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1. INTRODUCTION where x(O) and z(O2) are coordinates of oxygen atoms at positions (x,0,1/4) (xyz) and O2 oxygen atom at position (x, y, z), respectively (3). They have also noticed a progressive increase of Ni-O distance from La to Sm in contradiction to decreasing unit cell volume. Their interpretation of the effect was connected with the charge that is ’kept’ in a more covalent bonding of R-O, which would decrease the amount of charge between Ni-O and weaken (lengthen) the bond. Whereas for smaller rare earths, down to Ho, the accompanying reduction of unit cell volume influences the bondlengths more and forces a decrease of Ni- O distance. Using the Brown’s valence bond model one can estimate whether there is a correspondence between the Eu-O bond lengths and the octahedra tilting. This phenomenological model describes correlation between bond-length and a corresponding formal valence. In nonstrained crystals the rule of Valence Bond Sum (VBS) is satisfied, i.e. the formal charge of an ion is equal to the sum of the bond valences around this ion. When the stress that is introduced by coexistence of various structural units is not relieved, the VBS rule is a measure of stress existing in the structural bonds. The model enabled to estimate that Ni cations are slightly underbonded and under tensile stress. Whereas Eu ions are overbonded and under compressive stress, which causes the structure to be metastable. Brown and Shannon have formulated the so-called distortion theorem which states that deviation from of the individual bond-lengths in a coordination sphere, in which the average bondlength is kept constant, will increase the valence sum at the central ion (3). Following the theorem, with any Ni-O bond length change with respect to the average value a distortion of NiO6 octahedra will be caused due to the trend of underbonded Ni cations to increase their valence. In a rather different approach Frand et al. prepared a series of nickelates with mixture of neodymium and samarium in varying ratio (25). This resulted in ob- taining a series of nickelates with monotonically decreasing metal-insulator tran- sition temperature. This indicates that macroscopic variation of the compounds structure (proper distribution of particles or domains of constitutive compounds) may influence the transition temperature and electronic properties. The group also investigated dependence of the properties on grinding time of a chosen com- pound. They noticed that with the increase of grinding time the phase transition was less pronounced (investigated by DSC measurements) and completely disap- peared in the amorphous sample. Annealing in synthesis conditions of the sample ”restored” the transition however moderate temperature (400 ◦C for 16 hours) does not restore the transition. 10PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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