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Here, we propose an extension of the diabatic approach to photo-induced phase transitions and suggest three states model, which mimics a system with three local diabatic states or equivalently two states where one of them is unstable against a nonsymmetric distortion. The model allows to discuss symmetry breaking transformations and is intended to describe photo-induced transformations in molecular systems, where a path of structural relaxation involves two coordinates. The three states are mapped into a minimum version of Ising spin-1 model, which exhibits metastability. Conditions for switching between stable and metastable phases are discussed in terms of a competition between crystal field, changed by illumination, and dipolar coupling. Examples are neutral (N) to ionic (I) transformation in mixed-stack charge-transfer system and low-temperature photo-induced spin-crossover transformation in metal complexes.

Abstract. This paper presents different aspects of x-ray diffraction techniques for material science: investigation of symmetry breaking, electron density analysis, diffuse scattering, aperiodic systems and time-resolved experiments.

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Mixed-stack charge-transfer (CT) complexes undergoing the neutral-ionic (N-I) phase transition are molecular materials formed of stacks where electron donor (D) and acceptor (A) molecules regularly alternate. In the N phase, the CT is low and molecules are situated on inversion centers, while in the I phase, the increase of CT is accompanied by a lattice distortion (dimerization process and symmetry breaking). The one-dimensional (1D) architecture triggers the chain multistability by stabilizing lattice-relaxed (LR)-CT excitations ...D° A° D° A° $(D^+A^-)(D^+A^-)(D^+A^-)$ Do A" D° A° D°... These 1D nano-scale objects are at the heart of the equilibrium N-I phase transition and govern the fascinating physical properties such as giant dielectric response or photo-induced phase transformations. In this contribution, the 1D character of these critical excitations will be demonstrated by direct observation using high resolution X-Ray diffraction.