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1.2 Physical properties of the RNiO3 system Figure 1.5: Unit cell structure of selected Ruddlesden-Popper phases - Structure for the Lan+1NinO3n+1 compounds with for n=1, 2, 3 and infinity. Small solid circles correspond to Ni, while medium solid circles to La and open circles to oxygen ions (41) rare earth, fills the space between eight corner-sharing NiO6 octahedra in such a way that the distortion is minor, t=0.987 (66), and the structure is rhombohedral. Smaller rare earth ions follow the principle that the smaller the radius, the larger the distortion. It is due to the fact that the smaller R3+ cations are not able to sufficiently stabilize the regular configuration of the octahedra which therefore distort in various ways. It results in decreased electronic overlap of nickel ions and increase in the metal-insulator transition temperature. This simple picture was the basis for the investigation of the properties of nickelates. At the electronic transition there is a slight change of lattice parameters which results in 0.2-0.25% decrease in volume of metallic PrNiO3 unit cell (29, 66). For samarium nickelate the change was estimated to be about 0.15%. Detailed neu- tron powder diffraction and X-ray diffraction studies of R=Pr, Nd, Sm nickelates revealed that the distortion from cubic structure is mainly related to NiO6 oc- tahedra that tilt at electron localization (29). The octahedra were found to tilt around b and c orthorhombic axes (a0b+c+) (29) which leads to variation of the angles α = ∠Ni − O1 − Ni and β = ∠Ni − O2 − Ni (the superexchange angles). 7PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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