The systematic study of the metal-to-insulator (MI) transitions in the RNiO3 (R= rare earths) perovskites has constituted a central theme in our research. They find aplication in intelligent windows, since the metal state is not transparent to IR photons. The MI temperature can be tuned with the size of the R3+ ion.
MI transitions have been interpreted as a function of the reduction and closing of the charge-transfer gap between O2- and Ni3+ ions. The difficulty of preparation of these phases, associated with the presence of trivalent nickel, increases as the lanthanide radius diminishes, in such a way that the mentioned transport properties (and MI transitions) had only been investigated for R between La to Eu.
One of the most relevant results in precedent years was the preparation of the phases for R= Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, which had not been prepared since the pioneering work by Démazeau in 1971. In our laboratory, the full series of RNiO3 perovskites could be stabilized under moderate pressures of 2 GPa, in sufficiently large amounts to perform accurate neutron and synchrotron diffraction studies across the transitions.
For YNiO3 we described, for the first time, a charge disproportionation associated with the MI transition. In the crystal structure Ni atoms occupy two independent crystallographic sites with slightly different charge, 3+d and 3-d.
This effect is extensive to other small rare-earth members, from R= Ho to R= Lu. Recent results concern: i) high-pressure studies across the MI transitions by different techniques, showing a metallization of the high-T phase for LuNiO3; ii) Mössbauer results in 57Fe-doped Ni perovskites combined with neutron studies offering an additional evidence of the charge disproportionation in RNiO3 (R= Sm, Eu, Gd, Dy); iii) new features of the metallic state in NdNiO3 ; iv) the resolution of the magnetic structure of DyNiO3, v) the observation, for the first time, of magnetic peaks by synchrotron soft x-ray resonant magnetic powder diffraction and iv) the study of the evolution of the charge transfer between Ni3+d and Ni3-d for the whole RNiO3 series from x-ray absorption data.
Evidence of kinetically arrested supercooled phases in the perovskite oxide NdNiO3, Kumar, D Rajeev, KP, Alonso, JA, Martinez-Lope, MJ, JOURNAL OF PHYSICS-CONDENSED MATTER21, 485402 (2009)
On the magnetic structure of DyNiO3, Munoz, A, Alonso, JA, Martinez-Lope, MJ, Fernandez-Diaz, MT, JOURNAL OF SOLID STATE CHEMISTRY182, 1982-1989(2009).
Charge disproportionation in RNiO3 perovskites (R=rare earth) from high-resolution x-ray absorption spectroscopy, Medarde, M, Dallera, Cm,Grioni, M, Delley, B, Vernay, F, Mesot, J, Sikora, M, Alonso, JA, Martinez-Lope, MJ, PHYSICAL REVIEW B 80, 245105 (2009)
Pressure dependence of metal-insulator transition in perovskites RniO3 (R= Eu, Y, Lu). Cheng J.G., Zhou J.S., Goodenough J.B., Alonso J.A., Martinrz-Lope M.J, PHYSICAL REVIEW B 82, 085107 (2010)
Magnetic and structural features of the NdNi(1-x)Mn(x)O3 perovskite series investigated by neutron diffraction , Sanchez-Benitez, J.; Martinez-Lope, M. J.; Alonso, J. A.; Garcia-Munoz, J. L., JOURNAL OF PHYSICS-CONDENSED MATTER 23, 226001 (2011)