Magnetic susceptibility of tetrathiafulvalene tetracyanoquinodimethane under pressure

We report high-accuracy static magnetic-susceptibility data (T,P) for tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ) over the temperature range 3 400 K at both ambient and applied pressures to 10 kbar. The spin susceptibility s increases with temperature, saturating above 360 K at the value +(6.05 0.05)×10-4 emu/mol without passing through a maximum. s decreases rapidly under pressure at all temperatures. The value of the pressure derivative (-dln s/dP) in %/kbar decreases from +8.5 at 75 K to +7 at 300 K and +5 at 360 K. From these pressure dependencies it is possible to show that the thermal contraction of the lattice is sufficiently strong to account for most of the substantial temperature dependence of s(T) observed at ambient pressure. These results are discussed within the framework of various theoretical models of the susceptibility which take into account band-structure effects, electron-phonon interactions, and electron-electron interactions.