CHLORINE-NUCLEAR-QUADRUPOLE-RESONANCE STUDY OF THE NEUTRAL-TO-IONIC TRANSITION IN TETRATHIAFULVALENE-CHLORANIL

The temperature dependence of the chlorine-35 nuclear-quadrupole-reson ance (NQR) parameters as been studied in the mixed-stack charge-transf er complex tetrathiafulvalene-chloranil and discussed with reference t o spectroscopic and neutron-diffraction results. The NQR spectrum chan ges conspicuously at the neutral-to-ionic transition temperature, T(N- I) = 79.2 K. The changes have been analyzed in terms of the symmetry b reaking occurring at the transition. Below, coexistence of the neutral and ionic phases has been observed down to the limit of metastability of the neutral phase (T(c) approximately 69 K). Spin-lattice relaxati on rates reveal the existence of highly anisotropic critical fluctuati ons with a predominantly one-dimensional character.