Structural Phase Transition in (NMe₄)₂TCNQ₃

Abstract

The structural properties of a neutral/radical 1 : 2 mixed compound, (NMe 4 ) 2 TCNQ 3 , have been studied by the X-ray diffraction method in a temperature range between 25 and 150 °C. The experimental data show that a second order phase transition takes place at around 60 °C and that the high-temperature phase has a monoclinic C2/m structure. The molecular planes of dimerized TCNQ -⋅ radicals, being slipped against each other in the low temperature phase, are expected from the C2/m symmetry to be totally eclipsed in the high-temperature phase. A good fit of the power law (1 − T / T c ) α to the temperature variation of the intensity of the Bragg reflection arising from the slipped TCNQ -⋅ radicals is obtained with T c = 330.7± 0.8 K and α = 0.66± 0.05 for T ≤ T c .Influences of this novel phase transition on the electronic properties are discussed.

High-pressure infrared absorption in Cs2TCNQ3 crystals grown under magnetic field

“…This component couples with the molecular vibration to drive the EMV modes, which appear along the E//a polarization with much smaller intensity compared to the modes appear in E//b polarization. We have previously observed in the case of (NMe 4 ) 2 TCNQ 3 , which crystal structure is very similar to the present substance, that these modes disappear when the TCNQ − dimers are no longer slipped due to the phase transition [17,19]. The intensity of the EMV modes appear in E//a polarization, therefore, represents the amount of slip distance of the TCNQ − dimers.…”

Infrared study on structural phase transition in (NMe4)2TCNQ3

“…(1) to the experimental data yields T C = 340 K, and α = 0.62-0.70 for all the bands. The values of α are compared with α = 0.63 and 0.66, which are obtained for the intensity of the S 1 band and (018) Bragg reflection of the X-ray diffraction, respectively [2,3].…”

“…A structural phase transition in this substance was suggested from the temperature dependence of the satellite band (S 1 ) observed at 1.3 eV in the E//a polarization of the optical absorption spectrum, where E and a are the electric field of light and a-axis of the monoclinic crystal structure, respectively [2]. The S 1 band diminishes continuously with increasing temperature to disappear at T C = 65 o C. The Xray diffraction study has shown that second-order structural phase transition takes place at the same temperature [3]. In the low-temperature phase the two TCNQ anion radicals form a slipped dimer, in which they are displaced against each other in the direction of short molecular axis.…”

Infrared study on pressure-induced charge delocalization in Cs2TCNQ3

“…Consequently, the dipole moment of the charge transfer transition is not perfectly parallel to the b-axis but is slightly canted to have a weak component along the a-axis, and thus drives the S 1 abd emv bands for the Ella polarization. As temperature is elevated the second order phase transition takes place at 338 K to have the TCNQ − molecules be completely eclipsed [11], so that above 338 K both the S 1 and emv bands disappear for Ella. In this analogy, the present experimental result suggests strongly that the radical-like TCNQ − molecules are eclipsed in the high-pressure phase of Cs 2 TCNQ 3 .…”