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Phase transition of the |(C6H11)(C2H5)2NH|+(TCNQ)−2 anion radical salt
Volume 45A, number 4
PHYSICS LETTERS
8 October 1973
PHASE TRANSITION OF THE (C6H11)(C2H5)2NHJ~(TCNQ)~ ANION RADICAL SALT S. FLANDROIS, P. DELHAES, J.C. GIUNTINI Centre de Recherches PaulPascal, Domaine Universitaire, 33 Talence, France and
P.
DUPUIS
Laboratoire de Chimie Physique Macromoléculaire, ENSIC-INPN, 54 Nancy, France Received 13 July 1973 The TCNQ anion radical salt of diethylcyclohexylammonium undergoes a first-order phase transition at Ca. 347°K,where sharp discontinuities of the heat Capacity, electrical conductivity and paramagnetic suscepti-
bility are observed.
Much attention has been devoted to the solid anion radical salts of tetracyanoquinodimethane because of their high electrical conductivities. A few salts are known to undergo phase transitions: the triphenylmethy1phosphonium~(TCNQ)~[1, 2], the morpholinium~TCNQ— [3] and the simple alcali TCNQ salts [4]. We report here experimental determinations on a phase transition of the diethylcyclohexylammonium~ (TCNQ)~salt, which is distinguishable from the previous ones by large discontinuities of the observed physical properties. The salt was prepared following the methods of Acker and Melby [5] from para bi-(dicyanomethyl) benzene*. It was purified by recrystallisation in acetonitrile. The stoichiometry was controlled by elementary chemical analysis and spectroscopy. The microcrystalline powder samples were compared for specific-heat determination and d.c. electrical conductivity measurements. Preliminary observations of microcrystals with a polarizing light microscope exhibited a change of birefringence due to a phase transition above 345°K[6]. Differential scanning calorimetry measurements gave an endothermic signal with a value of the enthalpy of transition MI = 1160±100 cal/mole. The transition was found irreversible at least up to 77°K;however a new crystallisation in acetonitrile gives again the initial phase.
the amine on tetracyanoquinodimethane leads to a mixture of low and high temperature forms.
* Direct action of
Magnetic susceptibility measurements by static method have been briefly reported [7]. It was found that before the transition the paramagnetic susceptibiity can be interpreted by a singlet-triplet law, with an energy distance between the singlet and excited triplet statesJ ~ 0.08 eV. At the temperature of transition the paramagnetism increases by a factor of 2.5 and then obeys a pure Curie-law.
.~
jc.
~
180~ . 41,/
/
j
i~a
/
I
/
-
___________________________________________________ 300 320 340 T (K) Fig. 1. Thermal variation of the heat capacity of (diethylcyclohexylammonium)~ (TCNQ)1 round the temperature of transition.
339
Volume 45A, number 4
PHYSICS LETTERS
I
8 October 1973
p = 0.5; however this model did not agree quantitatively with experiment. In particular, the condition: transition centered about p = 0.5 was not full filled [10] The transition of the diethylcyclohexylammonium~ (TCNQ)~is the only one where this condition is obeyed: the triplet excitation density is equal to p = 0.25 just before and 0.75 after the transition. The temperature dependence of the magnetic susceptibility can therefore be explained by this model. Following Chesnut, the transition is characterized by an entropy increase: /s.S = ~R ~p ln3, where I.~pis the change of excitation density. For the studied transition, the spin entropy change is i~S= 1.1 cal/deg~mole. On the other hand the evaluation of i~Sfrom the enthalpy of transition as measured by DSC experiment gives a value of 3.3 cal/deg mole. The extra entropy variation (2.2 calf deg mole) should be due to the crystal structure changes. The determination of the structure of the low and high temperature forms is now in progress and should allow to clear up the behavior of the physical properties at the phase transition. Moreover the investigations of the pressure dependence** and the solvent influence might allow us to know the phase diagram of such a transition and to compare it with .
~
-
10’
-
1o~
-
.
10
i ..
I
1 2~0
1
•S_.
1
I
320
I 340
I
3&2
T (K)
Fig. 2. Thermal variation of the electrical resistivity cyclohexylammoniurn)~(TCNQ).
of
(diethyl-
those found in some oxydes [9]. **
The heat capacities were measured with an adiabatic calorimater [8] in the range 295 370°K.As it is shown in fig. 1 a sharp discontinuity of the heat capacity C~occurs at 347°K.This jump of C~,shows undoubtedly that the transition is of first order. The irreversibility of the transition is also exhibited in this experiment. The most interesting result is obtained from the thermal variation of the electrical resistivity (fig. 2). At the transition temperature the electrical resistivity 4, the salt going from increase by a factor of about i0 —
a state of good conductivity to an insultating one. This jump is exceptionnal compared to the other known phase transitions occuring in anion radical salts and can be compared to the metal-insulator transitions found in inorganic compounds [9]. In order to explain these phase transitions, a simple phenomenological model has been proposed by Chesnut [10] . He showed that a first-order transition occurs under certain conditions with a discontinuity of the excitation density p of the triplet state. In the model the transition must occur symmetrically about 340
A preliminary investigation under high pressure (P 40 kbar) to detect the pressure effect has been unsuccessful; between toom temperature and 420 K the sample exhibits always a large temperature dependent conductivity.
References [1] R.G. Kepler, J. Chem. Phys. 39(1963) 3528. [2] Y. lida, Bull. Chem. Soc. Japan 43 (1970) 3685; A. Kosaki et al., Bull. Chem. Soc. Japan 43 (1970) 2280, Y. Suzuki and Y. lida, Bull. Chem. Soc. Japan 46 (1973)
683.
[3] J.C. Bailey andD.B. Chesnut, J. Chem. Phys. 51(1969) 5118. [4] J.G. Vegter, T. Hibma and J. Kommandeur, Chem. Phys. Lett.Acker 3 (1969) 427. [5] D.S. et al, J. Amer. Chem. Soc. 82 (1960) 6408; L.R. Melby et al., J. Amer. Chem. Soc. 84 (1962) 3374. [6] 5. Flandois and G. Sauthoff, J. Phys. Chem. Solids, to be published. [7] 5. Flandois, P. Dupuis and J. Ned, C.R. Acad. Sci. (1969) 1091. [8] 269 F. Aly, Thesis, Bordeaux, 1973. 191 D. Adler, Rev. Mod. Phys. 40 (1968) 714; Solid State Phys. 21(1968)1. [10] D.B. Chesnut, J. Chem. Phys. 40(1964) 405.
PHYSICS LETTERS
8 October 1973
PHASE TRANSITION OF THE (C6H11)(C2H5)2NHJ~(TCNQ)~ ANION RADICAL SALT S. FLANDROIS, P. DELHAES, J.C. GIUNTINI Centre de Recherches PaulPascal, Domaine Universitaire, 33 Talence, France and
P.
DUPUIS
Laboratoire de Chimie Physique Macromoléculaire, ENSIC-INPN, 54 Nancy, France Received 13 July 1973 The TCNQ anion radical salt of diethylcyclohexylammonium undergoes a first-order phase transition at Ca. 347°K,where sharp discontinuities of the heat Capacity, electrical conductivity and paramagnetic suscepti-
bility are observed.
Much attention has been devoted to the solid anion radical salts of tetracyanoquinodimethane because of their high electrical conductivities. A few salts are known to undergo phase transitions: the triphenylmethy1phosphonium~(TCNQ)~[1, 2], the morpholinium~TCNQ— [3] and the simple alcali TCNQ salts [4]. We report here experimental determinations on a phase transition of the diethylcyclohexylammonium~ (TCNQ)~salt, which is distinguishable from the previous ones by large discontinuities of the observed physical properties. The salt was prepared following the methods of Acker and Melby [5] from para bi-(dicyanomethyl) benzene*. It was purified by recrystallisation in acetonitrile. The stoichiometry was controlled by elementary chemical analysis and spectroscopy. The microcrystalline powder samples were compared for specific-heat determination and d.c. electrical conductivity measurements. Preliminary observations of microcrystals with a polarizing light microscope exhibited a change of birefringence due to a phase transition above 345°K[6]. Differential scanning calorimetry measurements gave an endothermic signal with a value of the enthalpy of transition MI = 1160±100 cal/mole. The transition was found irreversible at least up to 77°K;however a new crystallisation in acetonitrile gives again the initial phase.
the amine on tetracyanoquinodimethane leads to a mixture of low and high temperature forms.
* Direct action of
Magnetic susceptibility measurements by static method have been briefly reported [7]. It was found that before the transition the paramagnetic susceptibiity can be interpreted by a singlet-triplet law, with an energy distance between the singlet and excited triplet statesJ ~ 0.08 eV. At the temperature of transition the paramagnetism increases by a factor of 2.5 and then obeys a pure Curie-law.
.~
jc.
~
180~ . 41,/
/
j
i~a
/
I
/
-
___________________________________________________ 300 320 340 T (K) Fig. 1. Thermal variation of the heat capacity of (diethylcyclohexylammonium)~ (TCNQ)1 round the temperature of transition.
339
Volume 45A, number 4
PHYSICS LETTERS
I
8 October 1973
p = 0.5; however this model did not agree quantitatively with experiment. In particular, the condition: transition centered about p = 0.5 was not full filled [10] The transition of the diethylcyclohexylammonium~ (TCNQ)~is the only one where this condition is obeyed: the triplet excitation density is equal to p = 0.25 just before and 0.75 after the transition. The temperature dependence of the magnetic susceptibility can therefore be explained by this model. Following Chesnut, the transition is characterized by an entropy increase: /s.S = ~R ~p ln3, where I.~pis the change of excitation density. For the studied transition, the spin entropy change is i~S= 1.1 cal/deg~mole. On the other hand the evaluation of i~Sfrom the enthalpy of transition as measured by DSC experiment gives a value of 3.3 cal/deg mole. The extra entropy variation (2.2 calf deg mole) should be due to the crystal structure changes. The determination of the structure of the low and high temperature forms is now in progress and should allow to clear up the behavior of the physical properties at the phase transition. Moreover the investigations of the pressure dependence** and the solvent influence might allow us to know the phase diagram of such a transition and to compare it with .
~
-
10’
-
1o~
-
.
10
i ..
I
1 2~0
1
•S_.
1
I
320
I 340
I
3&2
T (K)
Fig. 2. Thermal variation of the electrical resistivity cyclohexylammoniurn)~(TCNQ).
of
(diethyl-
those found in some oxydes [9]. **
The heat capacities were measured with an adiabatic calorimater [8] in the range 295 370°K.As it is shown in fig. 1 a sharp discontinuity of the heat capacity C~occurs at 347°K.This jump of C~,shows undoubtedly that the transition is of first order. The irreversibility of the transition is also exhibited in this experiment. The most interesting result is obtained from the thermal variation of the electrical resistivity (fig. 2). At the transition temperature the electrical resistivity 4, the salt going from increase by a factor of about i0 —
a state of good conductivity to an insultating one. This jump is exceptionnal compared to the other known phase transitions occuring in anion radical salts and can be compared to the metal-insulator transitions found in inorganic compounds [9]. In order to explain these phase transitions, a simple phenomenological model has been proposed by Chesnut [10] . He showed that a first-order transition occurs under certain conditions with a discontinuity of the excitation density p of the triplet state. In the model the transition must occur symmetrically about 340
A preliminary investigation under high pressure (P 40 kbar) to detect the pressure effect has been unsuccessful; between toom temperature and 420 K the sample exhibits always a large temperature dependent conductivity.
References [1] R.G. Kepler, J. Chem. Phys. 39(1963) 3528. [2] Y. lida, Bull. Chem. Soc. Japan 43 (1970) 3685; A. Kosaki et al., Bull. Chem. Soc. Japan 43 (1970) 2280, Y. Suzuki and Y. lida, Bull. Chem. Soc. Japan 46 (1973)
683.
[3] J.C. Bailey andD.B. Chesnut, J. Chem. Phys. 51(1969) 5118. [4] J.G. Vegter, T. Hibma and J. Kommandeur, Chem. Phys. Lett.Acker 3 (1969) 427. [5] D.S. et al, J. Amer. Chem. Soc. 82 (1960) 6408; L.R. Melby et al., J. Amer. Chem. Soc. 84 (1962) 3374. [6] 5. Flandois and G. Sauthoff, J. Phys. Chem. Solids, to be published. [7] 5. Flandois, P. Dupuis and J. Ned, C.R. Acad. Sci. (1969) 1091. [8] 269 F. Aly, Thesis, Bordeaux, 1973. 191 D. Adler, Rev. Mod. Phys. 40 (1968) 714; Solid State Phys. 21(1968)1. [10] D.B. Chesnut, J. Chem. Phys. 40(1964) 405.