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1. INTRODUCTION This in turn may coincide with variation of transfer integral between Ni eg and O 2p orbitals and allow for electron transfer. The discontinuous change of unit cell volume implies that the phase transition is of first order type for studied RNiO3 compounds (R = Pr, Nd, Sm). This change is mostly apparent due to abrupt decrease of b and c cell parameters at the transition temperature. However, the total linear expansion coefficient is the smallest for b parameter. For Nd and Pr it was possible to estimate that Ni-O bond lenghts as well as the NiO6 tilting suddenly increase at the TMI when compounds become insulating. The structural changes do not evidently impose that any change of symmetry occurs. The value of tolerance factor and superexchange angle θ are directly proportional to each other with an approximate relation (29) ∆θ ≈ 275∆t◦. (1.2) with respect to bond length variation the approximated relation is the follow- ing (29) ∆θ ≈ −275◦(d d2 √2)∆d . (1.3) R−O Ni−O Ni−O The authors of (29) stated that because no symmetry change is observed at the transition the driving force of the structural changes would be triggered by the electronic localization as a reaction to increased Ni-O distance. Application of Valence Bond Model (VBM) and results of crystallographic investigation re- vealed that 3+ valence of R and Ni (low-spin), and 2− of oxygen are a very good approximation of the real valence, with the discrepancy equal to about 2%. The dependence of electronic behaviour with respect to the structure was collected by Torrance et al. in a well-known phase diagram(92). The ∠Ni-O-Ni bond angle increases either with the increasing size of the rare earth ion or with the temperature, that is with the metallicity of the nickelate. As it was said by Torrance et al. this is a general steric effect specific for the structure and found for other distorted perovskites as well. The authors do not account it as a driving reason for the metal-insulator transition, in contrast to V2O3 where the volume change is significant (3.5%). Granados and Obradors (36, 82) suggested a coexistence of both metallic and insulating phase over a wide temperature range in the vicinity of the MI transition. This, as no structure symmetry change was observed, was said to be connected with appearance of strain due to volume change. The differences in growth rate of the two phases appears to influence the electronic behaviour. Obradors per- forming pressure dependent electronic investigation have found an almost linear 8PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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