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Electrical conduction properties of Ni(dpg)2I, Ni(dpg)2Br, and Pd(dpg)2I (where dpg = diphenylglyoxime)
INORG.
NUCL.
CHEM.
LETTERS
ELECTRICAL Ni(dpg)2Br ,
AND
Vol. 9, pp.
CONDUCTION Pd(dPg)2I
1269-1273,
1973. Pergamon
PROPERTIES (WHERE
OF
Press.
Printed in Great Britain.
Ni(dpg)2I ,
dpg = DIPHENYLSLYOXIME)
A.E. Underhill e and D.M. Watkins School of Physical and Molecular Sciences, and R. Pethig School of Electr~)nic and Engineering Science, University College of North Wales, Bangor, Caerns (Received 6 August 1973)
A recent review (1) described thmee distinct types of compound that may contain delocalised
'metallic' electrons in one-dimension and hence may act
as prototypes fop high-temperature
supemconductors.
These ame (a) organic
charge transfer complexes containing TCNQ, (b) non-inteETal oxidation state metal atom chain compounds of the F~roEmann type (2) such as K2Pt(CN)4BPo. 3 2.3H20 , and (c) compounds containing chains of halogen atoms, particularly iodine.
The compounds M(dPg)2X (where M = Ni or Pd;
X = BP or I) could
be unique in that they may possess two of these systenls within the same structume.
Previous womkems (3) have shown that the M(dpg)2X compounds possess tetmagonal structumes in which the square co-planam M(dpg) 2 units f o m columnam stacks with equivalent M-M distances.
The halogen atones are situated
in channels running parallel to the metal chains and ape surrounded by the phenyl groups of the ligands. Pd(dPg)2I contains the 13
Keller and Seibold (4) have r~cently shown that
ion and this indicates an average oxidation state 1269
1270
ELECTRICAL CONDUCTION
Vol. 9, No. 12
ofr2.33 for the Pd, which is similar to the oxidation state of Pt in the Kmogmann type compounds.
This increase in the oxidation state of the metal
is probably responsible for the reduction in the M-M distance in the M(dPg)2X compounds compared with the distance in the unoxidlsed M(dpg) 2 (see Table). The reduction is comparable to that observed for Krogmann type compounds but o the actual distances are ,~ 0.5A larger in the dpg series.
Although the
paramaEnetic properties of these compounds are the subject of controversy, effects have been observed (4) with pure Pd(dPg)2I which suE~est
the presence
of mobile conduction electrons in the solid complex.
Because of the obvious importance of these systems we have examined the electrical conduction properties of these compounds.
Unfortunately because
single crystals of sufficient size could not be obtained and because of the low thermal stability of the compounds all measumements were restricted to compressed pellets of polycrystalline samples at room temperatume.
The results
ape shown in the Table.
The dc conductivities of the M(dPg)2X samples are at least 105 times greater than those observed for the M(dpg) 2 compounds and ape compamable to those observed for compressed pellets of the KmoEmann type compounds measured under similar conditions (5).
The results for Ni(dPg)2I are in aEmeement
with the observations of Foust and Soderberg (3).
The M(dPg)2X compounds,
however, exhibit the following dc characteristics: a) results using silver paste are lower than those using aquadag (colloidal graphite) as the contact material, b) a much wider range of o values were obtained for the same compound using silver paste compared with using aquadag, c) the variation of current with applied voltage is non-ohmic and tends to a satumation value with silver paste and to an I = V x (where x > i.O) relationship using aquadag.
Vol. 9, No. 12
ELECTRICAL CONDUCTION
TABLE
1271
Specific Conductivities at Room-temperature
Specific Conductivity ~-icm-i
M-M
(relative to air)
distance
Ni(dpg) 2
Pd(dpg) 2
Ni(dpg)2I
3.547(a)
3.517(a)
Electrode material
dc
1592 Hz
Ag paste
<10 -12
<10 -8
Ag paste
Pd(dpg)2I
i0 GHz
4.08xlO -5
2.13
1.59x].0- 5
2.18
4.78xi0 -4
3.64
1.09x10 -3
4.16
<10 -8
3.28(b) Aquadag
Ni(dpg)2Br
<10 -12
lO GHz
3.36(b)
3.26(b)
3xlO
Ag paste Aquadag
Ag paste Aquadag
"i0-~ 1.7xl0-
"i0 -7 1.2 xlO- b
2.3xlO
'~10-6 8 .~xlO -b
"lO -6 1.6 xlO 3.23XI0 -3
a.
C.V. Banks and D.W. Barnum, J. Amer. Chem. Soc., 80, 4767 (1958).
h.
ref. 3.
l0.8
These ~esults suggest a combination of electmode effects and a space charge limitation of the cuPment and indicate that the observed dc conductivities may not be a true measure of the bulk conductivity of these materials.
The results
at 1592 Hz are comparable, within experimental ezTor, to those obtained using dc methods but these results were also obtained using silvem paste or aquadag as contact materials.
To overcome the possible effects of the electrodes and
intemcmystalline contact resistances we have measured the conductivities using an electmodeless measurement technique at i0 GHz (8).
The conductivities for
both the parent compounds and the M(dpg)2X series at i0 GHz are considerably greater than the dc and 1592 Hz values and may therefore represent the true bulk conductivities of the compounds.
It is however, also possible that the observed
1272
ELECTRICAL CONDUCTION
Vol. 9, No. 12
conductivities at i0 GHz are influenced by dielectric loss mechanisms.
There is
a large increase in the dielectric constant for the material when halogen is present and this increase is largest for the sample possessing the highest conductivity.
It is evident that the inclusion of halogen in these compounds has resulted in an enormous increase in the do conductivity, and a considerable incmease in the conductivity at l0 GHz, compared with the divalent M(dpg) 2 compounds.
It is
also evident that neither the ac nor the dc conductivity is appreciably dependent on the nature of the metal or halogen.
The conduction pathway could be associated
with the presence of a chain of metal atoms in a non-intes~al oxidation state as proposed for the K~ogmann type compounds.
However the reflectance spectra of
these complexes do not exhibit the strong absorption band, extending fTom the visible to the infrared r~gion, which is chamacteristic of the Kl-ogmann type compounds, although the position of this band may be dependent on the intermetallic distance.
Alternatively the conduction pathway could be associated
with the chains of halogen atoms present in the structure.
Poly-iodide chains
have been shown (7) to be responsible fop the high conductivity in the charge transfer complex formed between iodine and N,N'-diphenyl-p-phenylenediamine QS"= 3.4 x 10-1fl-lcm-1).
These results indicate that this type of compound may possess desirable conduction properties but measurements on single c~ystals ape required before the conduction mechanism can be established and fumther efforts to obtain these are in progress.
Precise crystallographic data for single crystals is also
required to attempt to determine whether distortions of the regulam metal-metal spacings exist due to the pmesence of either a Peierls instability (8) or a Kohn anomaly (9).
These effects result from the instability of the electronic
kinetic energy in a one-dimensional system and the resulting distortions appear to be present in the linear alkali-TCNQ salts (10) and in the linear metal atom chain compound K2Pt(CN)4Br0.3.3H20 (ll).
Vol. 9, No. 12
ELECTRICAL CONDUCTION
1273
Experimental
The compounds were prepamed as previously described (3) and their composition checked by elemental analysis. to prevent decomposition. described (12).
The compounds were stored at -20°C
Dc and 1592 Hz ac measurements were made as previously
iO GHz measumements were made using a microwave resonating
cavity pertuDbation technique described elsewhere (8) and corrections (13) were made to take into account effects arising from the slight cavity field distortions associated with the teflon sample holder. References
i.
E.B. Yagubskii and K.L. Khidekel, Russ. Chem. Rev., 41, i011 (1972).
2.
K. Kr~gmann, Angew. Chem. Internat. Edn., 8, 35 (1969).
3.
A.S. Foust and R.H. Soderberg, J. Amer. Chem. Soc., 89, 5507 (1987).
4.
H.J. Keller and K. Seibold, J. Amer. Chem. Soc., 98, 1310 (1971).
5.
P.S. Gomm and A.E. Underhill, J. Chem. Soc. (Dalton), 334 (1972).
6.
D.D. Eley and R. Pethig, Disc. Faraday Soc., No 51, 184 (1971).
7.
V. Hadek, J. Chem. Phys., 49, 5202 (1968).
8.
P.F. Peierls, Quantum Theory of Solids, p. 108, Clarendon Press, Oxford (1955).
9.
W. Kohn, Phys. Rev. Left., 2, 393 (1959).
lO.
J.G. Vegler and J. Kommandeur, 3rd Int. Symp. Chemistry of the Org. Solid State, Glasgow, Sept. 1972.
ll.
B. Renker, H. Rietschel, L. Pintschovius, W. Glaser, P. Br~esch, D. Kuse and M.J. Rice, Phys. Rev. Left., 30, 1144 (1973).
12.
P.S. Gomm, T.W. Thomas and A.E. Underhill, J. Chem. Soc. (A), 2154 (1971).
13.
T.E. Cross and R. Pethig, to he published.
NUCL.
CHEM.
LETTERS
ELECTRICAL Ni(dpg)2Br ,
AND
Vol. 9, pp.
CONDUCTION Pd(dPg)2I
1269-1273,
1973. Pergamon
PROPERTIES (WHERE
OF
Press.
Printed in Great Britain.
Ni(dpg)2I ,
dpg = DIPHENYLSLYOXIME)
A.E. Underhill e and D.M. Watkins School of Physical and Molecular Sciences, and R. Pethig School of Electr~)nic and Engineering Science, University College of North Wales, Bangor, Caerns (Received 6 August 1973)
A recent review (1) described thmee distinct types of compound that may contain delocalised
'metallic' electrons in one-dimension and hence may act
as prototypes fop high-temperature
supemconductors.
These ame (a) organic
charge transfer complexes containing TCNQ, (b) non-inteETal oxidation state metal atom chain compounds of the F~roEmann type (2) such as K2Pt(CN)4BPo. 3 2.3H20 , and (c) compounds containing chains of halogen atoms, particularly iodine.
The compounds M(dPg)2X (where M = Ni or Pd;
X = BP or I) could
be unique in that they may possess two of these systenls within the same structume.
Previous womkems (3) have shown that the M(dpg)2X compounds possess tetmagonal structumes in which the square co-planam M(dpg) 2 units f o m columnam stacks with equivalent M-M distances.
The halogen atones are situated
in channels running parallel to the metal chains and ape surrounded by the phenyl groups of the ligands. Pd(dPg)2I contains the 13
Keller and Seibold (4) have r~cently shown that
ion and this indicates an average oxidation state 1269
1270
ELECTRICAL CONDUCTION
Vol. 9, No. 12
ofr2.33 for the Pd, which is similar to the oxidation state of Pt in the Kmogmann type compounds.
This increase in the oxidation state of the metal
is probably responsible for the reduction in the M-M distance in the M(dPg)2X compounds compared with the distance in the unoxidlsed M(dpg) 2 (see Table). The reduction is comparable to that observed for Krogmann type compounds but o the actual distances are ,~ 0.5A larger in the dpg series.
Although the
paramaEnetic properties of these compounds are the subject of controversy, effects have been observed (4) with pure Pd(dPg)2I which suE~est
the presence
of mobile conduction electrons in the solid complex.
Because of the obvious importance of these systems we have examined the electrical conduction properties of these compounds.
Unfortunately because
single crystals of sufficient size could not be obtained and because of the low thermal stability of the compounds all measumements were restricted to compressed pellets of polycrystalline samples at room temperatume.
The results
ape shown in the Table.
The dc conductivities of the M(dPg)2X samples are at least 105 times greater than those observed for the M(dpg) 2 compounds and ape compamable to those observed for compressed pellets of the KmoEmann type compounds measured under similar conditions (5).
The results for Ni(dPg)2I are in aEmeement
with the observations of Foust and Soderberg (3).
The M(dPg)2X compounds,
however, exhibit the following dc characteristics: a) results using silver paste are lower than those using aquadag (colloidal graphite) as the contact material, b) a much wider range of o values were obtained for the same compound using silver paste compared with using aquadag, c) the variation of current with applied voltage is non-ohmic and tends to a satumation value with silver paste and to an I = V x (where x > i.O) relationship using aquadag.
Vol. 9, No. 12
ELECTRICAL CONDUCTION
TABLE
1271
Specific Conductivities at Room-temperature
Specific Conductivity ~-icm-i
M-M
(relative to air)
distance
Ni(dpg) 2
Pd(dpg) 2
Ni(dpg)2I
3.547(a)
3.517(a)
Electrode material
dc
1592 Hz
Ag paste
<10 -12
<10 -8
Ag paste
Pd(dpg)2I
i0 GHz
4.08xlO -5
2.13
1.59x].0- 5
2.18
4.78xi0 -4
3.64
1.09x10 -3
4.16
<10 -8
3.28(b) Aquadag
Ni(dpg)2Br
<10 -12
lO GHz
3.36(b)
3.26(b)
3xlO
Ag paste Aquadag
Ag paste Aquadag
"i0-~ 1.7xl0-
"i0 -7 1.2 xlO- b
2.3xlO
'~10-6 8 .~xlO -b
"lO -6 1.6 xlO 3.23XI0 -3
a.
C.V. Banks and D.W. Barnum, J. Amer. Chem. Soc., 80, 4767 (1958).
h.
ref. 3.
l0.8
These ~esults suggest a combination of electmode effects and a space charge limitation of the cuPment and indicate that the observed dc conductivities may not be a true measure of the bulk conductivity of these materials.
The results
at 1592 Hz are comparable, within experimental ezTor, to those obtained using dc methods but these results were also obtained using silvem paste or aquadag as contact materials.
To overcome the possible effects of the electrodes and
intemcmystalline contact resistances we have measured the conductivities using an electmodeless measurement technique at i0 GHz (8).
The conductivities for
both the parent compounds and the M(dpg)2X series at i0 GHz are considerably greater than the dc and 1592 Hz values and may therefore represent the true bulk conductivities of the compounds.
It is however, also possible that the observed
1272
ELECTRICAL CONDUCTION
Vol. 9, No. 12
conductivities at i0 GHz are influenced by dielectric loss mechanisms.
There is
a large increase in the dielectric constant for the material when halogen is present and this increase is largest for the sample possessing the highest conductivity.
It is evident that the inclusion of halogen in these compounds has resulted in an enormous increase in the do conductivity, and a considerable incmease in the conductivity at l0 GHz, compared with the divalent M(dpg) 2 compounds.
It is
also evident that neither the ac nor the dc conductivity is appreciably dependent on the nature of the metal or halogen.
The conduction pathway could be associated
with the presence of a chain of metal atoms in a non-intes~al oxidation state as proposed for the K~ogmann type compounds.
However the reflectance spectra of
these complexes do not exhibit the strong absorption band, extending fTom the visible to the infrared r~gion, which is chamacteristic of the Kl-ogmann type compounds, although the position of this band may be dependent on the intermetallic distance.
Alternatively the conduction pathway could be associated
with the chains of halogen atoms present in the structure.
Poly-iodide chains
have been shown (7) to be responsible fop the high conductivity in the charge transfer complex formed between iodine and N,N'-diphenyl-p-phenylenediamine QS"= 3.4 x 10-1fl-lcm-1).
These results indicate that this type of compound may possess desirable conduction properties but measurements on single c~ystals ape required before the conduction mechanism can be established and fumther efforts to obtain these are in progress.
Precise crystallographic data for single crystals is also
required to attempt to determine whether distortions of the regulam metal-metal spacings exist due to the pmesence of either a Peierls instability (8) or a Kohn anomaly (9).
These effects result from the instability of the electronic
kinetic energy in a one-dimensional system and the resulting distortions appear to be present in the linear alkali-TCNQ salts (10) and in the linear metal atom chain compound K2Pt(CN)4Br0.3.3H20 (ll).
Vol. 9, No. 12
ELECTRICAL CONDUCTION
1273
Experimental
The compounds were prepamed as previously described (3) and their composition checked by elemental analysis. to prevent decomposition. described (12).
The compounds were stored at -20°C
Dc and 1592 Hz ac measurements were made as previously
iO GHz measumements were made using a microwave resonating
cavity pertuDbation technique described elsewhere (8) and corrections (13) were made to take into account effects arising from the slight cavity field distortions associated with the teflon sample holder. References
i.
E.B. Yagubskii and K.L. Khidekel, Russ. Chem. Rev., 41, i011 (1972).
2.
K. Kr~gmann, Angew. Chem. Internat. Edn., 8, 35 (1969).
3.
A.S. Foust and R.H. Soderberg, J. Amer. Chem. Soc., 89, 5507 (1987).
4.
H.J. Keller and K. Seibold, J. Amer. Chem. Soc., 98, 1310 (1971).
5.
P.S. Gomm and A.E. Underhill, J. Chem. Soc. (Dalton), 334 (1972).
6.
D.D. Eley and R. Pethig, Disc. Faraday Soc., No 51, 184 (1971).
7.
V. Hadek, J. Chem. Phys., 49, 5202 (1968).
8.
P.F. Peierls, Quantum Theory of Solids, p. 108, Clarendon Press, Oxford (1955).
9.
W. Kohn, Phys. Rev. Left., 2, 393 (1959).
lO.
J.G. Vegler and J. Kommandeur, 3rd Int. Symp. Chemistry of the Org. Solid State, Glasgow, Sept. 1972.
ll.
B. Renker, H. Rietschel, L. Pintschovius, W. Glaser, P. Br~esch, D. Kuse and M.J. Rice, Phys. Rev. Left., 30, 1144 (1973).
12.
P.S. Gomm, T.W. Thomas and A.E. Underhill, J. Chem. Soc. (A), 2154 (1971).
13.
T.E. Cross and R. Pethig, to he published.