We report a study of the structural phase transitions induced by pressure in bulk black phosphorus (BP) using both synchrotron x-ray diffraction measurements up to 12.2 GPa and Raman spectroscopy up to 18.2 GPa . Since the 80's it is known that on increasing pressure this layered system undergoes two different phase transitions, in particular from a rhombohedral semimetallic phase to simple cubic metallic phase around 10 GPa . Very recently, BP attracted renewed attention since monolayer to few-layers samples (phosphorene) very appealing for applications can be readily prepared, but some basic questions are still open in the case of the bulk system. In particular, previous and present Raman measurements show that different structures can coexist, and that a broad and intense Raman spectrum can be observed above 10 GPa [1,3], while no Raman components are expected in an elemental simple cubic system. Deviations from the ideal hydrostatic conditions which may be occuring above 10 GPa may lead to the simple cubic phase of the BP to adopt a locally non-cubic structure. Such a scenario can explain the observation of two board Raman components entered around ~250 cm-1 in the pressure range 14-18 GPa.
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