Two main problems faced by the increasingly technological society are the huge amount of waste that humans generate and the scarcity or criticality of many of the materials used. In this sense, one of the EU's priorities is to promote the transition to a circular economy, where the materials and products manufactured with them are kept in the life cycle as long as possible. Among the strategic materials, Niobium, Tantalum and Rare Earth Elements (REE) are included in the 2020 year list of the 30 critical raw materials of European Union due to their importance for high-tech products and emerging innovations and the risk in the security of supply and economic importance.
In this work, the oxides of two of these strategic materials, Niobium and Tantalum, are recovered from the tailings of the Penouta Sn–Ta–Nb deposit (located in Galicia, Spain) via hydrometallurgical route [1,2]. The recovered oxides have been used to obtain micro- and nanostructures by a simple thermal evaporation method. The structures have been characterized by means of X-ray diffraction, scanning electron microscopy, luminescence and Raman spectroscopy. The possibility of using these micro- and nanostructures for optical, sensing and energy storage applications will be discussed and related to the crystal structure of the oxides obtained [3,4].
 F. A. López, et al. (2018) Minerals 8, 20.
 O. Rodríguez et al. (2020) RSC Adv., 10, 21406-21412
 C. Nico, et al. (2016) Progress in Materials Science 80, 1-37.
 S. Xia, et al. (2018) Nano Energy 45, 407-412.