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Fil: Jeanneau, Justin. Universite Grenoble Alpes; Francia. Centre National de la Recherche Scientifique; Francia

We perform an extensive study of Sr_{3}Cr_{2}O_{7}, the n=2 member of the Ruddlesden-Popper Sr_{n+1}Cr_{n}O_{3n+1} system. An antiferromagnetic ordering is clearly visible in the magnetization and the specific heat, which yields a huge transition entropy, Rln(6). By neutron diffraction as a function of temperature we have determined the antiferromagnetic structure that coincides with the one obtained from density functional theory calculations. It is accompanied by anomalous asymmetric distortions of the CrO_{6} octahedra. Strong coupling and Lanczos calculations on a derived Kugel-Khomskii Hamiltonian yield a simultaneous orbital and moment ordering. Our results favor an exotic ordered phase of orbital singlets not originated by frustration.

We show in this Letter that heating under high pressure drives ${\mathrm{C}}_{60}$ to new distorted crystalline phases that are metastable at room temperature and pressure. We report three different distortions that are essentially characterized by two nearest neighbor distances, ${d}_{0}^{\ensuremath{'}}\ensuremath{\sim}10$ and ${d}_{0}^{\ensuremath{'}\ensuremath{'}}\ensuremath{\sim}9.2$ \AA{}. The excellent accord with theoretical calculations supports the view that these new carbon phases can be understood as the long range order polymerization of ${\mathrm{C}}_{60}$ through cycloaddition reactions that are at the origin of the shorter intermolecular distances.