  The neutral-to-ionic phase transition in the mixed-stack charge-transfer
complex tetrathiafulvalene-p-chloranil (TTF-CA) has been studied by
pressure-dependent infrared spectroscopy up to p=11 kbar and down to low
temperatures, T = 10 K. By tracking the C=O antisymmetric stretching mode of CA
molecules, we accurately determine the ionicity of TTF-CA in the
pressure-temperature phase diagram. At any point the TTF-CA crystal bears only
a single ionicity; there is no coexistence region or an exotic high-pressure
phase. Our findings shed new light on the role of electron-phonon interaction
in the neutral-ionic transition.
