  We provide a general thermodynamic framework for the understanding of
guest-induced structural transitions in hybrid organic-inorganic materials. The
method is based on the analysis of experimental adsorption isotherms. It allows
the determination of the free energy differences between host structures
involved in guest-induced transitions, especially hard to obtain
experimentally. We discuss the general case of adsorption in flexible materials
and show how a few key quantities, such as pore volumes and adsorption
affinities, entirely determine the phenomenology of adsorption, including the
occurrence of structural transitions. On the basis of adsorption
thermodynamics, we then propose a taxonomy of guest-induced structural phase
transitions and the corresponding isotherms. In particular, we derive generic
conditions for observing a double structural transition upon adsorption, often
resulting in a two-step isotherm. Finally, we show the wide applicability and
the robustness of the model through three case studies of topical hybrid
organic-inorganic frameworks: the hysteretic hydrogen adsorption in
Co(1,4-benzenedipyrazolate), the guest-dependent gate-opening in
Cu(4,4'-bipyridine)(2,5-dihydroxybenzoate)2 and the CO2-induced "breathing" of
hybrid material MIL-53.
