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Charge transfer in the layered compound TaS2 intercalated with NH3 studied by perturbed angular correlations
Volume 67A, number 1
PHYSICS LETTERS
10 July 1978
CHARGE TRANSFER IN THE LAYERED COMPOUND laS2 INTERCALATED WITH NH3 STUDIED BY PERTURBED ANGULAR CORRELATIONS 1 T. BUTZ and H. SAITOVITCH Physik-Department, Technische Universität Manchen, D-8046 Garching, Germany A. LERF Zentralinstitut fi1r Tieftemperaturforachungder Bayerischen Akademieder Wissenschaften, D-8046 Garching, Germany
and H.-D. ZAGEFKA and R. SCHöLLHORN Anorganisch-chemischesInstitut der Universität Münster, D.4400 Münster, Germany Received 29 March 1977
A comparison of tantalum quadrupolar split hyperfine spectra of 2H-TaS 2 with TaS2(NH3) and TaS2(NH3)j/3(H20)213 yields convincing evidence for the ionic model of these compounds. We thus consider these compounds as polyelectrolytes: (NH~)u3(NHah,3[TaS2J_h/3 and (NH)l/3 (H20)2/3 [TaS2] 1j3.
The layered compound TaS2 intercalated with NH3 has been widely studied in the past [1—8]and it is generally accepted that charge transfer from the intercalate the metal layer However, there is sometocontroversy abouttakes the place. mechanism of this charge transfer. Acrivos et al. [3] propose a model of ammonia lone pair orbitals overlapping with the metal conduction electron wave functions. Gamble and Silbernagel [5] showed that the lone pair orbital of NH3 is oriented parallel to the planes and thus propose a weak covalent interaction between the lone pair orbital and the sulfide layers. Schollhorn and Zagefka [8] recently presented evidence for an ionic model with NH~ions (about 10%) solvated by neutral NH3. The protons required for the NH~formation are supplied by partial oxydation of NH3 to N2 on intercalation. An equivalent amount of electrons furnished by this reaction is taken up by the dichalcogenide layers. In order to study the mechanism of charge transfer in these compounds we measured the nuclear quadru1
On leave from CBPF, Rio de Janeiro; supported by CNPq, Brasil and KFA, Jiilich, Germany.
74
pole interaction at Ta in 2H-TaS2, TaS2(NH3), and TaS2(NH3 )~,i3(H2 0)213 by time differential perturbed angular correlations (TDPAC) on j3-decay the 133—482 keY 181Ta which is fed via from ‘8111f. cascade in The samples were prepared from the elements by iodine vapour transport and were doped with about 200 ppm neutron irradiated Hf. 2H-TaS 2 was then intercalated with NH3 at —70°Cunder exclusion of 02 and H20 and kept in a sealed ampoule under NH3 atmosphere. The partially hydrated ammonia intercalate forms when TaS2 (NH3) reacts with water [7]. The room temperature TDPAC spectra of TaS2(NH3) and TaS2(NH3)113(H20)213 are compared with the spectrum of 2H-TaS2 in fig. 1. The latter exhibits a unique frequency with negligible damping; the spectrum for laS2 (NH3) consists of two closely spaced frequency components with unequal intensity and the spectrum for TaS2(NH3)113(H20)213 shows one component with a small frequency distribution (which leads to damping). quadrupole frequencies PQ = 2qQ/h, the relativeThe widths of the frequency distribu. etions (assumed lorentzian) 2cr/w (the precession frequency ~ is related to PQ by ~ 37rPQI1O), and the
Volume 67A, number 1
PHYSICS LETTERS
2H-TaS
being multiplied by appropriate Sternheimer antishielding factors. Wethe assumeR = 0.1 andTa—Ta take (1dis~ 5~[10]. Since the intralayer = 62 for tance andTa the la—S bonding angle does not change upon intercalation [11], qv~is expected to remain constant. Similarly, ql~t~ will not change for an increase in the interlayer distance [12]. The effects re-
2
0
TaS
sponsible for i~qare the following: (i) the local con-
2 (NH3)
: 0
—
2 orbitals duction electron density increases if charge is transferred and yields e.g. for filling up of d5 ~n, where 1~ndenotes the number of3> electrons= +25.6 transferred per TaS 2 molecule, assuming (r = 50 A—3 for Ta4~ [13]; (ii) all distant conduction
(~J
____________________________
TOS 2(NH&i,3(H20)213
I
;~r~fVW\M
-15
-2C0
3C)
70
TIME Insi
Fig. 1. TDPAC spectra for the nuclear quadrupole interaction at Ta in 2H-TaS2, TaS2(NH3), and TaS2(NH3)l,3(H20)2/3 at 300 K. The solid lines represent least squares fitted functions as described in the text.
deduced electric field gradients q are compiled in table 1. We note, that q changes upon intercalation for 3 (prominent comTaS2(NH3) by L~q = —2.08(3) A— ponent) and for TaS 2(NH3)1~3(H20)2~3 by ~q = —2.47~3)A~. The electric field gradient can be written as [9]: q = qlatt(l ‘i~,)+ q~(1 R) + qce(l R) + q~(l—7,~), where qlatt’ q~,~ and q1 denote the contributions from the Ta and S ions, the valence and local conduction electrons, and the intercalate, if present, each —
10 July 1978
—
electrons reduce the effective Ta charge and yield for a narrow conduction band I~q= —9.7~n;(iii) the intercalate itself, if charged as in the ionic model, contributes directly L~q= —5.9 ~n, assuming a rapid proton exchange frequency ~ 1 o~ sl)to with equal Item population (i) is too (jump of all and large interstitial of opposite sites. sign account for the observed /~q.Hence, we expect strong mixing of different d-orbitals. Probably all the above effects play a role. Although we cannot calculate the charge transfer ~ from ~q unambiguously, a comparison with plasma frequency data for pyridine intercalates [14] with our hyperfme spectroscopic data [15,16] yields Lin ~ 0.3 for Ta52(NH3). We note, that i~nis very ~imi1arfor TaS2(NH3)1,3(H20)2~3,small differences in ~1qprobably being minor of the lattice parameters [7].due Thetovalue of variations An ~ 0.3 is strongly supported by the fact, that we find exactly the same ~q for hydratedTa52(NH3) as for fully hydrated TaS2(Li)1~3and laS2(Na)113, for which the ionic nature and the transfer of one electron per alcali atom is well established [17]. Thus the replacement of 2/3 of NH3 by H20 does not affect the amount of charge transferred nor the transfer mechanism. This
Table 1 Quadrupole frequencies, relative widths of the frequency distributions, and deduced electric field gradients for the spectra of fig. 1 as derived from least squares fits. 3) a) Compound VQ [MHz] 2a/~(%) q (A 2H-TaS 2 872.1 (1.3) 0 —9.903 (15) TaS2(NH3) 800.4 (1.4) 27(6)% 10.8 (4.4) —9.089 (16) 689.0 (1.4) 73% 1.0 (6) —7.824 (16) TaS2(NH3)i,~(H2O)2i~ 654.5 (1.2) 2.4 (3) —7.432 (14) a) The negative sign is taken from ref. [18].
75
Volume 67A, number 1
PHYSICS LETTERS
disagrees with the overlap model which predicts a lower charge transfer for the hydrated system. On the other hand, this feature is easily understandable in terms of the ionic model of Schollhorn and Zagefka [8] which assumes NH~solvated in neutral NH3 or H20. Our value for ~n, however, is three times larger than the value quoted by the above authors. The reason for this discrepancy is not yet entirely clear. However, problems connected with the particle size and homogeneity, as suggested by the two-component spectrum for laS2(NH3), could account for this difference, especially in view of the rather different reaction ternperature (25°C)in the former studies [8]. The ionic model is further supported by a pronounced anomalous temperature dependence of q which we found for laS2(NH~)and laS~(pyridine),,, 1, [16]. Theseanomahes seem to be explainable by direct contributions to q from charged intercalates only, contributions from the dipole moments being negligible small. Among other features, we find, e.g., inhomogenous static line broadening at sufficiently low ternperatures, especially for TaS2(NH3). We thus believe that we observe the dynamics of the charged intercalate in these polyelectrolyte systems via Ta quadrupole hyperfine spectroscopy. The ionic model offers a reasonable explanation for the expenmental observation that only a limited amount of NH3 and H20, respectively, can be removed at room temperature by pumping [1,2]: for reasons of charge balance solvate molecules only may leave the interlayer space reversibly, whereas the catiomc species canbe replaced without decomposition by cation reactions only [17]. Finally, we suggest to call the intercalation process an electron transfer reaction rather than to speak of charge transfer complexes.
Prof. J.Y. Acrivos are gratefully acknowledged. We thank the staff of the Forschunsgreaktor Karlsruhe FR2 for neutron irrathations of Hf. This work was supported by the Bundesministerium für Forschung und Technologie. References [1] M.B. Dines and R.B. Levy, J. Phys. Chem. 79 (1975) 1979. [2] J.V. Acrivos, C. Delios, N.Y. Topsøe and J.R. Salem, J. Ph’s. Chem. 79 (1975) 3003. [3] J.V. Acrivos, S.F. Meyer and T~H.Gebaile, in: Electrons in fluids, eds. J. Jortner and N.R. Koster (Springer, New
York, 1973) p. 341; J.V. Acrivos and J.R. Salem, Phil. Mag. 30 (1974) 603. [41B.G. Silbernagel and F.R. Gamble, Phys. Rev. Lett. 32 (1974) 1436. [5] F.R. Gamble (1975) 2544 and B.G. Silbernagel, J. Chem. Phys. 63 [61B.G. Silbernagel, M.B. Dines, F.R. Gamble, L.A. Gebhard, and M.S. Whittingham, J. Chem. Phys. 65 (1976) 1906. [7] M.S. Whittingham, Mat. Res. Bul. 9 (1974) 1681. [8] R. Schdilhorn and H.-D. Zagefka, Angew. Chem. mt. Ed. [9] 193. (1957) p. 321. [10] F.D. Feiock and W.R. Johnson, Phys. Rev. 187 (1969) 39. [11] R.R. Chianelli, J.C. Scanlon, M.S. Whittingham and F.R. Gamble, Inorg. Chem. 14 (1975) 1691. [12] E. Ehrenfreund, A.C. Gossaxd and F.R. Gamble, Phys. Rev. B5 (1972)1708. [13] A.J. Freeman, private communication. [14] A.R. Beal and W.Y. Liang, Phil. Mag. 27 (1973) 1397; J. Phys. C6 (1973) L482. [151T. A. Vasquez,Interactions H. Saitovitch, G.M. Kalvius A. Butz, Lerf, Hyperfine 4 (1978) 798. and [161T. Butz, A. Lerf, A. Vasquez and H. Saitovitch, 15th Ann. Solid state physics Conf. (Warwick, England, 1978). [17] A. Lerf and R. Schollhorn, Inorg.Chem. 16 (1977) 2950. [181L. Pfeiffer, M. EibschUtz and D. Salomon, Hyperfine Interactions 4 (1978) 803.
The continous interest and support of this work by Prof. G.M. Kalvius and stimulating discussions with
76
10 July 1978
PHYSICS LETTERS
10 July 1978
CHARGE TRANSFER IN THE LAYERED COMPOUND laS2 INTERCALATED WITH NH3 STUDIED BY PERTURBED ANGULAR CORRELATIONS 1 T. BUTZ and H. SAITOVITCH Physik-Department, Technische Universität Manchen, D-8046 Garching, Germany A. LERF Zentralinstitut fi1r Tieftemperaturforachungder Bayerischen Akademieder Wissenschaften, D-8046 Garching, Germany
and H.-D. ZAGEFKA and R. SCHöLLHORN Anorganisch-chemischesInstitut der Universität Münster, D.4400 Münster, Germany Received 29 March 1977
A comparison of tantalum quadrupolar split hyperfine spectra of 2H-TaS 2 with TaS2(NH3) and TaS2(NH3)j/3(H20)213 yields convincing evidence for the ionic model of these compounds. We thus consider these compounds as polyelectrolytes: (NH~)u3(NHah,3[TaS2J_h/3 and (NH)l/3 (H20)2/3 [TaS2] 1j3.
The layered compound TaS2 intercalated with NH3 has been widely studied in the past [1—8]and it is generally accepted that charge transfer from the intercalate the metal layer However, there is sometocontroversy abouttakes the place. mechanism of this charge transfer. Acrivos et al. [3] propose a model of ammonia lone pair orbitals overlapping with the metal conduction electron wave functions. Gamble and Silbernagel [5] showed that the lone pair orbital of NH3 is oriented parallel to the planes and thus propose a weak covalent interaction between the lone pair orbital and the sulfide layers. Schollhorn and Zagefka [8] recently presented evidence for an ionic model with NH~ions (about 10%) solvated by neutral NH3. The protons required for the NH~formation are supplied by partial oxydation of NH3 to N2 on intercalation. An equivalent amount of electrons furnished by this reaction is taken up by the dichalcogenide layers. In order to study the mechanism of charge transfer in these compounds we measured the nuclear quadru1
On leave from CBPF, Rio de Janeiro; supported by CNPq, Brasil and KFA, Jiilich, Germany.
74
pole interaction at Ta in 2H-TaS2, TaS2(NH3), and TaS2(NH3 )~,i3(H2 0)213 by time differential perturbed angular correlations (TDPAC) on j3-decay the 133—482 keY 181Ta which is fed via from ‘8111f. cascade in The samples were prepared from the elements by iodine vapour transport and were doped with about 200 ppm neutron irradiated Hf. 2H-TaS 2 was then intercalated with NH3 at —70°Cunder exclusion of 02 and H20 and kept in a sealed ampoule under NH3 atmosphere. The partially hydrated ammonia intercalate forms when TaS2 (NH3) reacts with water [7]. The room temperature TDPAC spectra of TaS2(NH3) and TaS2(NH3)113(H20)213 are compared with the spectrum of 2H-TaS2 in fig. 1. The latter exhibits a unique frequency with negligible damping; the spectrum for laS2 (NH3) consists of two closely spaced frequency components with unequal intensity and the spectrum for TaS2(NH3)113(H20)213 shows one component with a small frequency distribution (which leads to damping). quadrupole frequencies PQ = 2qQ/h, the relativeThe widths of the frequency distribu. etions (assumed lorentzian) 2cr/w (the precession frequency ~ is related to PQ by ~ 37rPQI1O), and the
Volume 67A, number 1
PHYSICS LETTERS
2H-TaS
being multiplied by appropriate Sternheimer antishielding factors. Wethe assumeR = 0.1 andTa—Ta take (1dis~ 5~[10]. Since the intralayer = 62 for tance andTa the la—S bonding angle does not change upon intercalation [11], qv~is expected to remain constant. Similarly, ql~t~ will not change for an increase in the interlayer distance [12]. The effects re-
2
0
TaS
sponsible for i~qare the following: (i) the local con-
2 (NH3)
: 0
—
2 orbitals duction electron density increases if charge is transferred and yields e.g. for filling up of d5 ~n, where 1~ndenotes the number of3> electrons= +25.6 transferred per TaS 2 molecule, assuming (r = 50 A—3 for Ta4~ [13]; (ii) all distant conduction
(~J
____________________________
TOS 2(NH&i,3(H20)213
I
;~r~fVW\M
-15
-2C0
3C)
70
TIME Insi
Fig. 1. TDPAC spectra for the nuclear quadrupole interaction at Ta in 2H-TaS2, TaS2(NH3), and TaS2(NH3)l,3(H20)2/3 at 300 K. The solid lines represent least squares fitted functions as described in the text.
deduced electric field gradients q are compiled in table 1. We note, that q changes upon intercalation for 3 (prominent comTaS2(NH3) by L~q = —2.08(3) A— ponent) and for TaS 2(NH3)1~3(H20)2~3 by ~q = —2.47~3)A~. The electric field gradient can be written as [9]: q = qlatt(l ‘i~,)+ q~(1 R) + qce(l R) + q~(l—7,~), where qlatt’ q~,~ and q1 denote the contributions from the Ta and S ions, the valence and local conduction electrons, and the intercalate, if present, each —
10 July 1978
—
electrons reduce the effective Ta charge and yield for a narrow conduction band I~q= —9.7~n;(iii) the intercalate itself, if charged as in the ionic model, contributes directly L~q= —5.9 ~n, assuming a rapid proton exchange frequency ~ 1 o~ sl)to with equal Item population (i) is too (jump of all and large interstitial of opposite sites. sign account for the observed /~q.Hence, we expect strong mixing of different d-orbitals. Probably all the above effects play a role. Although we cannot calculate the charge transfer ~ from ~q unambiguously, a comparison with plasma frequency data for pyridine intercalates [14] with our hyperfme spectroscopic data [15,16] yields Lin ~ 0.3 for Ta52(NH3). We note, that i~nis very ~imi1arfor TaS2(NH3)1,3(H20)2~3,small differences in ~1qprobably being minor of the lattice parameters [7].due Thetovalue of variations An ~ 0.3 is strongly supported by the fact, that we find exactly the same ~q for hydratedTa52(NH3) as for fully hydrated TaS2(Li)1~3and laS2(Na)113, for which the ionic nature and the transfer of one electron per alcali atom is well established [17]. Thus the replacement of 2/3 of NH3 by H20 does not affect the amount of charge transferred nor the transfer mechanism. This
Table 1 Quadrupole frequencies, relative widths of the frequency distributions, and deduced electric field gradients for the spectra of fig. 1 as derived from least squares fits. 3) a) Compound VQ [MHz] 2a/~(%) q (A 2H-TaS 2 872.1 (1.3) 0 —9.903 (15) TaS2(NH3) 800.4 (1.4) 27(6)% 10.8 (4.4) —9.089 (16) 689.0 (1.4) 73% 1.0 (6) —7.824 (16) TaS2(NH3)i,~(H2O)2i~ 654.5 (1.2) 2.4 (3) —7.432 (14) a) The negative sign is taken from ref. [18].
75
Volume 67A, number 1
PHYSICS LETTERS
disagrees with the overlap model which predicts a lower charge transfer for the hydrated system. On the other hand, this feature is easily understandable in terms of the ionic model of Schollhorn and Zagefka [8] which assumes NH~solvated in neutral NH3 or H20. Our value for ~n, however, is three times larger than the value quoted by the above authors. The reason for this discrepancy is not yet entirely clear. However, problems connected with the particle size and homogeneity, as suggested by the two-component spectrum for laS2(NH3), could account for this difference, especially in view of the rather different reaction ternperature (25°C)in the former studies [8]. The ionic model is further supported by a pronounced anomalous temperature dependence of q which we found for laS2(NH~)and laS~(pyridine),,, 1, [16]. Theseanomahes seem to be explainable by direct contributions to q from charged intercalates only, contributions from the dipole moments being negligible small. Among other features, we find, e.g., inhomogenous static line broadening at sufficiently low ternperatures, especially for TaS2(NH3). We thus believe that we observe the dynamics of the charged intercalate in these polyelectrolyte systems via Ta quadrupole hyperfine spectroscopy. The ionic model offers a reasonable explanation for the expenmental observation that only a limited amount of NH3 and H20, respectively, can be removed at room temperature by pumping [1,2]: for reasons of charge balance solvate molecules only may leave the interlayer space reversibly, whereas the catiomc species canbe replaced without decomposition by cation reactions only [17]. Finally, we suggest to call the intercalation process an electron transfer reaction rather than to speak of charge transfer complexes.
Prof. J.Y. Acrivos are gratefully acknowledged. We thank the staff of the Forschunsgreaktor Karlsruhe FR2 for neutron irrathations of Hf. This work was supported by the Bundesministerium für Forschung und Technologie. References [1] M.B. Dines and R.B. Levy, J. Phys. Chem. 79 (1975) 1979. [2] J.V. Acrivos, C. Delios, N.Y. Topsøe and J.R. Salem, J. Ph’s. Chem. 79 (1975) 3003. [3] J.V. Acrivos, S.F. Meyer and T~H.Gebaile, in: Electrons in fluids, eds. J. Jortner and N.R. Koster (Springer, New
York, 1973) p. 341; J.V. Acrivos and J.R. Salem, Phil. Mag. 30 (1974) 603. [41B.G. Silbernagel and F.R. Gamble, Phys. Rev. Lett. 32 (1974) 1436. [5] F.R. Gamble (1975) 2544 and B.G. Silbernagel, J. Chem. Phys. 63 [61B.G. Silbernagel, M.B. Dines, F.R. Gamble, L.A. Gebhard, and M.S. Whittingham, J. Chem. Phys. 65 (1976) 1906. [7] M.S. Whittingham, Mat. Res. Bul. 9 (1974) 1681. [8] R. Schdilhorn and H.-D. Zagefka, Angew. Chem. mt. Ed. [9] 193. (1957) p. 321. [10] F.D. Feiock and W.R. Johnson, Phys. Rev. 187 (1969) 39. [11] R.R. Chianelli, J.C. Scanlon, M.S. Whittingham and F.R. Gamble, Inorg. Chem. 14 (1975) 1691. [12] E. Ehrenfreund, A.C. Gossaxd and F.R. Gamble, Phys. Rev. B5 (1972)1708. [13] A.J. Freeman, private communication. [14] A.R. Beal and W.Y. Liang, Phil. Mag. 27 (1973) 1397; J. Phys. C6 (1973) L482. [151T. A. Vasquez,Interactions H. Saitovitch, G.M. Kalvius A. Butz, Lerf, Hyperfine 4 (1978) 798. and [161T. Butz, A. Lerf, A. Vasquez and H. Saitovitch, 15th Ann. Solid state physics Conf. (Warwick, England, 1978). [17] A. Lerf and R. Schollhorn, Inorg.Chem. 16 (1977) 2950. [181L. Pfeiffer, M. EibschUtz and D. Salomon, Hyperfine Interactions 4 (1978) 803.
The continous interest and support of this work by Prof. G.M. Kalvius and stimulating discussions with
76
10 July 1978