Synthesis, Characterization and properties of non Molecular solids

The group established by Prof. Alario-Franco back in 1973 has been qualified as "excellent" in 2018 by the Spanish Research Agency. Along this period, the focus has been the synthesis, structure and microstructure of materials in the search of outstanding physical-chemical properties. The main research lines include: study of magnetic/magnetocaloric compounds, superconductors, multiferroics, electrodes and electrolytes of batteries, supercaps and SOFCs. The group benefits from visits of scientists and actively collaborates with other prestigious national/international researchers. Regarding experimental aspects such as the synthesis, the group has expertise in alternative methods, being pioneer in high pressure/high temperature methods, also called "extreme conditions" through the unique "High-Pressure UCM Lab". The group has also pioneered in Spain the microstructural characterization of solids by means of Transmission Electron Microscopy and ancillary techniques along with late Atomic Resolution HRTEM and STEM having the instrumental facilities at the UCM. The group has expertise in transport measurements and frequent access to neutron diffraction and synchrotron facilities is granted. Currently, the group is composed by 21 researchers, including the permanent staff, postdocs and PhD students. The scientific output of the group in the last 5 years includes more than 120 research papers in indexed journals, 3 patents over 500 communications at conferences and 10 PhD Theses.

The crystal structure adopted by a material is strongly dependent on the chemical composition and synthesis conditions. In this sense, high pressure and high temperature syntheses (HPHT) have been widely used not only to prepare new materials but also for inducing phase transformations.In particular, the (HPHT) method is traditionally used to synthesise metastable structures, as it is the case of the highly distorted perovskites and the tetragonal zircon-type (S. G. I41/amd) of RCrO4 oxides (R = rare earth element); transforms into the tetragonal scheelite-type structure (S. G. I41/a). In the case of the perovskites HPHT conditions allow the stabilisation of small cations at the A-perovskite sites, even for chemical compositions where the tolerance factor t is below 0.75. Among the ABO3 systems treated under HPHT conditions, Mn-containing double perovskites have attracted an especial interest during the last years, due to its spin, charge and orbital degrees of freedom, which induce complex orderings featuring their properties. Mn can be accommodated into the A or B sites depending on its oxidation state,. for instance, Mn4+ and Mn3+ (rVI = 0.53 Å and r VI = 0.645 Å, respectively are only stable at the B position, but Mn2+ can be stabilised under high pressure conditions in the highly coordinated A site of the perovskite structure despite its relatively small ionic radius .By combining A and B site orders in AA"BB"O6 given rise to a so called double-double perovskites (DDPv) obtained in . our previous results on the HP synthesis of the stabilisation of small A"site cations has lately motivated the study of Mn2+2RSbO6 DPv polymorphs. Within this family of oxides several types of magnetic and transport properties appear. Research concerning the polymorphism on the RCrO4 phases (R= rare earth) under HPHT conditions have been studied in our research group. We have reported that the ferromagnetic RCrO4-zircon phases show large magnetocaloric effect at low temperatures which make these compounds potential candidates to be used in magnetic refrigeration processes.