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17Zn and 33S nuclear magnetic shielding in zinc chalcogenides
~
Solid State Con~uunications, Voi.33, pp.]051-I053. Pergamon Press Ltd. ]980. Printed in Great Britain,
67Zn AND 33S NUCLEAR MAGNETIC SHIELDING IN ZINC CHALCOGENIDES M. Haller, W.E. Hertler, O. Lutz and A. Nolle Physikalisches Institut der Universit~t T~bingen, D - 7400 T[bingen, Federal Republic of Germany (Received 7
January 1980 by E. Mollwo)
Narrow 67Zn and 33S NMR signals have been detected in powdered ZnS, ZnSe, and ZnTe. Large chemical shifts relative to aqueous reference solutions have been found. The nuclear magnetic shielding of 67Zn in these solids is given in the absolute atomic reference scale together with the Knight-shift for the metallic state taken from literature.
Introduction Due to modern sensitive instrumentation, investigations of NMR signals in solid llb-chalcogenides are possible. This has recently been shown e.g. for 77Se, 113Cd, and 125Te (I-4) in a variety of compounds. Chemical shift tensors, absolute shielding constants, direct and isotropic and anisotropic indirect spin-spincouplings have been evaluated in solid compounds. Besides these spin-I/2-nuclei, investigations of the quadrupolar nuclei 67Zn and 33S are of interest in llb-chalcogenides. NMR investigations of 67 Zn, however, are not easy to perform due to the low gyromagnetic ratio (5), the small natural abundance of 4 , 1 1 % and the quadrupole moment Q = 0.16,10-28m 2 (5). ~ ~7 Zn in a queou s solutions have Some stu d ies o i been reported (6,7). Signals in the metallic statehaverecently been observed for liquid (8,9) and solid zinc (10). For other solid zinc compounds~ as far as we know, only two short remarks upon 67Zn NMR signals have been published (11,12). Nearly the same situation is found for the nuclide 33S, which also has a small gyromagnetic ratio (5), a low natural abundance pf 0.75 % and a quadrupole moment Q = -0.055.10-25m2(5). 33S NMR signals have been observed in some simple compounds in the liquid state (13,14). Further a remark is given upon a 33S signal in sphalerite (ZnS) in ref. (13). We tried to extend our systematic NMR studies in solid ll-Vl-compounds (I-4) also to 67Zn and 33S. For zinc an atomic shielding scale (6) is available as in the case of 113Cd (2). Therefore the measured chemical shifts of the solids can be given in this scale. This fact is also very important for a comparison of experimental Knight-shifts given in this scale with theoretical values.
a 90 ° pulse were accumulated and Fourier-transformed by a B-NC 12 data unit. Cylindrical sample tubes of 10 mm o.d. with a sample volume of ~bout 1.5 cm3 were used. A typical signal of 7Zn in ZnTe is given in Fig. I. The temperature was (298 ± 2)K. ZnTe (99.998%) and ZnSe (99.99%) were purchased from Zinsser, Frankfurt. ZnS (99.99%) was delivered by Cerac, Milwaukee. The chemical shifts of 33S and 67Zn in the solids are referred to liquid samples: A 4.0 molal solution of Cs2SO 4 in D20 for 33S and a 2.5 molal solution of ZnSO 4 in H20 for 67Zn. The chemical shifts 6 are given by:
Experimental In a magnetic field of 2.114 T, which was externally stabilized by a IH-stahilization unit, the Larmor frequencies of the spin-3/2-nucleus 33S and the spin-5/ 2 -nu cl eus 67Zn are near 6.91M}{z resp. 5.63 MHz. The, recep%ivities are 1.7.10 -5 for 33S and 1.2"10 -4 for 67Zn compared with I for the proton for an equal number of naturally abundant hydrogen and sulfur or zinc atoms. The NMR signals were observed with a multi-nuclei pulse NMR spectrometer Bruker SXP 4-100. The free induction decays following
Fi 5. I: 67Zn NMR signal of ZnTe powder near 5 . 6 3 1 M H z with a linewidth of 56 Hz. Experimental spectrum width: 20 kHz, number of pulses: 11350, measuring time: 15 h 46 min, 1000 data points were accumulated, followed by 15384 ~oints of zerofilling before Fou~'ier transformation.
6 = (~sample - Vreference)/~reference" The absolute shielding constant of the ZnSO 4 reference sample in the atomic scale is: q*(ZnSO 4 in H~O) = - 690 (10).10 -6 (see ref. (6)).
Results The chemical shifts and linewidths of the 67Zn signals in ZnS, ZnSe, and ZnTe and also of 33S in ZnS are given in table I. 1051
I052
Table
No.
I0
I: Results of the 67Zn and 33S NMR measurements
Linewidth in Hz
Shielding Constant q*. lO-U
Nucleus
Sample
Chemical Shift in ppm
67Zn
ZnSO 4 2.5 M in H20
0
67Zn
ZnS
67Zn
ZnSe
67Zn
ZnTe
33S
Cs2SO 4 4.0 M in D20
0
2o
33S
ZnS
- 562 ± I
53
690 ± 10
82
378 ± 2
1068 ± 10
52
273 ± 2
963 ± 10
5O
85 • 3
775 ± 10
The linewidths are very small for both uclei in the solid powder samples. For 7Zn even the signal in the reference solution is broader (6). Zn-chalcogenides are dilute nuclear spin systems, therefore the magnetic dipole-dipole interaction is small. Further, all three compounds are cubic, this results in a very weak electric field gradient at the position of the quadrupolar nuclei 67Zn and 33S. If the solid state linewidths are as small as shown in table I, it might be possible to narrow the signals by an usual sample spinning with about 50 Hz. The rotation around an axis perpendicular to the magnetic field resulted in a narrowing of the 67 Zn signal in ZnS from 52 Hz to 39 Hz, showing, that the linewidth is at least partly due to an anisotropic interaction. The contribution of the inhomogeneity of the magnetic field has been estimated to less equal 6 Hz by a separate 2H measurement. A careful inspection of the lineshape in the case of ZnTe showed no hint on a dipoledipole interaction with 125Te as has been found recently in other llb-chalcogenides (1,3,4). A possible influence has been studied theoretically by a computer program. It could be shown that the lineshape is only a little bit changed. Because it is not known, whether a Gaussian or a Lorentzian lineshape occurs for 67Zn in ZnTe, the rather small influence of a direct dipole-dipole interaction between 67Zn and 125Te could not be established. The longitudinal relaxation times, estimated by pulse sequence variation, are some seconds as mentioned in ref. (12). Due to the narrow lines and the good signal-to-noise ratios up to 40 : I within I h measuring time, the chemical shifts can be given very accurately. They are also given in table I. In the case of 67Zn, the shielding constants of the solids are given in the atomic scale. Further, the shielding constants for all known 67Zn NMR signals are reported in fig. 2. The amount of the shielding constants is about the same for the chalcogenides as for aqueous ionic solutions. The two at the first sight different results of 67Zn in liquid metallic zinc are identical if one takes into account the different reference samples used (8,9). Further, the shielding of the diamagnetic reference is not at all negligible in comparison with the Knightshift. For a comparison with theoretical results,
~
Vol. 33,
NUCLEAR MAGNETIC SHIELDING IN ZINC CHALCOGENIDES
57
6"/10-6 Shielding Constant
-
-
67Zn at 5.63MHz
Free Atom
Aqueous Soluhons -
Solids
500 ~
-1000
-
Z
n
Zn(ClO&}2 CI2
ZnTe ZnSe
0 1 2 3 4 5 Moles Salt kg Solution
ZnS
-1500
-2000
-2500 LiZn -3000
-3500
-4000 Zn Metal
hquid
Fi~. 2: Atomic reference scale of the nuclear magnetic shielding of 67Zn. Data from this work and of Refs. (6,8,9,11) were used. the Knight-shift referred to the free atom is a more reliable basis. The amount of the shielding constants of 67Zn in Zn-chalcogenides decreases with increasing atomic number of the chalcogenes (see fig. 3). The same behaviour was found in the case of 113Cd in Cd-chalcogen~des (2) with the exception of Cd0, where the amount of the shielding constant is smaller than for Cdg. Therefore the shielding constant of ZnO would be rather interesting. But unfortunately, in the ZnO powder of Wurtzite type no signal was detected. For 33S a signal in ZnS has been observed.
Vol. 33, No.
10
N U C L E A R M A G N E T I C SHIELDING IN ZINC C H A L C O G E N I D E S
|053
¢,i061Shielding Constant , 20 (3
4,0
S
60
Se
Te
Z
Atomic Number
-400
-800
J e"
/
ZincChalcogenides
•
/
-1200
/ -1600
"~
J
•/
Cadmium Chatcogenides
Fig. 3: Dependence of the absolute nuclear magnetic shielding of 67Zn and 113Cd (Ref.2) in chalcogenides as a function of the atomic number. The lines connecting the experimental points are given for clearness and have no theoretical meaning. A signal-to-noise ratio of 14 : I has been achieved within 13 h. This is one of the weakest NMR signals of a heteronucleus observed so far in solid state by a direct measurement. Unfortunately, no atomic shielding scale is available at the moment for 33S. These examples show, that the observation of very weak NMR signals of quadrupolar hetero-
nuclei is possible in favourable cases and that certain applications seem to be feasible. Acknowledgement We thank Prof. H. KrUger for his support of this work and the Deutsche Forsehungsgemeinschaft for their financial support.
References (I) (2) (3) (4) (5) (6) (7) (8) (9) 10) 11) 12) 13) 14)
Koch, W., Lutz, 0., and Nolle, A., Z. Physik A 28__9, 17 (1978) Nolle, A., Z. Naturforsch. 33__aa,666 (1978) Nolle, A., Z. Physik B 3_~4, 175 (1979) Balz, R., Haller, M., Hertler, W.E., Lutz, 0., Nolle, A., and Schafitel, R., J. Magn. Res., in press Fuller, G.H., J. Phys. Chem. Ref. Data 5, 835 (1976) Epperlein, B.W., Krdger, H., Lutz, 0., and Schwenk A., Z. Naturforsch. 29a, 1553 (1974) Maciel, G.E., Simeral, L., and Ackermann, J.J.H., J. Phys. Chem. 8_!, 263 (1977) Kerlin, A.L., and McNeil, J.A., Sol. State Comm. 27, 757 (1978) Bucklisch, R.,and Ploumbidis, D., Phys. Rev. B17, 4160 (1978) Herberg, H., Abaft, J., and Voigtl&nder, J., Z. Naturforsch. 34a, 1029 (1979) Bennet, L.H., Phys. Rev. 150, 418 (1966) Hayes, C.E., and Pound, R.V., Bull. Amer. Phys. Soc. I_~7,330 (1972) Retcofsky,H.L., and Friedel, R.A., J. Amer. Chem. Soc. 94, 6579 (1972) Lutz, 0., Nolle, A., and Schwenk, A., Z. Naturforsch. 28___a, 1370 (1973)
Solid State Con~uunications, Voi.33, pp.]051-I053. Pergamon Press Ltd. ]980. Printed in Great Britain,
67Zn AND 33S NUCLEAR MAGNETIC SHIELDING IN ZINC CHALCOGENIDES M. Haller, W.E. Hertler, O. Lutz and A. Nolle Physikalisches Institut der Universit~t T~bingen, D - 7400 T[bingen, Federal Republic of Germany (Received 7
January 1980 by E. Mollwo)
Narrow 67Zn and 33S NMR signals have been detected in powdered ZnS, ZnSe, and ZnTe. Large chemical shifts relative to aqueous reference solutions have been found. The nuclear magnetic shielding of 67Zn in these solids is given in the absolute atomic reference scale together with the Knight-shift for the metallic state taken from literature.
Introduction Due to modern sensitive instrumentation, investigations of NMR signals in solid llb-chalcogenides are possible. This has recently been shown e.g. for 77Se, 113Cd, and 125Te (I-4) in a variety of compounds. Chemical shift tensors, absolute shielding constants, direct and isotropic and anisotropic indirect spin-spincouplings have been evaluated in solid compounds. Besides these spin-I/2-nuclei, investigations of the quadrupolar nuclei 67Zn and 33S are of interest in llb-chalcogenides. NMR investigations of 67 Zn, however, are not easy to perform due to the low gyromagnetic ratio (5), the small natural abundance of 4 , 1 1 % and the quadrupole moment Q = 0.16,10-28m 2 (5). ~ ~7 Zn in a queou s solutions have Some stu d ies o i been reported (6,7). Signals in the metallic statehaverecently been observed for liquid (8,9) and solid zinc (10). For other solid zinc compounds~ as far as we know, only two short remarks upon 67Zn NMR signals have been published (11,12). Nearly the same situation is found for the nuclide 33S, which also has a small gyromagnetic ratio (5), a low natural abundance pf 0.75 % and a quadrupole moment Q = -0.055.10-25m2(5). 33S NMR signals have been observed in some simple compounds in the liquid state (13,14). Further a remark is given upon a 33S signal in sphalerite (ZnS) in ref. (13). We tried to extend our systematic NMR studies in solid ll-Vl-compounds (I-4) also to 67Zn and 33S. For zinc an atomic shielding scale (6) is available as in the case of 113Cd (2). Therefore the measured chemical shifts of the solids can be given in this scale. This fact is also very important for a comparison of experimental Knight-shifts given in this scale with theoretical values.
a 90 ° pulse were accumulated and Fourier-transformed by a B-NC 12 data unit. Cylindrical sample tubes of 10 mm o.d. with a sample volume of ~bout 1.5 cm3 were used. A typical signal of 7Zn in ZnTe is given in Fig. I. The temperature was (298 ± 2)K. ZnTe (99.998%) and ZnSe (99.99%) were purchased from Zinsser, Frankfurt. ZnS (99.99%) was delivered by Cerac, Milwaukee. The chemical shifts of 33S and 67Zn in the solids are referred to liquid samples: A 4.0 molal solution of Cs2SO 4 in D20 for 33S and a 2.5 molal solution of ZnSO 4 in H20 for 67Zn. The chemical shifts 6 are given by:
Experimental In a magnetic field of 2.114 T, which was externally stabilized by a IH-stahilization unit, the Larmor frequencies of the spin-3/2-nucleus 33S and the spin-5/ 2 -nu cl eus 67Zn are near 6.91M}{z resp. 5.63 MHz. The, recep%ivities are 1.7.10 -5 for 33S and 1.2"10 -4 for 67Zn compared with I for the proton for an equal number of naturally abundant hydrogen and sulfur or zinc atoms. The NMR signals were observed with a multi-nuclei pulse NMR spectrometer Bruker SXP 4-100. The free induction decays following
Fi 5. I: 67Zn NMR signal of ZnTe powder near 5 . 6 3 1 M H z with a linewidth of 56 Hz. Experimental spectrum width: 20 kHz, number of pulses: 11350, measuring time: 15 h 46 min, 1000 data points were accumulated, followed by 15384 ~oints of zerofilling before Fou~'ier transformation.
6 = (~sample - Vreference)/~reference" The absolute shielding constant of the ZnSO 4 reference sample in the atomic scale is: q*(ZnSO 4 in H~O) = - 690 (10).10 -6 (see ref. (6)).
Results The chemical shifts and linewidths of the 67Zn signals in ZnS, ZnSe, and ZnTe and also of 33S in ZnS are given in table I. 1051
I052
Table
No.
I0
I: Results of the 67Zn and 33S NMR measurements
Linewidth in Hz
Shielding Constant q*. lO-U
Nucleus
Sample
Chemical Shift in ppm
67Zn
ZnSO 4 2.5 M in H20
0
67Zn
ZnS
67Zn
ZnSe
67Zn
ZnTe
33S
Cs2SO 4 4.0 M in D20
0
2o
33S
ZnS
- 562 ± I
53
690 ± 10
82
378 ± 2
1068 ± 10
52
273 ± 2
963 ± 10
5O
85 • 3
775 ± 10
The linewidths are very small for both uclei in the solid powder samples. For 7Zn even the signal in the reference solution is broader (6). Zn-chalcogenides are dilute nuclear spin systems, therefore the magnetic dipole-dipole interaction is small. Further, all three compounds are cubic, this results in a very weak electric field gradient at the position of the quadrupolar nuclei 67Zn and 33S. If the solid state linewidths are as small as shown in table I, it might be possible to narrow the signals by an usual sample spinning with about 50 Hz. The rotation around an axis perpendicular to the magnetic field resulted in a narrowing of the 67 Zn signal in ZnS from 52 Hz to 39 Hz, showing, that the linewidth is at least partly due to an anisotropic interaction. The contribution of the inhomogeneity of the magnetic field has been estimated to less equal 6 Hz by a separate 2H measurement. A careful inspection of the lineshape in the case of ZnTe showed no hint on a dipoledipole interaction with 125Te as has been found recently in other llb-chalcogenides (1,3,4). A possible influence has been studied theoretically by a computer program. It could be shown that the lineshape is only a little bit changed. Because it is not known, whether a Gaussian or a Lorentzian lineshape occurs for 67Zn in ZnTe, the rather small influence of a direct dipole-dipole interaction between 67Zn and 125Te could not be established. The longitudinal relaxation times, estimated by pulse sequence variation, are some seconds as mentioned in ref. (12). Due to the narrow lines and the good signal-to-noise ratios up to 40 : I within I h measuring time, the chemical shifts can be given very accurately. They are also given in table I. In the case of 67Zn, the shielding constants of the solids are given in the atomic scale. Further, the shielding constants for all known 67Zn NMR signals are reported in fig. 2. The amount of the shielding constants is about the same for the chalcogenides as for aqueous ionic solutions. The two at the first sight different results of 67Zn in liquid metallic zinc are identical if one takes into account the different reference samples used (8,9). Further, the shielding of the diamagnetic reference is not at all negligible in comparison with the Knightshift. For a comparison with theoretical results,
~
Vol. 33,
NUCLEAR MAGNETIC SHIELDING IN ZINC CHALCOGENIDES
57
6"/10-6 Shielding Constant
-
-
67Zn at 5.63MHz
Free Atom
Aqueous Soluhons -
Solids
500 ~
-1000
-
Z
n
Zn(ClO&}2 CI2
ZnTe ZnSe
0 1 2 3 4 5 Moles Salt kg Solution
ZnS
-1500
-2000
-2500 LiZn -3000
-3500
-4000 Zn Metal
hquid
Fi~. 2: Atomic reference scale of the nuclear magnetic shielding of 67Zn. Data from this work and of Refs. (6,8,9,11) were used. the Knight-shift referred to the free atom is a more reliable basis. The amount of the shielding constants of 67Zn in Zn-chalcogenides decreases with increasing atomic number of the chalcogenes (see fig. 3). The same behaviour was found in the case of 113Cd in Cd-chalcogen~des (2) with the exception of Cd0, where the amount of the shielding constant is smaller than for Cdg. Therefore the shielding constant of ZnO would be rather interesting. But unfortunately, in the ZnO powder of Wurtzite type no signal was detected. For 33S a signal in ZnS has been observed.
Vol. 33, No.
10
N U C L E A R M A G N E T I C SHIELDING IN ZINC C H A L C O G E N I D E S
|053
¢,i061Shielding Constant , 20 (3
4,0
S
60
Se
Te
Z
Atomic Number
-400
-800
J e"
/
ZincChalcogenides
•
/
-1200
/ -1600
"~
J
•/
Cadmium Chatcogenides
Fig. 3: Dependence of the absolute nuclear magnetic shielding of 67Zn and 113Cd (Ref.2) in chalcogenides as a function of the atomic number. The lines connecting the experimental points are given for clearness and have no theoretical meaning. A signal-to-noise ratio of 14 : I has been achieved within 13 h. This is one of the weakest NMR signals of a heteronucleus observed so far in solid state by a direct measurement. Unfortunately, no atomic shielding scale is available at the moment for 33S. These examples show, that the observation of very weak NMR signals of quadrupolar hetero-
nuclei is possible in favourable cases and that certain applications seem to be feasible. Acknowledgement We thank Prof. H. KrUger for his support of this work and the Deutsche Forsehungsgemeinschaft for their financial support.
References (I) (2) (3) (4) (5) (6) (7) (8) (9) 10) 11) 12) 13) 14)
Koch, W., Lutz, 0., and Nolle, A., Z. Physik A 28__9, 17 (1978) Nolle, A., Z. Naturforsch. 33__aa,666 (1978) Nolle, A., Z. Physik B 3_~4, 175 (1979) Balz, R., Haller, M., Hertler, W.E., Lutz, 0., Nolle, A., and Schafitel, R., J. Magn. Res., in press Fuller, G.H., J. Phys. Chem. Ref. Data 5, 835 (1976) Epperlein, B.W., Krdger, H., Lutz, 0., and Schwenk A., Z. Naturforsch. 29a, 1553 (1974) Maciel, G.E., Simeral, L., and Ackermann, J.J.H., J. Phys. Chem. 8_!, 263 (1977) Kerlin, A.L., and McNeil, J.A., Sol. State Comm. 27, 757 (1978) Bucklisch, R.,and Ploumbidis, D., Phys. Rev. B17, 4160 (1978) Herberg, H., Abaft, J., and Voigtl&nder, J., Z. Naturforsch. 34a, 1029 (1979) Bennet, L.H., Phys. Rev. 150, 418 (1966) Hayes, C.E., and Pound, R.V., Bull. Amer. Phys. Soc. I_~7,330 (1972) Retcofsky,H.L., and Friedel, R.A., J. Amer. Chem. Soc. 94, 6579 (1972) Lutz, 0., Nolle, A., and Schwenk, A., Z. Naturforsch. 28___a, 1370 (1973)