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Electrical conduction studies of a series of tetracyanoplatinum compounds
Solid State Communications, Vol. 29, pp. 557—560. Pergamon Press Ltd. 1979. Printed in Great Britain. ELECTRICAL CONDUCTION STUDIES OF ASERIES OF TETRACYANOPLATINUM COMPOUNDS J.H. O’Neill and A.E. Underhill School of Physical and Molecular Sciences, University College of North Wales, Bangor, Gwynedd, Wales and G.A. Toombs Department ofPhysics, University of Nottingham, University Park, Nottingham NG7 2RD, England (Received 24 October 1978 by R.A. Cowley) The first experimental study of the electrical~conductionpropertiesof a series of tetracyanoplatinum(U) salt single crystals is presented. The results are discussed in conjunction with recent work on the electrical conductivity of KCP. Itis shown that there Is a systematicvariation in the electrical conductivity of the materialswith the intrachainPt—Pt spacing. THE TETRACYANOPLATINUM (II) [TCP(II)]salts of general formulae M2 [Pt(CN)4]nH2O (where Mis a
series [8] We have now made a detailed study of the .
monovalent cation) and N[Pt(CN)4 ] nH~O(where N is a divalent cation) probably represent the most extensive range of integral oxidation state metal atom chain compounds known at present. The structure of many members of this series are known and they have been shown to possess a chain-like structure with the planar ‘t(CN’)4 ]2_ ions stacked one above the other along the c-axis of the crystal [1—3].The Intrachain Pt—Pt distance varies from 3.1 A to over 3.6 A depending on the nature of the cation and also on the degree of hydration. The colours of these compounds also vary dramatically from colourless for those with long Pt—Pt distances to reds and violets for those possessing the shortest Pt—Pt distances [1]. Recently a detailed study of the relationship between the position of the emission spectra bands and the Pt—Pt distance has been made [4]. There is also a series of non-Integral oxidation state platinum atom chain compounds which possess the same basic chain-like structure of planar [Pt(CN)4] units. The archetypical member of this series is 1(2 [Pt(CN)4 J Br0~303H2O[KCP]which has been studied extensively because of its high electrical conductivity [5] at room temperature. The Pt—Pt spacing for compounds in the series has been shown [6] to vary with the degree of partial oxidation. We present first an experimental study of the electrical conduction properties of a series of TCP(ll) salts and then relate the results obtainedto those previously reported for KCP. Electrical conduction studies on the TCP(II) salts have been restricted to a value7 quoted for a single and to studies on crystal of K2 [Pt(CN)4 polycrystalline discs for] xH2O a number of members of this ,
d.c. electrical conduction properties of single crystals of this series of TCP(II) salts. The d.c. conductivity was determined for all the crystals using a 2-probe method. Gold wires were attached to the crystals using colloidal graphite (Aquadag) as the contact material. The results are summarized in Table 1. For certain crystals a 4-probe method was also used and this was found to give a value two to three times greater than the 2-probe vaiue. This increase is smaller than the scatter of results obtained from different crystals of the same material. Seven TCP(II) salts were studied in the course of this investigation. Because these compounds can be obtained with varying degrees of hydration the compounds were carefully analyzed to establish which hydrate was being studied. The electrical conductivity parallel to the metal atom chain direction (au) was determined for all the compounds studied. Itwas found that a~could vary by up to one order of magnitude from crystal to crystal from the same preparation. The spread of results and the number of crystals examined for each compound is shown in Table 1. Whenever possible the activation energy for conduction was determined over the temperature range 100— 270K. However, for the potassium and strontium compounds it was found that the crystals were so brittle that they Invariably broke on cooling thus preventing the measurement of the activation energy. it is noteworthy that those two compounds have the longest Pt—Pt distances of all the compounds studied and therefore might be expected to be more susceptible to fracture perpendicular to the c-axisFor of the lithium crystal. and barium salts the crystals were sufficiently large for the electrodes to be attached
557
558
STUDIES OF ASERIES OF TETRACYANOPLATINUM COMPOUNDS
VoL 29, No.7
Table 1. Intra Pt—Pt distances and d.c. electriazl conductionproperties Intxa-Pt-.Pt distance (R)
Compound
hR (A’)
o~room-temp. ((r’ cnr l)
aj room-temp. (rr’ cnr’)
Activation
Number
energy (H)
crystals
(eV)
examined
100
0.41
10
100
0.47 0.27
2 4 9 1 2
o~:a
1
(A)
Li, [Pt(CN)4 I 3H,0
3.18’
b2E~(~’O4I -3H,0’ Ba[Pt(CN)4].4H20
3.18
0.3144
3 x 10’ —3 X 10~ 3 X 10~’
0.301
4 x 10’—3 x 10~ 8x 10~—5X 10-’
—
2x10’ Ba[Pt(CN)4 I .4H,0’
3.32
(NH4), IPt(CN)41 .2H,O Rb2 [Pt(CN)4].3H,0 MgIPt(CN)4 I 7H,O Sr[Pt(CN)4J.5H,O
3.26°
0.3067
3.40’
0.294
315b
0.3175
3~60b
2.88’
-
K, [Pt(CN)4 I 3H,0 K, [Pt(CN)4 ]Br,~,.311,0 *
10-’
0.26 1.0 0.3 0.32
0.2777
1—8x 10_s 5 X 10’—7 X 10’ 7 X 10-’ —2 X 10’ 4 X 10’—l X 10’
0.2873 0.3472
8 x 10’ -.4 x 10’ 1o2_4 x io2~
2—4
X
5 5
4 3 5
io-’t
io~
Crystals obtained from solutions containing Pt(IV) salts.
‘M.L. Moreau-Colin, BulL Soc. Royale Sciences Liege 34,778(1935). b Reference [2]. C Reference [1]. d Reference [3]. e C. Peters & C.F. Eagen, Phys. Rev. Lett. 34, 1132(1975). ~ H.R. Zeller & A. Beck, I. Chem. Phys. Solids 35,77(1974).
Iog~ cm
perpendicular to the platinum atom chain direction. At room temperature thePtconductivity In the direction perpendicular to the atom chain (aj) was about 102 lower than that found in the Pt atom chain direction. A
+
similar degree of anisotropy has been found in Magnus’s Green salt ([Pt(NH)) 12+ [Pt(~’M J2_, MGS) and other integral oxidation state metal atom chain
2
compounds [9]. Crystals of the lithium and barium compounds were also grown from solutions containing a 20% molar amount of Li2 [Pt(CN)4Cl2]and Ba[Pt(CN)4Cl2] respectively. It has previously been shown that the conductivity in MGS can be enhanced by several orders of magnitude by doping the crystals with an appropriate Pt(IV) compound [101. It can be seen from Table 1 that the lithium compound grown from the Pt(IV) doped solution exhibits a conductivity about one order of magnitude higher than those grown from “undoped” solutions. For the barium compound, crystals grown from “doped” solutions exhibit conductivities similar
-2
d
I
~
I
6 ~
a
i
_I~~
028
0-36
032
1 ~ 1) ~ (AFig. 1. Variation of room temperature conductivity in platinum atom chain direction fGr a series of tetrac 1anoplatinurn (II) salts. (a, Sr [Pt(CN)4151120; b, K2,~t(CN)4] 3H20, c, Rb2 I~t(CN~4~ 3H20, o~ -
-
• KCP under pressure [12]).
71120; 2
‘
“undoped” Increase in conductivity is to the most solutions. conductingThis crystals obtained from small compared with the large differences observed on changing the cation. Scott et ci. [10] found that it was possible to increase the Pt(IV) content and the electrical conductivity of MGS by instantaneous precipitation of the
salt from solutions containing Pt(IV) ions. However, the salmples of MGS formed slowly in gels contained a
VoL 29, No.7
STUDIES OF ASERIES OF TETRACYANOPLATINUM COMPOUNDS
much lower Pt(IV) content (Pt”: Pt” iO~:1) because of a complex redox equilibrium which occurs between Pt” and Pt” species in solution and which reduces the Pt”’ concentration in solution and hence the Pt(IV) doping in the crystals. This equilibrium is not established In the case of Instantaneous precipitation and a non-metastable phase is obtainedwith a high Pt(IV) content. All the [Pt(CN)4]2_ compounds, including those grown from solutions containing a high Pt(IV) content, were obtainedby slow crystallization in which equilibrium conditions would be expected. Thus it is likely that all the compounds contain only a very low Pt(IV) content. This is supported by work on KCP where only one stable phase has been shown to exist for a wide mole ratio of K2 [Pt(CN)4I and K2 [Pt(CN)4Br2] in solution [10]. The results in Table 1 for a11 at room temperature show that the conductivity is strongly dependent on the cation present. In Fig. 1, log all is plotted against hR for all the compounds studied. R is the intrachain Pt—Pt spacing. It can be seen that there is a systematicvariation of a11 with hR. Aband model such as that proposed to explain the variation of emission peaks [4] with R appears most appropriate for considering the electrical conduction properties of these compounds. A filled band resulting from overlap of hybridized (Pt 5d~,6s) orbitals and an empty band from overlap of (Pt 6Pm; CNir’) orbitals are separated by an energy gap which decreases as the Pt—Pt separation decreases. The energy gap is at least 2 eV and this is too large to allow significant carrier formation by promotion of electrons across the gap, even at room temperature. EPR measurements on MGS have shown the existence of a small contribution of bound 5d~holes due to Pt(III) lying 0.6 eV above the Sd~orbitals [11]. The Pt(III) holes occur due to the presence of Pt(IV) complexes during sample preparation and extend over several metal chain lattice sites. It seems likely that in the [Pt(CN)4)J2 series that the electrical conduction properties arise extrinsically due to the presence of Pt(IV) impurities, as in MGS [10]. Electrons will be thermally promoted from the top of the (5d~,6s) band into these localized impurity states and hence carriers (holes) will be generated in the top of the valence band. Therefore we interpret the observed activation energies E as the difference between the Fermi energy and the energy at the top of the valence band. The measured E range from 0.26 to 0.47 eV, excluding the ammonium compound, and we estimate that they are accurate to about ±0.05 eV. The reason for the very much higher activation energy for the ammonium compound is not known. Avariation in E of 0.2 eV, implies a change in the number of carriersby three orders of magnitude at room temperature.
559
However the variation of a11 from compound to compound does not correlate at all with the differences in the activation energies. Therefore we suspect that the activation energies are, in fact, approximately independent (i.e., to within about 0.1 eV) of the cation or Pt—Pt distance for all the compounds except the ammonium one. If the variation of a11 for the TCP(II) salts is not due to a significant change in the number of carriers, it must be due to a variation of the mobility. In order to investigate this possibility we have included in the figure the experimental results for the incremental electrical conductivity ofKCP under pressure obtained by Thielemans etci. [12] The Pt—Pt spacing used is inferred from the data given by Kuse [13] It can be seen that the slope of the curve is similar to that for the TCP(ll) salts, even though the origin of the carriers is different. Therefore, we believe that the variation of a11 with hR is primarily due to a strong dependence of the mobility on the Pt—Pt spacing. The large increase in au on going to KCP is due to the difference us the mechanism of carrier production. For the TCP(II) salts, the carriers are thermally activated. However, KCP has a conduction band which is not full owing to the presence of 0.3 bromines per platinum atom. This causes partial oxidation and a 0.85 full conduction band. Therefore, the qualitative difference in the ratio of the number of carriers for the TCP(II) compounds and KCP should be given by exp (—E/kB 7). Using the observed change in a11, we obtain E ~ 0.3 eV, in reasonable agreement with the observed activation energies. To summarize, we have shown that there is a systematic variation in the electrical conductivity of a wide range of Pt chain materials containing the [Pt(CN)4]unit. Further work is necessary in order to ascertain precisely the mechanisms responsible for this behaviour. ..
.
We wish to thank Johnson Matthey and Co. Ltd. for the loan of platinum salts and the Science Research Council for support (to J.H.O’N).
Acknowledgements
—
REFERENCES 1. 2. 3. ~ ~
M.L.Moreau-Colin, Structure and Bonding 10, K. Krogmann, Angew. Chem. Intern. Ed. 8,35 (1969). D.M. Wascheck, S.W. Peterson, A.H. Reis & J.M. Williams, Inorg. Chem. 15, 74(1976). ~Yersin, G1 915(1977) U. Rossler, Solid State H.R. Zelier, Low-dimensional Co-operative Phenomena (Edited by HJ. Keller), p. 215. Plenum Press, New York (1975).
560 6.
STUDIES OF ASERIES OF TETRACYANOPLATINUM COMPOUNDS
JM. W1lIi2m~,Inorg. NucL C’hem. Lett. 12,651 (1976). 7. MJ. Minot & J.H. Perlsteln,Phys. Rev. Lett. 26, 371 (1971). 8. Y. Hara, I. Shirotani & S. Minomura, Chem. Lett. 579(1973); Y. Hara, I. Shirotani & A. Onodera, Solid State Commun. 17,827(1975). 9. PS. Gomm, T.W. Thomas & A.E. Underhill, /. C’hem. Soc(a). 2154 (1971).
10.
Vol. 29, No.7
B.A. Scott, R. Mehran & BD. Silverman, Extended InteractIons between MetalIons(Edited by LV. Interrante), p. 331. Amer. Chem. Soc. Symp. Ser. No. 5 (1974). 11. F. Mehran & B.A. Scott, Phys. Rev. Lett. 31,99 (1973). 12. M. Thielemans, R. Deltour, D. Jerome & J.R. Cooper, Solid State Commun. 19,21(1976). 13. D. Kuse,Soiki State Commun. 13,885(1973).
series [8] We have now made a detailed study of the .
monovalent cation) and N[Pt(CN)4 ] nH~O(where N is a divalent cation) probably represent the most extensive range of integral oxidation state metal atom chain compounds known at present. The structure of many members of this series are known and they have been shown to possess a chain-like structure with the planar ‘t(CN’)4 ]2_ ions stacked one above the other along the c-axis of the crystal [1—3].The Intrachain Pt—Pt distance varies from 3.1 A to over 3.6 A depending on the nature of the cation and also on the degree of hydration. The colours of these compounds also vary dramatically from colourless for those with long Pt—Pt distances to reds and violets for those possessing the shortest Pt—Pt distances [1]. Recently a detailed study of the relationship between the position of the emission spectra bands and the Pt—Pt distance has been made [4]. There is also a series of non-Integral oxidation state platinum atom chain compounds which possess the same basic chain-like structure of planar [Pt(CN)4] units. The archetypical member of this series is 1(2 [Pt(CN)4 J Br0~303H2O[KCP]which has been studied extensively because of its high electrical conductivity [5] at room temperature. The Pt—Pt spacing for compounds in the series has been shown [6] to vary with the degree of partial oxidation. We present first an experimental study of the electrical conduction properties of a series of TCP(ll) salts and then relate the results obtainedto those previously reported for KCP. Electrical conduction studies on the TCP(II) salts have been restricted to a value7 quoted for a single and to studies on crystal of K2 [Pt(CN)4 polycrystalline discs for] xH2O a number of members of this ,
d.c. electrical conduction properties of single crystals of this series of TCP(II) salts. The d.c. conductivity was determined for all the crystals using a 2-probe method. Gold wires were attached to the crystals using colloidal graphite (Aquadag) as the contact material. The results are summarized in Table 1. For certain crystals a 4-probe method was also used and this was found to give a value two to three times greater than the 2-probe vaiue. This increase is smaller than the scatter of results obtained from different crystals of the same material. Seven TCP(II) salts were studied in the course of this investigation. Because these compounds can be obtained with varying degrees of hydration the compounds were carefully analyzed to establish which hydrate was being studied. The electrical conductivity parallel to the metal atom chain direction (au) was determined for all the compounds studied. Itwas found that a~could vary by up to one order of magnitude from crystal to crystal from the same preparation. The spread of results and the number of crystals examined for each compound is shown in Table 1. Whenever possible the activation energy for conduction was determined over the temperature range 100— 270K. However, for the potassium and strontium compounds it was found that the crystals were so brittle that they Invariably broke on cooling thus preventing the measurement of the activation energy. it is noteworthy that those two compounds have the longest Pt—Pt distances of all the compounds studied and therefore might be expected to be more susceptible to fracture perpendicular to the c-axisFor of the lithium crystal. and barium salts the crystals were sufficiently large for the electrodes to be attached
557
558
STUDIES OF ASERIES OF TETRACYANOPLATINUM COMPOUNDS
VoL 29, No.7
Table 1. Intra Pt—Pt distances and d.c. electriazl conductionproperties Intxa-Pt-.Pt distance (R)
Compound
hR (A’)
o~room-temp. ((r’ cnr l)
aj room-temp. (rr’ cnr’)
Activation
Number
energy (H)
crystals
(eV)
examined
100
0.41
10
100
0.47 0.27
2 4 9 1 2
o~:a
1
(A)
Li, [Pt(CN)4 I 3H,0
3.18’
b2E~(~’O4I -3H,0’ Ba[Pt(CN)4].4H20
3.18
0.3144
3 x 10’ —3 X 10~ 3 X 10~’
0.301
4 x 10’—3 x 10~ 8x 10~—5X 10-’
—
2x10’ Ba[Pt(CN)4 I .4H,0’
3.32
(NH4), IPt(CN)41 .2H,O Rb2 [Pt(CN)4].3H,0 MgIPt(CN)4 I 7H,O Sr[Pt(CN)4J.5H,O
3.26°
0.3067
3.40’
0.294
315b
0.3175
3~60b
2.88’
-
K, [Pt(CN)4 I 3H,0 K, [Pt(CN)4 ]Br,~,.311,0 *
10-’
0.26 1.0 0.3 0.32
0.2777
1—8x 10_s 5 X 10’—7 X 10’ 7 X 10-’ —2 X 10’ 4 X 10’—l X 10’
0.2873 0.3472
8 x 10’ -.4 x 10’ 1o2_4 x io2~
2—4
X
5 5
4 3 5
io-’t
io~
Crystals obtained from solutions containing Pt(IV) salts.
‘M.L. Moreau-Colin, BulL Soc. Royale Sciences Liege 34,778(1935). b Reference [2]. C Reference [1]. d Reference [3]. e C. Peters & C.F. Eagen, Phys. Rev. Lett. 34, 1132(1975). ~ H.R. Zeller & A. Beck, I. Chem. Phys. Solids 35,77(1974).
Iog~ cm
perpendicular to the platinum atom chain direction. At room temperature thePtconductivity In the direction perpendicular to the atom chain (aj) was about 102 lower than that found in the Pt atom chain direction. A
+
similar degree of anisotropy has been found in Magnus’s Green salt ([Pt(NH)) 12+ [Pt(~’M J2_, MGS) and other integral oxidation state metal atom chain
2
compounds [9]. Crystals of the lithium and barium compounds were also grown from solutions containing a 20% molar amount of Li2 [Pt(CN)4Cl2]and Ba[Pt(CN)4Cl2] respectively. It has previously been shown that the conductivity in MGS can be enhanced by several orders of magnitude by doping the crystals with an appropriate Pt(IV) compound [101. It can be seen from Table 1 that the lithium compound grown from the Pt(IV) doped solution exhibits a conductivity about one order of magnitude higher than those grown from “undoped” solutions. For the barium compound, crystals grown from “doped” solutions exhibit conductivities similar
-2
d
I
~
I
6 ~
a
i
_I~~
028
0-36
032
1 ~ 1) ~ (AFig. 1. Variation of room temperature conductivity in platinum atom chain direction fGr a series of tetrac 1anoplatinurn (II) salts. (a, Sr [Pt(CN)4151120; b, K2,~t(CN)4] 3H20, c, Rb2 I~t(CN~4~ 3H20, o~ -
-
• KCP under pressure [12]).
71120; 2
‘
“undoped” Increase in conductivity is to the most solutions. conductingThis crystals obtained from small compared with the large differences observed on changing the cation. Scott et ci. [10] found that it was possible to increase the Pt(IV) content and the electrical conductivity of MGS by instantaneous precipitation of the
salt from solutions containing Pt(IV) ions. However, the salmples of MGS formed slowly in gels contained a
VoL 29, No.7
STUDIES OF ASERIES OF TETRACYANOPLATINUM COMPOUNDS
much lower Pt(IV) content (Pt”: Pt” iO~:1) because of a complex redox equilibrium which occurs between Pt” and Pt” species in solution and which reduces the Pt”’ concentration in solution and hence the Pt(IV) doping in the crystals. This equilibrium is not established In the case of Instantaneous precipitation and a non-metastable phase is obtainedwith a high Pt(IV) content. All the [Pt(CN)4]2_ compounds, including those grown from solutions containing a high Pt(IV) content, were obtainedby slow crystallization in which equilibrium conditions would be expected. Thus it is likely that all the compounds contain only a very low Pt(IV) content. This is supported by work on KCP where only one stable phase has been shown to exist for a wide mole ratio of K2 [Pt(CN)4I and K2 [Pt(CN)4Br2] in solution [10]. The results in Table 1 for a11 at room temperature show that the conductivity is strongly dependent on the cation present. In Fig. 1, log all is plotted against hR for all the compounds studied. R is the intrachain Pt—Pt spacing. It can be seen that there is a systematicvariation of a11 with hR. Aband model such as that proposed to explain the variation of emission peaks [4] with R appears most appropriate for considering the electrical conduction properties of these compounds. A filled band resulting from overlap of hybridized (Pt 5d~,6s) orbitals and an empty band from overlap of (Pt 6Pm; CNir’) orbitals are separated by an energy gap which decreases as the Pt—Pt separation decreases. The energy gap is at least 2 eV and this is too large to allow significant carrier formation by promotion of electrons across the gap, even at room temperature. EPR measurements on MGS have shown the existence of a small contribution of bound 5d~holes due to Pt(III) lying 0.6 eV above the Sd~orbitals [11]. The Pt(III) holes occur due to the presence of Pt(IV) complexes during sample preparation and extend over several metal chain lattice sites. It seems likely that in the [Pt(CN)4)J2 series that the electrical conduction properties arise extrinsically due to the presence of Pt(IV) impurities, as in MGS [10]. Electrons will be thermally promoted from the top of the (5d~,6s) band into these localized impurity states and hence carriers (holes) will be generated in the top of the valence band. Therefore we interpret the observed activation energies E as the difference between the Fermi energy and the energy at the top of the valence band. The measured E range from 0.26 to 0.47 eV, excluding the ammonium compound, and we estimate that they are accurate to about ±0.05 eV. The reason for the very much higher activation energy for the ammonium compound is not known. Avariation in E of 0.2 eV, implies a change in the number of carriersby three orders of magnitude at room temperature.
559
However the variation of a11 from compound to compound does not correlate at all with the differences in the activation energies. Therefore we suspect that the activation energies are, in fact, approximately independent (i.e., to within about 0.1 eV) of the cation or Pt—Pt distance for all the compounds except the ammonium one. If the variation of a11 for the TCP(II) salts is not due to a significant change in the number of carriers, it must be due to a variation of the mobility. In order to investigate this possibility we have included in the figure the experimental results for the incremental electrical conductivity ofKCP under pressure obtained by Thielemans etci. [12] The Pt—Pt spacing used is inferred from the data given by Kuse [13] It can be seen that the slope of the curve is similar to that for the TCP(ll) salts, even though the origin of the carriers is different. Therefore, we believe that the variation of a11 with hR is primarily due to a strong dependence of the mobility on the Pt—Pt spacing. The large increase in au on going to KCP is due to the difference us the mechanism of carrier production. For the TCP(II) salts, the carriers are thermally activated. However, KCP has a conduction band which is not full owing to the presence of 0.3 bromines per platinum atom. This causes partial oxidation and a 0.85 full conduction band. Therefore, the qualitative difference in the ratio of the number of carriers for the TCP(II) compounds and KCP should be given by exp (—E/kB 7). Using the observed change in a11, we obtain E ~ 0.3 eV, in reasonable agreement with the observed activation energies. To summarize, we have shown that there is a systematic variation in the electrical conductivity of a wide range of Pt chain materials containing the [Pt(CN)4]unit. Further work is necessary in order to ascertain precisely the mechanisms responsible for this behaviour. ..
.
We wish to thank Johnson Matthey and Co. Ltd. for the loan of platinum salts and the Science Research Council for support (to J.H.O’N).
Acknowledgements
—
REFERENCES 1. 2. 3. ~ ~
M.L.Moreau-Colin, Structure and Bonding 10, K. Krogmann, Angew. Chem. Intern. Ed. 8,35 (1969). D.M. Wascheck, S.W. Peterson, A.H. Reis & J.M. Williams, Inorg. Chem. 15, 74(1976). ~Yersin, G1 915(1977) U. Rossler, Solid State H.R. Zelier, Low-dimensional Co-operative Phenomena (Edited by HJ. Keller), p. 215. Plenum Press, New York (1975).
560 6.
STUDIES OF ASERIES OF TETRACYANOPLATINUM COMPOUNDS
JM. W1lIi2m~,Inorg. NucL C’hem. Lett. 12,651 (1976). 7. MJ. Minot & J.H. Perlsteln,Phys. Rev. Lett. 26, 371 (1971). 8. Y. Hara, I. Shirotani & S. Minomura, Chem. Lett. 579(1973); Y. Hara, I. Shirotani & A. Onodera, Solid State Commun. 17,827(1975). 9. PS. Gomm, T.W. Thomas & A.E. Underhill, /. C’hem. Soc(a). 2154 (1971).
10.
Vol. 29, No.7
B.A. Scott, R. Mehran & BD. Silverman, Extended InteractIons between MetalIons(Edited by LV. Interrante), p. 331. Amer. Chem. Soc. Symp. Ser. No. 5 (1974). 11. F. Mehran & B.A. Scott, Phys. Rev. Lett. 31,99 (1973). 12. M. Thielemans, R. Deltour, D. Jerome & J.R. Cooper, Solid State Commun. 19,21(1976). 13. D. Kuse,Soiki State Commun. 13,885(1973).