Crystals of tetracene have been studied by means of lattice phonon Raman spectroscopy as a function of temperature and pressure. Two different phases (polymorphs I and II) have been obtained, depending on sample preparation and history. Polymorph I is the most frequently grown phase, stable at ambient conditions. Application of pressure above 1 GPa yields polymorph II, which is also obtained by cooling the sample below 140 K. However, the conditions for inducing the phase transitions depend on sample preparation and hystory, and polymorph II can also be maintained at ambient conditions. We have calculated the crystallographic structures and phonon frequencies as a function of temperature, starting from the configurations of the energy minima found by exploring the potential energy surface of crystalline tetracene. The spectra calculated for the first and second deepest minima match satisfactorily those measured for polymorphs I and II, respectively. The temperature dependence of the spectra is described correctly. All published x-ray structures, once assigned to the appropriate polymorph, are also reproduced.

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Novel chemistries for secondary batteries are investigated worldwide in order to boost the development of next-generation rechargeable storage systems and especially of lithium-devices. High capacity anode materials for Li-ion cells are at the center stage of R&D in order to improve the performances. In this view, conversion materials are an exciting playground. Among the various proposed class of conversion anodes, metal hydrides are probably the most challenging and promising due to the high theoretical capacities, instability toward the standard carbonate-based electrolytes, large volume variations upon cycling, and moderately low working voltages. Among them lightweight hydrides, like alkaline alanates, are an almost unexplored family of materials. In this study, we present a fundamental study on the electrochemical conversion reaction of sodium alanates: NaAlH4, Na3AlH6, and Na2LiAlH6. Our goal is to improve the understanding of the basic solid-state electrochemistry that drives the conversion reactions of these materials in lithium cells. Samples have been prepared mechanochemically and characterized by X-ray diffraction (XRD), infrared spectroscopy, and transmission electron microscopy. All materials have been assembled in lithium cells with a commercial liquid electrolyte to test their electrochemical activity. The Li incorporation/deincorporation mechanism for all materials has been explored by in situ XRD and interpreted also in view of density functional theory thermodynamic calculations.