site does not present a local cubic symmetry in some of the irtvestigated compounds [4], there was no evidence for any 423
: Volume
35, number
3
CHEMICAL PHYSICS LETTERS
Table 1 : IL1 Sb isomer shi.% at 4.2 K, Raman frequerxies NaSbF6 2.89 674
and Sb-F
bond lengths
LiSbFs (3) (1)
1.74
2.73.
(3) (1)
1.877
2.59 658.5
1975
[4] (the error on the last figure is given in brxkets)
KSbF6
668
1.5 September
RbSbFB (3) (1)
1.77
CsSbF6
2.58 (3) 654.5
2.63
(1)
1.97
NH4SbF6
652.5
-
(3) (2)
1.97
2.53 654.5
(3) (I)
1.97
asymmetry in the resonance shape even for the thinnest absorbers. As an example, for KSbF, the experimental linewidth is 2.8 mm/s for a thickness of 2.3 mg Sb/cm’ and the zero-thickness extrapolated Knewidth is 2.5 mm/s (fiJr cubic InSb the extrapolated linewidth Is 2.4 mm/s). It has been checked ‘&at the fitted IS do not change with absorber thic.kness, i.e., that no fortuitous error due to the neglect of a. small quadrupole interaction i; introduced in the data analysis.
Furthermore,
spectral
shape
for small
values
has been simulated on computer and fitted as a single line; it appear; that coupling constants smaller thao 4.5 mm/s al-e detected only as a sLi@t of e2@
tine broadening [6] and cannot be meaningfully extracted from experimenta! data. The FLsnan spectra have been recorded at room temperature using a He-Ne laser. The speed was 5 cm-‘/mm, anclihe error on the A!, peak positiofi is less t!-IzIl 1 cm .
- 2.65
T &Kll
0.8
+a
0.9
2.55
1.0
1.1
ELECTRONEGATIVITY
3. Results
Fig. 2. (a) Change of the isomer shift of l*lSb in MSbFe znd (b) of Raman frequency y1 verw the electronegativity of the alkali cation.
and discus.sion
The experL?ental
dat;l are surnanarized in table 1. The IS of ‘IISb depends slightly on the nature of the cation (fig. 2). The IS of _KSbF, is in agreement with one of the previously reForted data [2,7], whereas the value for NaSbF6 mination 181.
disagrees
with an earlier deter-
The increase of IS from the Rb to the Na salt signifies a decrease of electron density at the nucleus in that order; this trend arises essentially from a de-
crease in the population of the 5s valer,ce orbital. It is understood fr.om simple chemical arguments: as the eiectronegatitity of the alktali cation increases [9], the aLkaIi-fluorine bond cocalency increases at the expenses of the Sb-F bc’nd covalency. Thus, the pcpuiation OF the Sb hybridized. orbitals diminishes; 424.
..
‘.
.,
the direct ef%ct of the decrease of the 5s orbital
population is decisive in determining the change of IS [IO]. The frequency of the symmetrical stretching vibration increases tith the electronegativity of the alkali cation (fig. 2), in agreement.with the corresponding shortening of the Sb-F bond [4]. A similar correlation between bond ionicity and bond length has already been reported in other systems [ 1 I 1. it may be noticed that the diffkrences in ZS as well as in frequency ~1 for the Cs, Rb and K salts barely exceed the experimental error; this may be accounted for bj, the small difference bf electronegativity of these cations.
Vohnne
35, number 3
CHEMICAL P~SICS
References [l]
SH. Bowen, in: MG:sbauer Effect Data Index, cds. J.C. Stevens and V.E. Ste.vens (1972) p. 71. [2] S.L. Ruby, G.M. Kalvius, G.B. Bardand R.E. Snyder, Phys. Rev. 159 (1967) 239. [3] D.I. Baltrunas, S.P. Ionov. A. Aleksandrov and E.F. &lakarov. Chem. Phys. Letters 20 (1973) 55. [4] V. Gutmxm, Haagen chcrnistry, VoL 2 (Academic Press, New York, 1967) p. 120; N. Schrewetius, Z. Anorg. AU& Chcm. 238 (1938) 246; If. Bode and E. Voss, Z. Anor& Al!%. Chem. 264 (1951) 144; J.H. Burns, Acta Cryst. 15 (1962) 1058; R.W.G. Wyckoff, Crystal structures, Vol. 3 (Interscience. New York, 1965) p. 324.
LETTERS
15 September
1975
Z. *org. A!_lg.Chem. [51 V.W. Lznge and K. Askitopoulos, 223 (1935) 379. I61 G.K. Shenoy and SM. Friedt, Nucl. Instr. hfethods 116 (1974) 573. I.71 T. Birchnll and B. Dellavalle, Can. J. Chem. 49 (1971) 2808. 181 V.A. Brukhanov, B-Z. Iofa, V. Kotkhckar. %I. Semenov snd V.S. Shpinel, Soviet Phys. JBTP 26 (1968) 912. 191 A.L. Allrcd and E.G. Rochow, J. inorg. Nucl. Chem. 5 (1958) 2644. [!Ol J.hf. Friedt, G.K. Shenoy and hf. Burgard, J. Chem. Phys. 59 (1973) 4468. illI C.N.R. Rae and J.R. Ferraro, in: Spectroscopy in inorganic chemistry (Academic Press, New York, 1970) p. 85; R.D. Shannon and H. Vincent, Structure Bonding 19 (i374! 32.
425