Abstract
In this work, multifunctional oxide NdNiO3 (NNO) thin films grown on a SrTiO3 (STO) substrate using pulsed-laser deposition are studied. Temperature dependent resistivity measurements revealed that NNO/STO samples exhibit a sharp thickness dependent metal-insulator transition (MIT) over a range of 150-200 K. It is known that the electronic properties of correlated oxides are extremely complex and sensitive to changes in orbital occupancy. To evaluate the changes in the electronic and/or crystallographic structure responsible for the MIT, a site-selective (O, Ni and Nd) X-ray absorption near edge structure (XANES) analysis is performed above and below the transition temperature. Analysis of XANES spectra suggests that: (i) in NNO films nominally trivalent Ni ions exhibit multiple valency (bond disproportionation), (ii) intermetallic hybridization plays an important role, (iii) the presence of strong O 2p-O 2p hole correlation at low temperature results in the opening of the p-p gap and (iv) the valency of Nd ions matches well with that of Nd3+. For NNO films exhibiting a sharp MIT, Ni 3d electron localization and concurrent existence of Ni 3d8 and Ni 3d8L[combining low line]2 states are responsible for the observed transition. At temperatures below the MIT the O 2p-O 2p hole correlation is strong enough to split the O 2p band stabilizing insulating phase. Temperature and thickness dependent differences observed in the site-selective XANES data are discussed in terms of possible mechanisms for the MIT (negative charge-transfer type).
Highlights
Rare earth (R) nickel perovskites (RNiO3) undergo a first-order metal to insulator transition (MIT) as the temperature decreases.[1,2] Bulk NdNiO3 (NNO), for example, exhibits the metal–insulator transition (MIT) at ∼200 K.3 Traditionally, the origin of the MIT was strongly linked to structural modifications by the lowering of the symmetry from orthorhombic to monoclinic,[4] charge ordering[5] and a complex antiferromagnetic state.[6]
XRD data verify that the 200 nm NNO film is completely relaxed; we can consider the 200 nm NNO film to be a representative of bulk NNO
For NNO films grown on a STO substrate, the sharpness of the MIT in this work is comparable with the data reported earlier
Summary
Rare earth (R) nickel perovskites (RNiO3) undergo a first-order metal to insulator transition (MIT) as the temperature decreases.[1,2] Bulk NdNiO3 (NNO), for example, exhibits the MIT at ∼200 K.3 Traditionally, the origin of the MIT was strongly linked to structural modifications by the lowering of the symmetry from orthorhombic to monoclinic,[4] charge ordering[5] and a complex antiferromagnetic state.[6]. Rare earth (R) nickel perovskites (RNiO3) undergo a first-order metal to insulator transition (MIT) as the temperature decreases.[1,2] Bulk NdNiO3 (NNO), for example, exhibits the MIT at ∼200 K.3. The origin of the MIT was strongly linked to structural modifications by the lowering of the symmetry from orthorhombic to monoclinic,[4] charge ordering[5] and a complex antiferromagnetic state.[6] RNiO3 thin films grown on bulk substrates are even more complex systems as their properties are affected by the heterointerface formed, 4f electrons, localized and hybridized, with the other valence electrons (4f band) can be present in the neodymium nicke-. Temperature and thickness dependent analyses of XANES data are able to provide the information needed to understand mechanisms responsible for the MIT in NNO films. The strength of XANES analysis has been proved to reveal new phenomena at the heterointerface in the LaAlO3/SrTiO3 system.[18]
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