Preparation of a Mott insulator based on a BEDT-TTF charge transfer complex of hydrogen cyananilate: α′-(BEDT-TTF)2HCNAL

Abstract

BEDT-TTF (ET) and cyananilic acid (H 2 CNAL) afforded charge transfer crystals by a diffusion method. The chemical formula of the complex was deduced based on all the information concerning the elemental, structural, electric, magnetic and optical analyses. The ET molecules form one-dimensional columnar stacks composed of twisted dimers with a face-to-face overlap (a'-type stack). The acid works as an oxidant and is deprotonated to form the monoanion, HCNAL 12 , which forms ribbon-like aggregation by means of hydrogenbonds. The ribbons form anion layers that sandwich the ET layer. The crystal is semiconductive with a room temperature conductivity of 0.20-0.83 Scm 21 and activation energy of 0.15 eV along the stacking direction, though the band calculation by the extended Hu ¨ckel method suggests a metallic nature, indicating strong electron-correlation in this system. The complex is a Mott insulator and its magnetic susceptibility is described by the one-dimensional S~1 2 antiferromagnetic Heisenberg chain model with J/k B ~252¡3 K.

Thermoelectric Properties of Organic Charge-Transfer Compounds

“…(ET) 2 (HCNAL) was reported to be a Mott insulator. 6 This behavior may be reasonably explained in the case that the absolute value of S in the high-temperature limit is determined only by the entropy of spin and charge configurations in the Mott insulator. Chaikin and Beni generalized 9 the Heikes formula for the materials with localized spins to determine S. Table I summarizes some of the representative formulae (Eqs.…”

“…These values are considered to be significantly lower than those of the single crystals; e.g., the r value of the (ET) 2 (HCNAL) pellet was about two orders of magnitude lower than that of single crystals (83 S/m at 300 K: along the a-axis). 6 Although the absolute value of r would be inaccurate in this study because of the two-probe measurement, temperature dependences of r for (TTF)(TCNQ), (BO)(TCNQ), and (ET) 2 (HCNAL) pellets suggest hopping conduction at a temperature range higher than $190 K, $240 K, and $60 K, respectively (Fig. 3b).…”

“…4), which is comparable to those of the conventional inorganic TE materials. Because (ET) 2 (HCNAL) has the crystal structure of alternatively stacked ET molecule layers (a¢-type packing structure of ET molecules) and closed-shell counter-anion layers, 6 ET molecules are considered to serve as the conduction path.…”

“…6 The former two and the latter were purified by sublimation and recrystallization, respectively. BO and HCNAL were prepared as previously reported.…”

“…(ET) 2 (HCNAL) was prepared by the diffusion method in benzonitrile-CS 2 mixed solvent as described in Ref. 6. TE property measurements: Each powder sample was compressed (at $2 MPa) into a cylindricalshaped pellet with $3 mm diameter and $2 mm height.…”