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1.2 Physical properties of the RNiO3 system Influence of structure on electronic properties was also evidenced by room tem- perature X-ray diffraction measurements. When increasing x in the Sm1−xNdxNiO3 series there appears a discontinuous drop in lattice parameters values at x = 0.45, and thus unit cell volume. The doping level of x = 0.45 is the boundary that di- vides metallic and insulating Sm1−xNdxNiO3 compounds. This variation in a way reflects the temperature dependence of lattice parameters of other nickelates. No change of crystallographic structure has been observed in case of PrNiO3 up to 4.7 kbar hydrostatic pressure (77). A step-like characteristics of Ni-O distance and Ni-O-Ni superexchange angle was observed with respect to temperature, with the step appearing at the MIT temperature. Reentrant metallic behaviour at low temperatures observed by other groups was suggested by Medarde to be an effect of incomplete relaxation of the system due to different experimental conditions. The crystallinity of the compounds is highest when a high oxygen pressure annealing is used as a part of synthesis in order to stabilize the perovskite phase (2). Although the authors noted that other techniques using atmospheric pressure can produce nickelates with poorer crystallinity. The group prepared Sr/Th-doped La and Nd nickelates. Incorporation of larger and smaller than La ions into the La nickelate lattice surprisingly resulted in compression and expansion of the unit cell. This was explained by increase/decrease of mean oxidation state of nickel and thus a decrease/increase (opposite) change of Ni-O distance. On the other hand in Nd nickelates the variation of lattice parameters and unit cell volume is non-monotonic which was interpreted as a result of interplay between steric (due to size difference between R and A) and electronic factors (due to variation of Ni-O distances). In Nd0.95Sr0.05NiO3 the NiO6 octahedra are flattened in the ab plane due to contraction of Ni-O2ii (by 0.017 ̊A) and expansion of Ni-O2i distances. At the same time the in-plane Ni-O-Ni angle increases. In Nd0.95Th0.05NiO3 the octahe- dra are flattened as well however the variation of the Ni-O distances is opposite (2). Studies done on bulk NdNiO3−δ with a variable oxygen deficiency δ allowed an observation of the non-stoichiometry influence on Ni-O-Ni bond angles (91). The sol-gel prepared single phase orthorhombic polycrystalline samples had variation in oxygen deficiency from 0.08 to 0.22. The Ni-O-Ni bond angles φ and θ estimated by Hayashi method were found to decrease with increasing oxygen deficiency (see table 1.3 and figure 1.7). Hayashi formula for θ: θ = 135 + 13.4(θ0 − 135), (1.6) where cosθ0 = 1 − a2/b2, and a and b are the orthorhombic lattice parameters. 11PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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