Chapter 1 Introduction to Highly Conducting Quasi-One-Dimensional Organic Crystals

Abstract

The term “quasi-one-dimensional” is used to describe materials with structure so anisotropic that overlap of electron wavefunctions and, therefore, conductivity is larger in one crystallographic direction. This chapter introduces to highly conducting quasi-one-dimensional organic crystals such as TCNQ (tetracyanoquinodimethane), TTF (tetrathiafulvalene), or TMTSF (tetramethyltetraselenafulvalene). In the crystal, the TTF and TCNQ molecules are stacked on top of each other, with relatively close spacing in the stack or chain direction and much larger spacing between molecules in the other directions. At room temperature, the conductivity of TTF–TCNQ along the chain direction is 600 to 900 ohm –1 cm –1 . The ratio of the b-axis conductivity to that along the a direction, where the current goes between unlike molecules, is 10 3 at 300 K and increases to a maximum close to 6 × 10 3 near 60 K. The two-chain structure, with molecules of one type segregated in each chain, is characteristic for the class of highly conducting quasi-one-dimensional organic crystals. Generally, the molecules of each chain are closed shell when isolated. In the crystal, however, there is a transfer of electrons from one type of molecule to the other, leaving at least one set of chains with partially filled electron energy bands. In compounds containing TTF or TCNQ, TTF acts as a donor of electrons, while TCNQ is an acceptor.