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Magnetic behavior of copper diethyldithiocarbamate at low temperatures: A reinvestigation
15 March 1976’
CHEMICAL PHYSICS LETTERS
Volume 38, number 3
MAGNETIC BEHAVIOR OF COPPER DIETHYLDITHIOCARBAMATE
AT- LOW TEMPEUTURES:
A REiNVESTIGATION
A.J. VAN DDYNEVELDT, J.A. VAN SANTEN and RL. CARLIN* Kamerii~~il Onnes Ln~or~~or~urnof the University of Leyden, Leyden, Tke NetherLrnds Received 20 November 1975
The title compound, Cu&CNEt2)2, behaves at low temperatures (I-20 K) as a normal spin-?/2 motecule, with @ = 2.06 and the Curie-Weiss 6 =+a.25 K. This result contradicts an earher investigation that fed to the suggestion that the crystaliographically-occurring dimers are coupled ferromagnetically.
1. Introdnction The compound bis(N,N-diethyldithiocarbamato)copper( Cu(Sq CNEt&, has been investigated recentIy [ 1,2]. It has a binuclear structure, and a magnetic susceptibility, measured between 4.2 and 56 K, that was interpreted in terms of ferromagneticallycoupled dimers. That is, the experimental results were fitted to the we&known, modified BleaneyBowers equation 131, for a mole of interacting ions, x0 = [A!g2&3k(T--Ql
EI +fexp(-_2rjkll”)]-‘,
(1)
positive exchange constant, J. The. parameters reported as fitting the data are [from eq. (1 j], lg) = 2.041,2J = +24.0 cm -l (i.e., u/,% = 34.5 K), and Ii = -1.37 K. The resulting susceptibility is plotted as the dashed line in fig. 1. The negative Curie-Weiss constant was interpreted in terms of an interdimer antiferromagnetic interaction. At temperatures low with respect to U/k, eq. (I) leads to the Curie-Weiss law
with a
x = Cf(T-0)
(2)
for a system of N;/2 dimers with S = 1, so C = Ng2&3k.
(31
Fig_ 1. Reciprocal static susceptibility. x$, versus temperature. The closed symbols refer to the measurements on the
raw material. the open ones were obtained on a sample recrystallized from chloroform. --: xo = 0.397/U0.25). - - -: from Villa and Hatfield t 11.
romagnetic interaction in a dimer, led to this reinvestigation of the magnetic susceptibility of Cu(S2CNEQ2 at low temperatures as a function of magnetic field. It will be shown that Cu(S.$NEt& in fact does not exhibit any significant magnetic exchange interaction above 1 K.
An iriterest correspondence to this author at the Department
2. Exprimenfal
of Chemistry, University of Llhnois at Chicago Circle, ChiCagO, Winois 60680. USA.
ti(S$NEt-&
ulas prepared
as reported
El]_ No
585
: v&me
38;‘numbcr 3 -.. -. __.-.
.-
1..&ffeier&
in magnetic behavior was observed between ’ : the yaw mate&I and 4 sa~~&Ierecrystalfied from chldroform. .. ac differential-s&eptibilities, x(w). were studied . with a bridge m&hod’[4] _At zero external magnetic : ‘field and sufficientli lo+ frequencies, thjs technique gives the static susceptibility, x0, in arbitrary units. Our results are compared to the data reported above using a well-known paramagnet (manganese z.~mmoni&n Tutton salt) as a calibrant. The measurements were extended to strong magnetic fields (HITIm = SO kC?e). A study of x(o) tit these fields not only leads IS] to the pararntignetic spin--lattice relaxation times T, but also determines the isothermal susceptibility xT(o 4 7-1) and the adiabatic susceptibility X&W 3- 7-l). The effect of saturation on xT was studied and yields an independent check of the CurieWeiss f? and the effective spin, S. The adiabatic susceptibility can be related to the high temperature approximation of the magnetic specific heat [6j _ EPR measurements were performed as described previously
p13. Resdts and discussion In our first attempt to verify the possible occurrence of magnetically-coupled pairs in Cu(S,CNEt& we compared the zero field susceptibility of this compound with that of CuCs&XI&* 6 H,O. This copper Tutton salt is well-known to behave as a simple S = lf2 paramagnet and it was observed that Cu(S$NEti)2 behaves similarly. The actual susceptibilities x0, now using manganese Tutton salt as the refesence, are plotted in fig. 1, the inset showing an enfargement of the measurements below 4.2 K. The two samples mentioned above, in powdered form, were studied at liquid helium temperatures, but no significant difference was observed. It may be seen from fig. I that our measurements between I.2 and 20 K are weti described with a straight Iine. Applyivg the- Curie-Weiss law, eq. (2), to the data, the parameters for Cu&CNEt & are C 7 (0.397 +0.004) emu K/m& and 0. = (O-25 * O-OS)K. This value of the Curie con&+ feads to bo, = 2.06 + 0.01 if the system is assumed to cons&t of N ions with S = l/2, while eq. (3), if if were applicable here, would _resuft _ in the rather unusual value of l-78 k 0.01.
1; hfarch 1976
CHEMICAL FHYSfCS LETTERS
,
r
f
I
I
Fig. 2. Isothermal susceptibility as a function of external magnetic field at T = 2 K. The lines are calculated according to ref. 181, - - - : S = l/2,0 = 0 K:--I S = l/2,0 = 0.25 K;
---:S=l,e=OK;---rS=1,3=0.25K.Thepointsare experimental.
The magnetization of a paramagnetis known to reach a constant, saturation value at sufficiently strong external magnetic fields. The isothermat susceptibility, xr = @hf/iH),, will thus become zero a: these fields. It is interesting to notice that the way xT approaches zero is determined by the effective spin, S, and the Curie-Weiss constant, 6. The exact shape of the xT
versus H curves can be found by differentiation with respect to H of the BriUouin function describing the magnetization ES]:.Four such curves are plotted in fig. 2, all withg = 2.06 and T = 2 K, and with 0 being 0 and 0.25 K for each of S = l/2 and S = I. (In this regard, it should be observed that the universal curves of magnetization versus H/T which were presented earlier [2] for Cu(S,CXEt 2)~ cannot be drawn when B is non-zero.) Together with these calculated lines are shown the isothermal susceptibilities as determined from the x versus w plots at various fields. An immediate conclusion from this figure must be that the description with S = 1 is impossible, 3s such an effective spin value wouId Iead to much smaller xT values. The line with S = 112 and B = 0.25 K describes the measurements very well; in fact, the accuracy in determining 8 in t&is way is about *O-03 K. So, measuring XT yiefded an ~depend~nt. co~~~at~n of the spin and 8 values reported above from the zero Geld susceptibility measurements. The isothermai susceptibilities were all observed at ‘frequencies w 4 T- 1. This condition was verified by
Volume
38. cumber
3
CHEMICAL PHYSICS LETTERS
determining x over a large range-of frequencies, [9]. From such measurements the spin-lattice relaxation times, r, can al& be obtained. It is interesting to mention that the relaxation times of Cu(SS2CNEt2), at liquid helium temperatures clearly showed the direct relaxation process occurring at magnetic fields above 10 kOe. This relaxation process can be characterized by 3 time constant that usually follows IlO]: 7-l =AT#,
(4)
with A % 1.b X lOA s-l K-1 kOeA for isolated copper ions [9] _ The observed time constants can indeed be described by eq. (4) and the given value of A. Since a
spin-l system provides an If2 dependence in eq. (4) and a different A value, this is another confirmation of the fact that Cu(S&NEtd2 behaves as a spin-!/2 system. Another interesting result obtained from ac susceptibility measurements is &d, which is studied if the frequency of the oscillating field meets w > T-!. The adiabatic susceptibility as a function of external magnetic field is dirtictly related to the magnetic specific heat, chI, as can be seen from applying simple thermodynamics to a magnetic system 161. Our measurements reveal an internal field, (b/C)1/2, of 7.1 kOe, which corresponds to cM Tz/R = 0.24 K2 down to 1.2 K. This result may encourage a further analysis of the magnetic specific heat at lower temperatures. An independent check of the above reportedg value was obtained from the EPR study at 9.6 GHz. In the literature g values were reported for copper in similar compounds [J 1,I 21. We studied a single crystal of Cu(S2CNEt2)2, using a spectrometer that enables measurements in any direction of H with respect to the crystal axes [7]. The resonance fields were determined at T = 20.4 K in about 30 arbitrary directions, and the effective g values were then used to fiid the direction of the three principal axes of the g tensor (accuracy 1”). Along these axesg was found to be: g1 = 2.037,gz = 2.046 and g3 = 2.059, the accuracy being 1 in the last number. These g values are very similar to the v&es reported for the magneticaUy di-
15 March 1976
luted ‘compou$s. The results su%est cp>= 2.057 for a randomly oriented powder, which is a nice confirmation of the above g-value as dbtained from the measurements with S = l/2. Returning to fig. 1, the line that describes the susceptibility measurements of Villa and Hatfield [I], using eq. (1) is also illustrated. A considerable difference with our present results exists. These authors [ 1] concluded, on the basis of their data, that there is an intrapair ferromagnetic interaction bf some 30 K. Such an interaction would indeed be apparent at 20 K and below. Our measurements show that magnetic exchange, and particularly a ferromagnetic coupling in the dimers, is not important in this temperature interval. The positive Curie-Weiss constant; in contradiction to the earlier report, suggests that a ferromagnetic interaction occurs below 1 K, as frequently happens with other copper compounds.
References [I]
J.F.
Villa and W.E. Hatfield, Inorg. Chem. 10 (1971) 2038. [ 21 K.T. McGregor, D-J. Hodgson and W-E. Hatfield, Inorg. Chcm. 12 (1973) 731. [3] G. Kokoszka and G. Gordon, Transition Metal Chem. 5 (1969) 181. [4] A.J. de Vries and J.W.M. Livius, Appl. Sci. Res. 17 (1967) 31. (51 H.B.G. Casimir and F.K. Du Pr6, Physica S (1938) 507. [6] C.J. Gorter, Pammagnetic relaxation (Elsevier, Amsterdam, 1947). [ 71 N.J. Zimmerman,Thesis, Leydcn (1974); NJ. Zimmerman, F.P. van der hlark and J. van den lfandel, Physica 46 (1970) 204; J.A. van Santen, A.J. van Duyneveldt and R.L. Carlin, Physica 79B (1975) 91. [8] J. van den Broek, Thesis, Leyden (1960). [9] J. Soeteman, A.J. van Duyneveldt, C.L.M. Pouw and W. Brew, Physica 66 (1973) 63. [lo] R. Orbach, Proc. Roy. Sot. A264 (1961) 458. [II] A.K. Gregson and S. hfitra, J. Chem. Phys. 49 (1968) 3696. 1121 M.J. Weeks and J-P. Fackler, Inorg. Chem. 7 (1968) 2548.
587
CHEMICAL PHYSICS LETTERS
Volume 38, number 3
MAGNETIC BEHAVIOR OF COPPER DIETHYLDITHIOCARBAMATE
AT- LOW TEMPEUTURES:
A REiNVESTIGATION
A.J. VAN DDYNEVELDT, J.A. VAN SANTEN and RL. CARLIN* Kamerii~~il Onnes Ln~or~~or~urnof the University of Leyden, Leyden, Tke NetherLrnds Received 20 November 1975
The title compound, Cu&CNEt2)2, behaves at low temperatures (I-20 K) as a normal spin-?/2 motecule, with @ = 2.06 and the Curie-Weiss 6 =+a.25 K. This result contradicts an earher investigation that fed to the suggestion that the crystaliographically-occurring dimers are coupled ferromagnetically.
1. Introdnction The compound bis(N,N-diethyldithiocarbamato)copper( Cu(Sq CNEt&, has been investigated recentIy [ 1,2]. It has a binuclear structure, and a magnetic susceptibility, measured between 4.2 and 56 K, that was interpreted in terms of ferromagneticallycoupled dimers. That is, the experimental results were fitted to the we&known, modified BleaneyBowers equation 131, for a mole of interacting ions, x0 = [A!g2&3k(T--Ql
EI +fexp(-_2rjkll”)]-‘,
(1)
positive exchange constant, J. The. parameters reported as fitting the data are [from eq. (1 j], lg) = 2.041,2J = +24.0 cm -l (i.e., u/,% = 34.5 K), and Ii = -1.37 K. The resulting susceptibility is plotted as the dashed line in fig. 1. The negative Curie-Weiss constant was interpreted in terms of an interdimer antiferromagnetic interaction. At temperatures low with respect to U/k, eq. (I) leads to the Curie-Weiss law
with a
x = Cf(T-0)
(2)
for a system of N;/2 dimers with S = 1, so C = Ng2&3k.
(31
Fig_ 1. Reciprocal static susceptibility. x$, versus temperature. The closed symbols refer to the measurements on the
raw material. the open ones were obtained on a sample recrystallized from chloroform. --: xo = 0.397/U0.25). - - -: from Villa and Hatfield t 11.
romagnetic interaction in a dimer, led to this reinvestigation of the magnetic susceptibility of Cu(S2CNEQ2 at low temperatures as a function of magnetic field. It will be shown that Cu(S.$NEt& in fact does not exhibit any significant magnetic exchange interaction above 1 K.
An iriterest correspondence to this author at the Department
2. Exprimenfal
of Chemistry, University of Llhnois at Chicago Circle, ChiCagO, Winois 60680. USA.
ti(S$NEt-&
ulas prepared
as reported
El]_ No
585
: v&me
38;‘numbcr 3 -.. -. __.-.
.-
1..&ffeier&
in magnetic behavior was observed between ’ : the yaw mate&I and 4 sa~~&Ierecrystalfied from chldroform. .. ac differential-s&eptibilities, x(w). were studied . with a bridge m&hod’[4] _At zero external magnetic : ‘field and sufficientli lo+ frequencies, thjs technique gives the static susceptibility, x0, in arbitrary units. Our results are compared to the data reported above using a well-known paramagnet (manganese z.~mmoni&n Tutton salt) as a calibrant. The measurements were extended to strong magnetic fields (HITIm = SO kC?e). A study of x(o) tit these fields not only leads IS] to the pararntignetic spin--lattice relaxation times T, but also determines the isothermal susceptibility xT(o 4 7-1) and the adiabatic susceptibility X&W 3- 7-l). The effect of saturation on xT was studied and yields an independent check of the CurieWeiss f? and the effective spin, S. The adiabatic susceptibility can be related to the high temperature approximation of the magnetic specific heat [6j _ EPR measurements were performed as described previously
p13. Resdts and discussion In our first attempt to verify the possible occurrence of magnetically-coupled pairs in Cu(S,CNEt& we compared the zero field susceptibility of this compound with that of CuCs&XI&* 6 H,O. This copper Tutton salt is well-known to behave as a simple S = lf2 paramagnet and it was observed that Cu(S$NEti)2 behaves similarly. The actual susceptibilities x0, now using manganese Tutton salt as the refesence, are plotted in fig. 1, the inset showing an enfargement of the measurements below 4.2 K. The two samples mentioned above, in powdered form, were studied at liquid helium temperatures, but no significant difference was observed. It may be seen from fig. I that our measurements between I.2 and 20 K are weti described with a straight Iine. Applyivg the- Curie-Weiss law, eq. (2), to the data, the parameters for Cu&CNEt & are C 7 (0.397 +0.004) emu K/m& and 0. = (O-25 * O-OS)K. This value of the Curie con&+ feads to bo, = 2.06 + 0.01 if the system is assumed to cons&t of N ions with S = l/2, while eq. (3), if if were applicable here, would _resuft _ in the rather unusual value of l-78 k 0.01.
1; hfarch 1976
CHEMICAL FHYSfCS LETTERS
,
r
f
I
I
Fig. 2. Isothermal susceptibility as a function of external magnetic field at T = 2 K. The lines are calculated according to ref. 181, - - - : S = l/2,0 = 0 K:--I S = l/2,0 = 0.25 K;
---:S=l,e=OK;---rS=1,3=0.25K.Thepointsare experimental.
The magnetization of a paramagnetis known to reach a constant, saturation value at sufficiently strong external magnetic fields. The isothermat susceptibility, xr = @hf/iH),, will thus become zero a: these fields. It is interesting to notice that the way xT approaches zero is determined by the effective spin, S, and the Curie-Weiss constant, 6. The exact shape of the xT
versus H curves can be found by differentiation with respect to H of the BriUouin function describing the magnetization ES]:.Four such curves are plotted in fig. 2, all withg = 2.06 and T = 2 K, and with 0 being 0 and 0.25 K for each of S = l/2 and S = I. (In this regard, it should be observed that the universal curves of magnetization versus H/T which were presented earlier [2] for Cu(S,CXEt 2)~ cannot be drawn when B is non-zero.) Together with these calculated lines are shown the isothermal susceptibilities as determined from the x versus w plots at various fields. An immediate conclusion from this figure must be that the description with S = 1 is impossible, 3s such an effective spin value wouId Iead to much smaller xT values. The line with S = 112 and B = 0.25 K describes the measurements very well; in fact, the accuracy in determining 8 in t&is way is about *O-03 K. So, measuring XT yiefded an ~depend~nt. co~~~at~n of the spin and 8 values reported above from the zero Geld susceptibility measurements. The isothermai susceptibilities were all observed at ‘frequencies w 4 T- 1. This condition was verified by
Volume
38. cumber
3
CHEMICAL PHYSICS LETTERS
determining x over a large range-of frequencies, [9]. From such measurements the spin-lattice relaxation times, r, can al& be obtained. It is interesting to mention that the relaxation times of Cu(SS2CNEt2), at liquid helium temperatures clearly showed the direct relaxation process occurring at magnetic fields above 10 kOe. This relaxation process can be characterized by 3 time constant that usually follows IlO]: 7-l =AT#,
(4)
with A % 1.b X lOA s-l K-1 kOeA for isolated copper ions [9] _ The observed time constants can indeed be described by eq. (4) and the given value of A. Since a
spin-l system provides an If2 dependence in eq. (4) and a different A value, this is another confirmation of the fact that Cu(S&NEtd2 behaves as a spin-!/2 system. Another interesting result obtained from ac susceptibility measurements is &d, which is studied if the frequency of the oscillating field meets w > T-!. The adiabatic susceptibility as a function of external magnetic field is dirtictly related to the magnetic specific heat, chI, as can be seen from applying simple thermodynamics to a magnetic system 161. Our measurements reveal an internal field, (b/C)1/2, of 7.1 kOe, which corresponds to cM Tz/R = 0.24 K2 down to 1.2 K. This result may encourage a further analysis of the magnetic specific heat at lower temperatures. An independent check of the above reportedg value was obtained from the EPR study at 9.6 GHz. In the literature g values were reported for copper in similar compounds [J 1,I 21. We studied a single crystal of Cu(S2CNEt2)2, using a spectrometer that enables measurements in any direction of H with respect to the crystal axes [7]. The resonance fields were determined at T = 20.4 K in about 30 arbitrary directions, and the effective g values were then used to fiid the direction of the three principal axes of the g tensor (accuracy 1”). Along these axesg was found to be: g1 = 2.037,gz = 2.046 and g3 = 2.059, the accuracy being 1 in the last number. These g values are very similar to the v&es reported for the magneticaUy di-
15 March 1976
luted ‘compou$s. The results su%est cp>= 2.057 for a randomly oriented powder, which is a nice confirmation of the above g-value as dbtained from the measurements with S = l/2. Returning to fig. 1, the line that describes the susceptibility measurements of Villa and Hatfield [I], using eq. (1) is also illustrated. A considerable difference with our present results exists. These authors [ 1] concluded, on the basis of their data, that there is an intrapair ferromagnetic interaction bf some 30 K. Such an interaction would indeed be apparent at 20 K and below. Our measurements show that magnetic exchange, and particularly a ferromagnetic coupling in the dimers, is not important in this temperature interval. The positive Curie-Weiss constant; in contradiction to the earlier report, suggests that a ferromagnetic interaction occurs below 1 K, as frequently happens with other copper compounds.
References [I]
J.F.
Villa and W.E. Hatfield, Inorg. Chem. 10 (1971) 2038. [ 21 K.T. McGregor, D-J. Hodgson and W-E. Hatfield, Inorg. Chcm. 12 (1973) 731. [3] G. Kokoszka and G. Gordon, Transition Metal Chem. 5 (1969) 181. [4] A.J. de Vries and J.W.M. Livius, Appl. Sci. Res. 17 (1967) 31. (51 H.B.G. Casimir and F.K. Du Pr6, Physica S (1938) 507. [6] C.J. Gorter, Pammagnetic relaxation (Elsevier, Amsterdam, 1947). [ 71 N.J. Zimmerman,Thesis, Leydcn (1974); NJ. Zimmerman, F.P. van der hlark and J. van den lfandel, Physica 46 (1970) 204; J.A. van Santen, A.J. van Duyneveldt and R.L. Carlin, Physica 79B (1975) 91. [8] J. van den Broek, Thesis, Leyden (1960). [9] J. Soeteman, A.J. van Duyneveldt, C.L.M. Pouw and W. Brew, Physica 66 (1973) 63. [lo] R. Orbach, Proc. Roy. Sot. A264 (1961) 458. [II] A.K. Gregson and S. hfitra, J. Chem. Phys. 49 (1968) 3696. 1121 M.J. Weeks and J-P. Fackler, Inorg. Chem. 7 (1968) 2548.
587