Order‐Disorder Phase Transition in TEA+(TCNQ)₂‐

Abstract

The results of measurements of cell parameters by X-ray diffraction and of differential scanning calorimetry, versus temperature, on TEA -TCNQ, are presented and compared to the 0, do conductivity behaviour. The differential scanning calorimetry measurements show a first order phase transition at 220 K attributed to the TEA disorder. From the thermal expansion a dispersion is observed of the cell parameter a for temperatures below 220 K, attributed to a certain amount of cation "ordering" at low temperatures. The variation of the cell parameter c (the direction of the TCNQ chains), versus temperature, permits the calculation of the contribution of the TCNQ columns to the total heat capacity of the TEA TCNQ, by a simple linear model of coupled oscillators. It is found t o be dominated by intramolecular vibrational TCNQ modes and by the cation movements. The chain axis component of the Debye temperature and of the Gruneisen coefficient are B0 Y 150 K and yc x 7.5.Nous presentom une etude, par la diffraction des rayons X, de la dilatation thermique de la maille du TEA TCNQ, ainsi que des resultats d'analyse thermique diffbrentielle qui confirment l'existence d'une transition vers 220 K. L'Bvolution thermique du parambtre a montre une dispersion (-0,2%) des seules valeurs mesurees en dessous de 220 K. La chaleur spbcifique met en Bvidence une trhs forte hyst6rbsis de la transition, semblable ? i , celle observbe anterieurement en conductivite 6lectri-que. L'ensemble de ces resultats s'interprhte essentiellement comme une manifestation du desordre du cation TEA. Le comportement du parametre de maille c (axe des empilements des TCNQ) permet de calculer, par I'utilisation d'un modble unidimensionnel simple d'oscillateurs coupl6s, la contribution des chaines de TCNQ la chaleur spbcifique totale du TEA * TCNQ,. On obtient aussi le coefficient de Gruneisen (yc x 7,5) et la temperature de Debye (0, Y 150 K) pour la direction c.

Temperature Study of the Polarized Reflectivity of TEA(TCNQ)2Single Crystal

“…4b). As it appears the presence of bands at T < Tpc o r r o b o r a t e s t h e m o d e l o f t h e p h a s e t r a n s i t i o n in TEA(TCNQ) 2 given by Brau and Farges [16], attributing the transition to the TEA disorder. At high temperature phase the TEA cations are randomly disordered in rotation, while at low temperature phase the disorder is only over two equipopulated positions sharing a hydrogen bond with either a TCNQ(A) or a TCNQ(B) anion [4].…”

“…The temperature at which the first conductivity step occurs varies randomly from one thermal cycle to the next, and from one single crystal to another. It is scattered in the T-range from 268 to 196 K [8,16]. Furthermore, some authors have postulated that this transition is of the metal-insulator type, while others tend to see a semiconductor-semiconductor mechanism.…”

“…The DSC measurements have shown a first order phase transition at 220 K attributed to the TEA disorder [16]. In fact, at 300 K, each TEA cation is randomly disordered in rotation (dynamic disorder) and, at low temperature, the disorder is only over two equipopulated positions (static disorder) sharing a hydrogen bond with either a TCNQ (A) or a TCNQ (B) anion on two adjacent TCNQ chains [4].…”

Infrared Studies of TEA(TCNQ)2 Crystals Damaged by Electron Irradiation and Mechanical Treatment

“…The DSC measurements have shown a first-order phase transition at 220 K attributed to the TEA disorder [3]. NMR results [4] together with electric conductivity [5], thermopower [6], thermal conductivity [7], and infrared spectra [8] confirm the mechanism of the phase transition.…”

Dipolar glass and magneto-electric coupling within a π-stacked organic system

“…10,11 As the temperature decreases, there is a gradual increase in the tetramerisation of the TCNQ units along the stack. The TEA cations fluctuate between two different configurations within the crystal structure and at 220 K, they freeze into a disordered state 10,[12][13][14] that also effects the TCNQ stack and each dimer behaves as an S = 1/2 entity. The magnetism is dominated by singlet-triplet excitations and this crossover temperature is from B120 K. (This transition is from herein denoted as T ST ), the system shows strong antiferromagnetic coupling (i.e.…”