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The effect of high pressure on the nuclear quadrupole interaction at tantalum in 1T-TaS2 and 2H-TaS2
Volume 65A, number 2
PHYSICS LETFERS
20 February 1978
THE EFFECT OF HIGH PRESSURE ON THE NUCLEAR QUADRUPOLE INTERACTION AT TANTALUM IN IT-TaS2 AND 2H-TaS2 2 T. BUTZ, A. VASQUEZ’ H. SAITOVITCH Physik-Department, Technischeand Universitât Munchen, 8046 Garching, Germany and A. LERF Zentralinstitut fi~rTieftemperarurforschung der BayerischenAkademie der Wissenschaften, 8046 Garching, Germany Received 21 November 1977
The effect of high pressure on the nuclear quadrupole interaction at Ta in TaS 3/kbar for iT-laS 2 was found 3/kbax to befor very 2H-TaS small: (d In q/ dP)T373K = +2.1(9) x 10 2 and (din q/dP)~,,3~Q~ = +1.2(6) X 10 2 in qualitative agreement with lattice sum estimates.
Recently, charge density wave (CDW) effects have been studied in tantalum dichalcogenides by hyperfine spectroscopic methods [1—3] Particularly large differences of the order of 15% in the electric field gradient (EFG) experienced by Ta nuclei at inequivalent lattice sites were found in the commensurate CDW state of 1T-TaS2 [1] The aim of the present high pressure study is to investigate the sensitivity of the EFG to changes of the intralayer Ta—Ta distance of the Ta—S bonding angle. We have studied the nuclear quadrupole interaction at Ta in 1T-TaS2 at 373 K (incommensurate CDW) and in 2H-TaS2 at 300 K (no CDW) as a function of hydrostatic pressure up to 6 kbar using a conventional oil press equipped with a Bourdon type gauge. The measurements were performed by time differential perturbed angular correlations on the 133 keV 482 181Hf. The samples prepared from the elements keV cascade m Tawere which is fed via 13-decay from by iodine vapour transport and were doped with about
j 3.0 25
.
1TTOs~ at 373 K
j 2H-TaS2 at 300 K
2.0
.
—
~
1.5
‘~
to
~,
0.5 0 -0.5 -1.0
~.
__________
_____________
~ ~PR~SS~RE° Uarl 6 Fig. 1. Relative change of quadrupole frequency versus pressure
for
1T-TaS 2 at 373 K and for 2H-TaS2 at 300 K.
200 ppm neutron irradiated Hf. 1
On leave from address: UFRS, Porto Alegre, supported by CNPq, Brasil. Present Carnegie Mellon Umversity, Schenley Paxk, Pittsburgh, PA 15213, USA.
2
On leave from CBPF, Rio de Janeiro, supported by CNPq, Brasil and by KFA, Jülich, Germany,
2qQ/h with pressure for both poiyFig. 1 shows frequency v(2 = the e relative change of the quadrupole types of TaS Apparently, VQ and hence the EFG changes very2.little with pressure. The straight lines in fig. I represent fitted slopes of magnitude: (d In 159
Volume 65A, number 2
PHYSICS LETTERS
20 February 1978
iT
~ and d~2 ~2 character as has been calculated for NbSe2 [6] and hence qce is not expected to depend
2H(o)
strongly on c/a. Contrarily, qval depends critically on the bonding angle and vanishes for ideal octahedral close packing. Consequently, the rather weak pressure
-
30
20
1T 2H (a)
-
~o.
~ Z
w
adependence sum of q~a~t ofand the qce~ EFGqva~ implies playing thata minor qtotai isrole. primarily If qvai would contribute significantly, we would expect
2H(b)
0
‘~
\.~1
0
2H (b)
•
CHALCOGEN
METAL
a sharp decrease of the EFG with pressure, opposite to our observations. Since q~tt(l y~,)is-calculated—
to be as negative as q~otalbut larger in magnitude we assume the same sign for TaS2 as for TaSe2 [31
0 -J
—
20
qce is expected to be of comparable magnitude to q~a~t (1 y,~)but of opposite sign. A positive sign of —
~ -30
—
C-)
LiJ
qee is consistent with the assumption of ~ and dx2 ..~2conduction electron wave functions.
W -40 I
0.8
1.0
I
1.2
1.4 1.6 1.8 2.0 AXIAL RATIO c/a (iT), C/a: 2 (2H)
Fig. 2. The lattice electric field gradient versus c/a for iT, 211(a), and 2H(b) structures with Z = +5 for the metal and Z = —2 for the chalcogen ions.
3/kbar for 1T-TaS dP)T373 K = +2.1(9) X 10 and 3/kbar for2 2H(d ln VQ/dP)T=300K = + 1.2(6) X 10 Ta5 2. These results are surprising in view of the fact that the compressibility of Ta52 is large and above all rather anisotropic [4]. The total EFG consists of several contributions: qtotal (1 7=)qlatt + (1 —R)q~~~ + (1 R)q~~. Here, qlatt, qya~’and qce denote the contributions from all neighbouring ions, from valence and conduction electrons, respectively, each being appropriately shielded by Sternheimer antishielding factors, y, and R [5] We assume one electron in the conduction band, the other four metal electrons form spd-hybrids with the sulfur electrons. In fig. 2 ~ is plotted versus the axial ratio c/a assuming Z = +5 for Ta and Z = —2 for S. For TaS2, the lattice sums are identical for the 2H- and the iT-modification (compare MoS2: 2H(b)). Since c/a = 1.755 (iT) and c/a = 1.822 (2H), qktt is seen to depend very weakly on either c/a or the Ta—S bonding angle. Lattice sums, using compressibilities for 1T-TaS2 at 300 K a~3/kbar = 2.27(23)X iO~/ [4], prekbar a1 a,~= 1.04(11) X 103/kbar in qualitative dict dand in q~~ 1/dP = +0.2(5) X 10 The conduction agreement with our observations. electron wave functions are assumed to be mainly of —
—
.
—
160
In conclusion, the interpretation of the differences of EFG’s at inequivalent lattice sites in the commensurate CDW state of Ta-dichalcogenides on the basis of different Ta—S bonding angles [7] appears to be in conflict with the present data. We rather believe that the observed hyperfine spectra essentially image the distortions in the metal plane. The continuous interest and support of this work by Prof. G.M. Kalvius and valuable discussionf with Prof. A.J. Freeman are gratefully acknowledged. This work was supported by the Bundesministerium für Forschung und Technologie. References [1] T. Butz, A. Vasquez, H. Ernst and A. Lerf, Phys. Lett. 58A (1976) 51. [21T. Vasquez, H.Conf. Saitovitch, G.M. Kalvius and A. Butz, Lerf, A. lVth Internat. on Hyperfine interactions (Madison, NJ, 1977), to be published in Hyperfine Interactions. [3] L. Pfeiffer, M. EibschOtz and D. Salomon, IVth Internat. Conf. on Hyperfine interactions (Madison, NJ, 1977) to be published in Hyperfine Interactions. [4] For 1T-TaS2: D. Schifert, private communication; for others: e.g. J.L. Feldman, to be published. [5] M .H. Cohen and F. Reif, Solid state physics, vol. 5, eds. F. Sejtz and D. Turnbull (Academic Press, New York, 1957). [6] C.Y. Fong and M.L. Cohen, Phys. Rev. Lett. 32 (1974) 720. [7] M. Eibschütz and F.J. di Salvo, Phys. Rev. B15 (1977) 5181.
PHYSICS LETFERS
20 February 1978
THE EFFECT OF HIGH PRESSURE ON THE NUCLEAR QUADRUPOLE INTERACTION AT TANTALUM IN IT-TaS2 AND 2H-TaS2 2 T. BUTZ, A. VASQUEZ’ H. SAITOVITCH Physik-Department, Technischeand Universitât Munchen, 8046 Garching, Germany and A. LERF Zentralinstitut fi~rTieftemperarurforschung der BayerischenAkademie der Wissenschaften, 8046 Garching, Germany Received 21 November 1977
The effect of high pressure on the nuclear quadrupole interaction at Ta in TaS 3/kbar for iT-laS 2 was found 3/kbax to befor very 2H-TaS small: (d In q/ dP)T373K = +2.1(9) x 10 2 and (din q/dP)~,,3~Q~ = +1.2(6) X 10 2 in qualitative agreement with lattice sum estimates.
Recently, charge density wave (CDW) effects have been studied in tantalum dichalcogenides by hyperfine spectroscopic methods [1—3] Particularly large differences of the order of 15% in the electric field gradient (EFG) experienced by Ta nuclei at inequivalent lattice sites were found in the commensurate CDW state of 1T-TaS2 [1] The aim of the present high pressure study is to investigate the sensitivity of the EFG to changes of the intralayer Ta—Ta distance of the Ta—S bonding angle. We have studied the nuclear quadrupole interaction at Ta in 1T-TaS2 at 373 K (incommensurate CDW) and in 2H-TaS2 at 300 K (no CDW) as a function of hydrostatic pressure up to 6 kbar using a conventional oil press equipped with a Bourdon type gauge. The measurements were performed by time differential perturbed angular correlations on the 133 keV 482 181Hf. The samples prepared from the elements keV cascade m Tawere which is fed via 13-decay from by iodine vapour transport and were doped with about
j 3.0 25
.
1TTOs~ at 373 K
j 2H-TaS2 at 300 K
2.0
.
—
~
1.5
‘~
to
~,
0.5 0 -0.5 -1.0
~.
__________
_____________
~ ~PR~SS~RE° Uarl 6 Fig. 1. Relative change of quadrupole frequency versus pressure
for
1T-TaS 2 at 373 K and for 2H-TaS2 at 300 K.
200 ppm neutron irradiated Hf. 1
On leave from address: UFRS, Porto Alegre, supported by CNPq, Brasil. Present Carnegie Mellon Umversity, Schenley Paxk, Pittsburgh, PA 15213, USA.
2
On leave from CBPF, Rio de Janeiro, supported by CNPq, Brasil and by KFA, Jülich, Germany,
2qQ/h with pressure for both poiyFig. 1 shows frequency v(2 = the e relative change of the quadrupole types of TaS Apparently, VQ and hence the EFG changes very2.little with pressure. The straight lines in fig. I represent fitted slopes of magnitude: (d In 159
Volume 65A, number 2
PHYSICS LETTERS
20 February 1978
iT
~ and d~2 ~2 character as has been calculated for NbSe2 [6] and hence qce is not expected to depend
2H(o)
strongly on c/a. Contrarily, qval depends critically on the bonding angle and vanishes for ideal octahedral close packing. Consequently, the rather weak pressure
-
30
20
1T 2H (a)
-
~o.
~ Z
w
adependence sum of q~a~t ofand the qce~ EFGqva~ implies playing thata minor qtotai isrole. primarily If qvai would contribute significantly, we would expect
2H(b)
0
‘~
\.~1
0
2H (b)
•
CHALCOGEN
METAL
a sharp decrease of the EFG with pressure, opposite to our observations. Since q~tt(l y~,)is-calculated—
to be as negative as q~otalbut larger in magnitude we assume the same sign for TaS2 as for TaSe2 [31
0 -J
—
20
qce is expected to be of comparable magnitude to q~a~t (1 y,~)but of opposite sign. A positive sign of —
~ -30
—
C-)
LiJ
qee is consistent with the assumption of ~ and dx2 ..~2conduction electron wave functions.
W -40 I
0.8
1.0
I
1.2
1.4 1.6 1.8 2.0 AXIAL RATIO c/a (iT), C/a: 2 (2H)
Fig. 2. The lattice electric field gradient versus c/a for iT, 211(a), and 2H(b) structures with Z = +5 for the metal and Z = —2 for the chalcogen ions.
3/kbar for 1T-TaS dP)T373 K = +2.1(9) X 10 and 3/kbar for2 2H(d ln VQ/dP)T=300K = + 1.2(6) X 10 Ta5 2. These results are surprising in view of the fact that the compressibility of Ta52 is large and above all rather anisotropic [4]. The total EFG consists of several contributions: qtotal (1 7=)qlatt + (1 —R)q~~~ + (1 R)q~~. Here, qlatt, qya~’and qce denote the contributions from all neighbouring ions, from valence and conduction electrons, respectively, each being appropriately shielded by Sternheimer antishielding factors, y, and R [5] We assume one electron in the conduction band, the other four metal electrons form spd-hybrids with the sulfur electrons. In fig. 2 ~ is plotted versus the axial ratio c/a assuming Z = +5 for Ta and Z = —2 for S. For TaS2, the lattice sums are identical for the 2H- and the iT-modification (compare MoS2: 2H(b)). Since c/a = 1.755 (iT) and c/a = 1.822 (2H), qktt is seen to depend very weakly on either c/a or the Ta—S bonding angle. Lattice sums, using compressibilities for 1T-TaS2 at 300 K a~3/kbar = 2.27(23)X iO~/ [4], prekbar a1 a,~= 1.04(11) X 103/kbar in qualitative dict dand in q~~ 1/dP = +0.2(5) X 10 The conduction agreement with our observations. electron wave functions are assumed to be mainly of —
—
.
—
160
In conclusion, the interpretation of the differences of EFG’s at inequivalent lattice sites in the commensurate CDW state of Ta-dichalcogenides on the basis of different Ta—S bonding angles [7] appears to be in conflict with the present data. We rather believe that the observed hyperfine spectra essentially image the distortions in the metal plane. The continuous interest and support of this work by Prof. G.M. Kalvius and valuable discussionf with Prof. A.J. Freeman are gratefully acknowledged. This work was supported by the Bundesministerium für Forschung und Technologie. References [1] T. Butz, A. Vasquez, H. Ernst and A. Lerf, Phys. Lett. 58A (1976) 51. [21T. Vasquez, H.Conf. Saitovitch, G.M. Kalvius and A. Butz, Lerf, A. lVth Internat. on Hyperfine interactions (Madison, NJ, 1977), to be published in Hyperfine Interactions. [3] L. Pfeiffer, M. EibschOtz and D. Salomon, IVth Internat. Conf. on Hyperfine interactions (Madison, NJ, 1977) to be published in Hyperfine Interactions. [4] For 1T-TaS2: D. Schifert, private communication; for others: e.g. J.L. Feldman, to be published. [5] M .H. Cohen and F. Reif, Solid state physics, vol. 5, eds. F. Sejtz and D. Turnbull (Academic Press, New York, 1957). [6] C.Y. Fong and M.L. Cohen, Phys. Rev. Lett. 32 (1974) 720. [7] M. Eibschütz and F.J. di Salvo, Phys. Rev. B15 (1977) 5181.