  We investigate the charge and lattice states in a quasi-one-dimensional
organic ferroelectric material, TTF-QCl$_{4}$, under pressures of up to 35 kbar
by nuclear quadrupole resonance experiments. The results reveal a global
pressure-temperature phase diagram, which spans the electronic and ionic
regimes of ferroelectric transitions, which have so far been studied
separately, in a single material. The revealed phase diagram clearly shows that
the charge-transfer instability and the lattice symmetry breaking, which
coincide in the electronic ferroelectric regime at low pressures, bifurcate at
a certain pressure, leading to the conventional ferroelectric regime. The
present results reveal that the crossover from electronic to ionic
ferroelectricity occurs through the separation of charge and lattice
instabilities.
