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Charge-transfer-induced change of 181Ta Mössbauer isomer shift in 2H-TaS1 intercalated with hydrated sodium
Volume
119, number
2,3
CHARGE-TRANSFER-INDUCED IN 2H-TaS= 1NTERCALATED
Reccwed
CHEMICAL
30 August 19B5
PHYSlCS LETTERS
CHANGE OF WITH HYDRATED
“‘Ta MeSSBAUER SODIUM
ISOMER
SHIFT
3 June 1955
Mdbsbauer spsan or the 6 2 kcV LKUISIIIO~or lH’Ta intcrcalatron or hydrated sodrum A single sire IS observed
were measured m 11x .sanc sample or 2H-TdS2 before and afler lor Tn II-INn ,,,J(H,O),TaS,. The yuadrupole lnrcraaion drcrcases upon InwrcaldGon. in exact agreement uith TDPAC rrsultr Th e isomer shirl mcrexcs 10 3 higher value. refleaing the 1ou.x valcncy 31 the 7-3 site. A louer limnr or 30 mm/s per e - is deduced ror the Isornsr shirl chnngc due 10 charge mansTcr LO Ta.
The hyperfine invcstlgatlon of mtercalated compounds of 2H-TaS2 using time-differential perturbed angular correlation (TDPAC) of 181Ta, has established a parabolic relation between the quadrupole frequency and the charge transfer e- per TaS2 [l] _ A single frequency value was observed for all the intercalated compounds studied. In particular, the same frequency was measured for the hydlated alkali intercalates Li+, Na+ and K+ for the same stoichiometry- Since at room temperature these intercalates diffuse rapidly enough [2] to average out Ta site incqulvalencles, an mvestigation of 2H-TaSZ intercalated compounds seems feasible in terms of isomer shift and charge uptake at the Ta site We show here that the 6.2 keV M6ssbauer transitlon of 181Ta [3] IS indeed suItable for the investigation of intercalated tantalum dichalcogenides, and not as hopeless an enterprise as one might anticipate, despite the long measuring times We chose hydrated sodium as an intercalate because the lithmm compound has a poor stability [4] and potassium has high photoabsorption at the 6.2 keV gamma energy. Among the Ta dichalcogenides, 2H-TaS2 forms the largest 238
class of intercalation compounds with the greatest stabtity [5 ] _The l*lTa MBssbauer spectrum for Nal/j(H20)2TaS2 reported here is the tist for an intercalated Ta dlchalcogenide. The Mossbauer spectra were obtained in transmission geometry with a conventional sinusoidal electromechanical drive_ The source was l8lW in tungsten [6 ] _The absorber consisted of a single crystal, prepared by the usual vapor-phase transport method [7], of 6 X 5 mm2 in size and carefully cleaved down to a thickness of 5 mg/cm2_ This crystal was electrochemically intercalated under galvanostatic conditions_ The electrolyte was an outgassed aqueous 0.1 M Na2S04 solution kept under argon_ The Na uptake was calculated by Faraday’s law The electrochemical potential, measured versus SCE (saturated calomel electrode), was in perfect agreement with that previously determined [g] and confirmed the calculated uptake. The intercalation was terminated at the uptake of n = 033/TaS2, shortly after the onset of the single-phase region (0.27 =GII G 0.42) [8] _The absorber was then kept “wet” in the electrolyte by sealing it in a thin polyethylene en0 009-2614/85/S (North-Holland
03.30 0 Elsevier Scrence Publishers B-VPhysics Publishing Division)
Volume 119, number 2.3
30 August 1985
CHEMICAL PHYSICS LETTERS I
1
2H -To& at 300
K
at 300K 60
70 VELOCITY
80
90
(mmlsl
Fig. I, Room-temperature Massbaucr spectra of lel Ta in ZH-Ta.!S,before and after intcrcalction of hydrated Na using approaimate& the same geomeW The arrow indicate tk isomer-shift positions.
velope In order to prevent dehydration.
The spectra obtamed before and after intercalation are shown in fig. I_ A dramatic increase in isomer shrft, indicated by arrows, and a decrease of the quadrupole sphtting is clearly visible. Moreover, the shape of the spectrum shows that the electnc field gradient (EFG) IS axially symmetric and its largest component IS perpendicular to the layers. The solid lines in fig. 1 correspond to a Fit of an axrally symmetric EFG with the c axis parallel to the y direction with a fured quad~pole moment ratio Q(9/2)/Q(7~2) = 1.13 15 [9] _The adjusted fit parameters, i-e_ isomer shift, linewidth and quadrupofe splitting, are summarized in table I_ Due to the long measurmg time of up to a month (and more) and the large velocity range used for the mtercalated absorber, a rather close geometry was used leading to additional line broadening. Another mea-
Table 1 Isomer shift (relative to lE1~~)
TaS2 and Na1,3 CHZO)~TZS Compound
a) A~rn~~(7/2)
=+3.28(6)
;f; The 23Na Knight shift in Na,TiSz WY found to bc very small in ref. [ 121; this shows that Na is essentially ionic. While the change from X.52 to TaSz should not change this sltoation, the hydration is expected to guarantee full ionicity in any case.
sou.=& linewidth (hwhm), qoadnrpole interaction, and deduced elect& field gradients for 28
Isomer shift (mm/s)
2H-TaSz Nn,,, W,0)2T~2
surement of the same absorber but with a more distant geometry yieIded identical hyperfine parameters with an improved linewidth (W (half width at half maximum) = 0.23 mm/s). The change of the isomer shift by 9-S mm/s to higher velocities is a direct evidence of charge transfer to the Ta site_ The Na+ is assumed to be completely ionic in Na1i3 (HZ0)2TaSZ *. Furthermore, intrasandWI& volume effects due to intercalation are negligible. Assummg a complete charge transfer to the Ta site Ieads to an isomer shift change of a30 mm/s per e-.
Linewid th Iv (hwhm) (mm/s)
e%Q(7/2) (lo-6 cv)
69.9(2)
O-36(4)
79.7(2)
-5.06 (5)
O-30(4)
--.267
(4)
eq a) (lo** V/cm2) present work
TDPAC result [l]
-1.54(4) -1 12(4)
2 1.53 (2) 2 l-12(2)
b [lOI and Q(5fZ) =2 36(5) b [ll]_
239
Volume
119, number 2.3
CHEMICAL
PHYSICS LEITERS
A direct comparison with isomer shift values of other Ta compounds is problematic. The isomer shifts for Ta compounds studied thus far span a range of 110 mm/s [2] _Shift for chemrcally four-valent Ta lie between 70 and 80 mm/s; for the five-valent Cu,TaXa (X = S, Se, Te) they span a large range of 15 to 56 mm/s ClS] and the five-valent alkali tantalates are at the lower end of the isomer shift range (-7 to -18 mm/s). These formally five-valent compounds are not well suited for comparrson because of the strong ISOmer-shift volume dependence [ 133 _ The only system suitable for comparison is TaH,(O
240
30 August 1985
fmed uptake can be prepared reproducibly This is indeed astonishing considering the Iarge size of the caystat used in the present experiments, and therefore renders future investigations with other intercalates promising_ The continuous interest in and support of this work by Professor K. Andres and Professor G-M. Kalvius is gratefully acknowledged. This work was supported in part by the Bundesministerium fur Forschung und Technologie and the Deutsche Forschungsgemeinschaft.
References [1] T. ButE and A_ Lmf, Rev. Chim. Min. 19 (1982) 496 [2] U. Ruder, W. M~~er-W~u~ and R Schollhorn, J. Chem. Phys 75 (1981) 412. [33 D Salomon. in: Trends in MZissbauer spectroscopy, eds. P- Gutlkh and G M. Kalvius (Mainz Umv. Press, Mainz, 1983). [4] A- Lerf, Thesis, Universitit Miinchen (1976) [S] G-V. Subba Rao and M.W. Shafer, in: Physics and chemistry ofmaterials with layered structores, Vol. 6. Intercalated layered materials, ed. F. Levy (Reidel, Dordrecht, 1979). [6] G. KaiodI, D. Salonron and G. Wortmann. . Phvs. _ Rev. B8 (1973) 1912. 171 H. SchZier, Chemische Traruportreaknonen (Verlag Chcmie, Weiuheim, 1962). [El W. Brberacher. Thesis, TU Nlrnchen (1984). [9] M Elbschiitz. D. SaIomon and FJ. DiSatvo, Phys. Letters 93A (1983) 259. (10) J. Koijn. W- van Desburg, G.T. Ewan, T JohansDn and G. Tibell. Nud. Phys A360 (1981) 187. [ll] T- Butz and A. Lerf, Phys.. Letters 97A (1983) 217_ [ 121 B-G. SrlbemaSeI and M S. Whlttingham. Mat. Res. Bull_ ll(l976) 29. K. Zitter, J. Schmand, K. Waer and P. Sch~i&om. Mat Res. Bull. 19 (1984) 801. A. Heidemaun. G. Kaindi, D. Salomon, H. Wipf and G. Wortmsnn, Phys. Rev. Letters 36 (1976) 213; U. Potzel. R. Saab. 1. Velkl, H. Wipf, G_Wortmann and D. Saloman. J. Less Common Met. 101(1984)343. G. KaindI, D. Salomon and G. Wortmatm. in: MBssbauer isomer shifts, eds. G. Shenoy and F. Wagner (North-Holland, Amsteniam, 1978). G. Alefeld and J. Vijlkl, eds:, To& in applied physic% Vols. IS and 29, Hydrogen in metals I and II (Springer. Berlin, 1978). B-G. Silbemagel, Solid State Commun. 17 (1975) 361_
119, number
2,3
CHARGE-TRANSFER-INDUCED IN 2H-TaS= 1NTERCALATED
Reccwed
CHEMICAL
30 August 19B5
PHYSlCS LETTERS
CHANGE OF WITH HYDRATED
“‘Ta MeSSBAUER SODIUM
ISOMER
SHIFT
3 June 1955
Mdbsbauer spsan or the 6 2 kcV LKUISIIIO~or lH’Ta intcrcalatron or hydrated sodrum A single sire IS observed
were measured m 11x .sanc sample or 2H-TdS2 before and afler lor Tn II-INn ,,,J(H,O),TaS,. The yuadrupole lnrcraaion drcrcases upon InwrcaldGon. in exact agreement uith TDPAC rrsultr Th e isomer shirl mcrexcs 10 3 higher value. refleaing the 1ou.x valcncy 31 the 7-3 site. A louer limnr or 30 mm/s per e - is deduced ror the Isornsr shirl chnngc due 10 charge mansTcr LO Ta.
The hyperfine invcstlgatlon of mtercalated compounds of 2H-TaS2 using time-differential perturbed angular correlation (TDPAC) of 181Ta, has established a parabolic relation between the quadrupole frequency and the charge transfer e- per TaS2 [l] _ A single frequency value was observed for all the intercalated compounds studied. In particular, the same frequency was measured for the hydlated alkali intercalates Li+, Na+ and K+ for the same stoichiometry- Since at room temperature these intercalates diffuse rapidly enough [2] to average out Ta site incqulvalencles, an mvestigation of 2H-TaSZ intercalated compounds seems feasible in terms of isomer shift and charge uptake at the Ta site We show here that the 6.2 keV M6ssbauer transitlon of 181Ta [3] IS indeed suItable for the investigation of intercalated tantalum dichalcogenides, and not as hopeless an enterprise as one might anticipate, despite the long measuring times We chose hydrated sodium as an intercalate because the lithmm compound has a poor stability [4] and potassium has high photoabsorption at the 6.2 keV gamma energy. Among the Ta dichalcogenides, 2H-TaS2 forms the largest 238
class of intercalation compounds with the greatest stabtity [5 ] _The l*lTa MBssbauer spectrum for Nal/j(H20)2TaS2 reported here is the tist for an intercalated Ta dlchalcogenide. The Mossbauer spectra were obtained in transmission geometry with a conventional sinusoidal electromechanical drive_ The source was l8lW in tungsten [6 ] _The absorber consisted of a single crystal, prepared by the usual vapor-phase transport method [7], of 6 X 5 mm2 in size and carefully cleaved down to a thickness of 5 mg/cm2_ This crystal was electrochemically intercalated under galvanostatic conditions_ The electrolyte was an outgassed aqueous 0.1 M Na2S04 solution kept under argon_ The Na uptake was calculated by Faraday’s law The electrochemical potential, measured versus SCE (saturated calomel electrode), was in perfect agreement with that previously determined [g] and confirmed the calculated uptake. The intercalation was terminated at the uptake of n = 033/TaS2, shortly after the onset of the single-phase region (0.27 =GII G 0.42) [8] _The absorber was then kept “wet” in the electrolyte by sealing it in a thin polyethylene en0 009-2614/85/S (North-Holland
03.30 0 Elsevier Scrence Publishers B-VPhysics Publishing Division)
Volume 119, number 2.3
30 August 1985
CHEMICAL PHYSICS LETTERS I
1
2H -To& at 300
K
at 300K 60
70 VELOCITY
80
90
(mmlsl
Fig. I, Room-temperature Massbaucr spectra of lel Ta in ZH-Ta.!S,before and after intcrcalction of hydrated Na using approaimate& the same geomeW The arrow indicate tk isomer-shift positions.
velope In order to prevent dehydration.
The spectra obtamed before and after intercalation are shown in fig. I_ A dramatic increase in isomer shrft, indicated by arrows, and a decrease of the quadrupole sphtting is clearly visible. Moreover, the shape of the spectrum shows that the electnc field gradient (EFG) IS axially symmetric and its largest component IS perpendicular to the layers. The solid lines in fig. 1 correspond to a Fit of an axrally symmetric EFG with the c axis parallel to the y direction with a fured quad~pole moment ratio Q(9/2)/Q(7~2) = 1.13 15 [9] _The adjusted fit parameters, i-e_ isomer shift, linewidth and quadrupofe splitting, are summarized in table I_ Due to the long measurmg time of up to a month (and more) and the large velocity range used for the mtercalated absorber, a rather close geometry was used leading to additional line broadening. Another mea-
Table 1 Isomer shift (relative to lE1~~)
TaS2 and Na1,3 CHZO)~TZS Compound
a) A~rn~~(7/2)
=+3.28(6)
;f; The 23Na Knight shift in Na,TiSz WY found to bc very small in ref. [ 121; this shows that Na is essentially ionic. While the change from X.52 to TaSz should not change this sltoation, the hydration is expected to guarantee full ionicity in any case.
sou.=& linewidth (hwhm), qoadnrpole interaction, and deduced elect& field gradients for 28
Isomer shift (mm/s)
2H-TaSz Nn,,, W,0)2T~2
surement of the same absorber but with a more distant geometry yieIded identical hyperfine parameters with an improved linewidth (W (half width at half maximum) = 0.23 mm/s). The change of the isomer shift by 9-S mm/s to higher velocities is a direct evidence of charge transfer to the Ta site_ The Na+ is assumed to be completely ionic in Na1i3 (HZ0)2TaSZ *. Furthermore, intrasandWI& volume effects due to intercalation are negligible. Assummg a complete charge transfer to the Ta site Ieads to an isomer shift change of a30 mm/s per e-.
Linewid th Iv (hwhm) (mm/s)
e%Q(7/2) (lo-6 cv)
69.9(2)
O-36(4)
79.7(2)
-5.06 (5)
O-30(4)
--.267
(4)
eq a) (lo** V/cm2) present work
TDPAC result [l]
-1.54(4) -1 12(4)
2 1.53 (2) 2 l-12(2)
b [lOI and Q(5fZ) =2 36(5) b [ll]_
239
Volume
119, number 2.3
CHEMICAL
PHYSICS LEITERS
A direct comparison with isomer shift values of other Ta compounds is problematic. The isomer shifts for Ta compounds studied thus far span a range of 110 mm/s [2] _Shift for chemrcally four-valent Ta lie between 70 and 80 mm/s; for the five-valent Cu,TaXa (X = S, Se, Te) they span a large range of 15 to 56 mm/s ClS] and the five-valent alkali tantalates are at the lower end of the isomer shift range (-7 to -18 mm/s). These formally five-valent compounds are not well suited for comparrson because of the strong ISOmer-shift volume dependence [ 133 _ The only system suitable for comparison is TaH,(O
240
30 August 1985
fmed uptake can be prepared reproducibly This is indeed astonishing considering the Iarge size of the caystat used in the present experiments, and therefore renders future investigations with other intercalates promising_ The continuous interest in and support of this work by Professor K. Andres and Professor G-M. Kalvius is gratefully acknowledged. This work was supported in part by the Bundesministerium fur Forschung und Technologie and the Deutsche Forschungsgemeinschaft.
References [1] T. ButE and A_ Lmf, Rev. Chim. Min. 19 (1982) 496 [2] U. Ruder, W. M~~er-W~u~ and R Schollhorn, J. Chem. Phys 75 (1981) 412. [33 D Salomon. in: Trends in MZissbauer spectroscopy, eds. P- Gutlkh and G M. Kalvius (Mainz Umv. Press, Mainz, 1983). [4] A- Lerf, Thesis, Universitit Miinchen (1976) [S] G-V. Subba Rao and M.W. Shafer, in: Physics and chemistry ofmaterials with layered structores, Vol. 6. Intercalated layered materials, ed. F. Levy (Reidel, Dordrecht, 1979). [6] G. KaiodI, D. Salonron and G. Wortmann. . Phvs. _ Rev. B8 (1973) 1912. 171 H. SchZier, Chemische Traruportreaknonen (Verlag Chcmie, Weiuheim, 1962). [El W. Brberacher. Thesis, TU Nlrnchen (1984). [9] M Elbschiitz. D. SaIomon and FJ. DiSatvo, Phys. Letters 93A (1983) 259. (10) J. Koijn. W- van Desburg, G.T. Ewan, T JohansDn and G. Tibell. Nud. Phys A360 (1981) 187. [ll] T- Butz and A. Lerf, Phys.. Letters 97A (1983) 217_ [ 121 B-G. SrlbemaSeI and M S. Whlttingham. Mat. Res. Bull_ ll(l976) 29. K. Zitter, J. Schmand, K. Waer and P. Sch~i&om. Mat Res. Bull. 19 (1984) 801. A. Heidemaun. G. Kaindi, D. Salomon, H. Wipf and G. Wortmsnn, Phys. Rev. Letters 36 (1976) 213; U. Potzel. R. Saab. 1. Velkl, H. Wipf, G_Wortmann and D. Saloman. J. Less Common Met. 101(1984)343. G. KaindI, D. Salomon and G. Wortmatm. in: MBssbauer isomer shifts, eds. G. Shenoy and F. Wagner (North-Holland, Amsteniam, 1978). G. Alefeld and J. Vijlkl, eds:, To& in applied physic% Vols. IS and 29, Hydrogen in metals I and II (Springer. Berlin, 1978). B-G. Silbemagel, Solid State Commun. 17 (1975) 361_