Phase transitions in the organic semiconductor dipropylamine tetracyanoquinodimethane: DPA TCNQ₂

Abstract

. Can. J. Chem. 66,938 (1988). Dipropylarnine tetracyanoquinodirnethane, DPA TCNQ2, is by stoichiometry a 1:2 organic salt giving rise to a f filled band quasi one dimensional electronic system. Conductivity measurements indicate the presence of at least three phase transitions. A dominant transition at 215 K shows simultaneous large discontinuities in both electric and magnetic properties. Infrared measurements suggest a structural change at 21 5 K related to conduction electron-molecular vibration interactions in view of the + appearance of A, TCNQ modes in the infrared spectrum. Deuteration of the donor N-H bond lowers the main transition temperature by 5 K but maintains the same physical properties as in the nondeuterated compound. Le dipropylarnine tCtracyanoquinodirnCthane (DPA TCNQ2) est un sel organique de stoechiomttrie 1:2 qui donne lieu a un systkme tlectronique quasi-unidimensionnel comportant une bande qui n'est remplie qu'au a. Des rnesures de conductivitC suggkrent la prisence d'au rnoins trois transitions de phases. Une transition dorninante a 215 K prtsente des discontinuitCs sirnultanCes irnportantes tant dans les propriCtCs Clectriques que magnktiques. Des mesures infrarouges perrnettent de voir apparaitre les modes A, du TCNQ et suggkrent qu'il se produit un changement de structure a 215 qui est reliC i d e s interactions de vibration de conduction Clectron-rnolCcule. La deutCration de la liaison donneur N-H diminue par 5 K la temperature d'apparition de la transition principale; toutefois, on retrouve les mernes propriCtCs physiques que celles du produit non-deutCrC.[Traduit par la revue]

A study of the phase transition in semiconducting 1:2 tetracyanoquinodimethane charge transfer complexes with dipropylamine derivatives: X-ray structure and polarized reflection spectra

“…This coefficient is very small (about 3%) for the polarization perpendicular to the stacking a axis, confirming thus that TCNQ Ag modes Previously reported electrical and magnetic properties of these semiconductors (7,8) pointed to an unusual phase transition. which we termed an inverse s~i n -~e i e r l s transition.…”

“…Preparation and spectra Single crystals of the materials were prepared by methods described elsewhere (7). Chemical analysis showed 1:2 stoichiometry.…”

“…The low reflectivity along the a axis indicates a markedly high electron-phonon scattering rate, which is related to the low electrical conductivity observed at room temperature in these complexes (7). Below 210 K, the spectra of all these complexes change dramatically, many bands splitting below Tc as in the case of TEA-(TCNQ)' (14,17).…”

“…In particular, a 1 :2 charge transfer series of crystals with dipropylarnine (DPA) and its derivatives as donors and 7,7/,8,g1-tetracyanoquinodi- 'Contribution from le Groupe de Recherches sur les Semiconducteurs et DiClectriques. methane (TCNQ) as acceptor exhibited unique electric, magnetic, and thermoelectric power (TEP) behaviour (7,8). A dominant transition around the temperature T, = 210 K showed an abrupt change of these properties.…”

“…Measurements of the electrical conductivity and magnetic properties suggested the occurrence of an "inverse spin -Peierls" transition (8) because the lower temperature phase exhibited an increase in conductivity. Preliminary infrared (IR) spectra showed that A, modes of the TCNQ anion became IR active and split into multiplets below the phase transition temperature T, (7).…”