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1. INTRODUCTION ered to monoclinic P21 space group for heavier RE (Ho, Y, Er, Tm, Yb, Lu), but the distortion is minor and for lighter of the members it is difficult to detect [RT structure - (5, 6, 7, 21, 24, 66), structure at the MIT- (4, 8, 29)]. LaNiO3 compound is the one with the least distortion and its structure is described by R3c rhombohedral space group. The distortion is considered with respect to an ideal cubic perovskite structure of for example CaTiO3 aristotype. The distortion is bound to the relative size of the rare earth ion which, if too small, does not sufficiently fill the space in the middle of the unit cell and the cubic structure is destabilized. Based on that, a measure of distortion was formulated by V. M. Goldschmidt and subsequently called Goldschmidt or tolerance factor t = √dR−O . (1.1) 2dN i−O For a perfect cubic crystal the factor has a value of one. Relative elongations of the unit cell in any direction will result in a change of distance between Ni-O and R-O, and decrease of the tolerance factor. Figure 1.4: Unit cell of RNiO3 crystals - a) perfect cubic perovskite structure (Pm3m), b) Rhombohedral distortion (R3c), c) Orthorhombic distortion (Pbnm) (76) Considering RNiO3 family of compounds it is generally difficult to stabilize the high symmetry structure(41). The compounds belong to a family of so-called Ruddlesden-Popper phases of oxides of An+1MnO3n+1 (or (AO)(AMO3)), where n is natural number. Perfectly stoichiometric RNiO3 compounds will correspond to n = ∞. It is very much connected with the difficulty of stabilizing Ni ions in the 3+ oxidation state and the fact that Ni2+ state is present in NiO and R2NiO4 compounds. This fact will be discussed in subsequent sections. The overview of the crystallographic structure of whole chain of rare earth nickelates can give a little insight into their electronic properties. At the very beginning Lacorre et al. noticed that the size of the rare earth is prominently important considering the metal-insulator transition (66). Lanthanum, the largest 6PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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