NMR Studies of Phase Transitions
Résumé
Understanding the mechanisms of transformations between different polymorphs, that is between phases of different symmetries at constant atomic and molecular composition, has always been the touchstone of modern research. In addition to structural methods, nuclear magnetic resonance (NMR) always brought complementary informations of primary importance for the study of phase transitions. For solid matter, the improved high-field NMR resolution and sensitivity, combined with technical advances in probes as well as progresses in NMR methodology and ab initio methods, give new opportunities in the study of phase transitions. For instance, one of the most modern breakthroughs in modern solid-state NMR concerns the quasiroutine study of quadrupolar nuclei, reflected by the huge amount of publications per year. This work is intended to present a pedagogical review of modern solid-state NMR studies of solid–solid phase transitions. This review will not be exhaustive, but rather focuses on some contributions which mentions methods that can be used or tried when a phase transition has to be studied or evidenced by NMR. Nevertherless, the most recent results obtained with modern NMR are reviewed, and about 300 articles over the period 2000–2005 are reported. After a general and pedagogical introduction of the different solid-state NMR signatures of phase transitions, and the experimental cautions, the different concepts of NMR with applications to phase transitions are illustrated. The interplays between continuous- and discontinuous-phase transitions, order–disorder or displacive models, pretransitional effects, coexistence, phase diagrams and NMR parameters (lineshape, inhomogeneous or homogeneous broadening, relaxation, etc.) that permit distinction between different types of disorders, dynamics and slow motions are considered. The examples cover molecular crystals, ferroelectrics, intergrowth compounds, H-bonded systems, etc. We also tried to emphasize, through the choice of the examples, the complement informations between NMR, X-ray diffraction and incoherent neutron scattering. Although this review deals primarily with solid–solid structural phase transitions, almost all the concepts introduced and reported can be directly applied to soft and liquid-like matter such as polymers, liquid crystals or micellar systems.