Phase competition and excitations in the one-dimensional neutral–ionic transition systems are theoretically studied comprehensively. From the semiclassical treatment of the bosonized Hamiltonian, we examine the competition among the neutral (N), ferroelectric-ionic (Iferro), and paraelectric-ionic (Ipara) states. The phase transitions between them can become first-order when the fluctuation-induced higher-order commensurability potential is considered. In particular, the description of the first-order phase boundary between N and Iferro enables us to analyze N–Iferro domain walls. Soliton excitations in the three phases are described explicitly and their formation energies are evaluated across the phase boundaries. The characters of the soliton and domain-wall excitations are classified in terms of the topological charge and spin. The relevance to the experimental observations in the molecular neutral–ionic transition systems is discussed. We ascribe the pressure-induced crossover in tetrathiafulvalene-p-chloranil (TTF-CA) at a high-temperature region to that from the N to the Ipara state, and discuss its consequence.
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