Properties of Simple Alkali-Tetracyanoquinodimethan Salts. I. Magnetic Behavior of Lithium Tetracyanoquinodimethan

The absolute value of the spin susceptibility of the radical-ion salt ${\mathrm{Li}}^{+}$TCN${\mathrm{Q}}^{-}$ (tetracyanoquinodimethan) has been accurately measured in the temperature range of 100-450 °K. It is shown that the magnitude and temperature dependence of the susceptibility can be quantitatively explained on the basis of an elementary symmetry-split band theory for a linear system. The splitting is due to the "dimerization" along the chain of TCN${\mathrm{Q}}^{-}$ ions. The resulting values for bandwidths and band gap (0.055 and 0.135 eV, respectively) are in fair agreement with the values obtained from a tight-binding band-theory calculation on ${\mathrm{Rb}}^{+}$TCN${\mathrm{Q}}^{-}$, for which the crystal structure is accurately known. The experimental results are also compared with other theoretical calculations. The results show band theory to be considerably superior in this particular case.