Strain-driven disproportionation at a correlated oxide metal-insulator transition

Abstract

Metal-to-insulator phase transitions in complex oxide thin films are exciting phenomena which may be useful for device applications, but in many cases the physical mechanism responsible for the transition is not fully understood. Here we demonstrate that epitaxial strain generates local disproportionation of the NiO6 octahedra, driven through changes in the oxygen stoichiometry, and that this directly modifies the metal-to-insulator phase transition in epitaxial (001) NdNiO3 thin films. Theoretically, we predict that the Ni-O-Ni bond angle decreases, while octahedral tilt and local disproportionation of the NiO6 octahedra increases resulting in a small band gap in otherwise metallic system. This is driven by an increase in oxygen vacancy concentration in the rare-earth nickelates with increasing in-plane biaxial tensile strain. Experimentally, we find an increase in pseudocubic unit-cell volume and resistivity with increasing biaxial tensile strain, corroborating our theoretical predictions. With electron energy loss spectroscopy and xray absorption, we find a reduction of the Ni valence with increasing tensile strain. These results indicate that epitaxial strain modifies the oxygen stoichiometry of rare-earth perovskite thin films and through this mechanism affect the metal-to-insulator phase transition in these compounds. PACS numbers: 68.55.Ln, 68.60.Bs, 73.50.-h, 73.20.-r 3 Metal-to-insulator phase transitions (MITs) in strongly-correlated electronic systems are fascinating phenomena which have attracted significant attention for decades [1]. Among complex oxide materials which exhibit MITs are rare-earth nickelates having the generic formula RNiO3, where the rare-earth element (R) is smaller than lanthanum, i.e. R = Pr, Nd ... [2]. The critical temperature of the MIT is dependent on the Ni-O-Ni bond angle: straightening the angle with a larger R cation stabilizes the metallic state over the insulating state and lowers the transition temperature [3-5]. For example, the MIT temperatures in bulk NdNiO3 and SmNiO3 (Ni-O-Ni bond angles of 157.1 and 153.4°, respectively) have been reported to be approximately 200 and 400 K, respectively. It should be also emphasized that a breathing order by disproportionation of the Ni-O bond length plays a crucial role in the MIT [6-10].In RNiO3 thin films, misfit strain arising from a lattice mismatch between the film layer and the underlying substrate affects the lattice volume, electrical conductivity and MIT temperature [11][12][13][14][15][16][17][18]. In particular, films under in-plane tensile strain are more insulating compared to those under in-plane compressive strain. The origin of this interesting phenomenon remains unclear, although a number of mechanisms have been proposed [19][20][21][22][23][24].We also note that the effect of oxygen non-stoichiomety on the MIT has been reported in bulk RNiO3 [25,26].It is widely accepted that epitaxial strain in transition metal oxide films can be accommodated through the formation of oxygen vacancy defects, resulting in offstoi...

Doping evolution of the Mott–Hubbard landscape in infinite-layer nickelates

“…More direct interpretation of these spectral changes will require comparison with careful and thorough calculations. Similar to the O-K edge, the spectral differences between bulk and thin film NdNiO3 in the L2,3 are likely due to strain effects imparted by the substrate (46). Unlike the Ni-L2,3 and O-K, the Nd-M4,5 edge (4f states) remains unchanged upon reduction from perovskite to infinite-layer ( Fig.…”

Spatially Controlled Octahedral Rotations and Metal–Insulator Transitions in Nickelate Superlattices

“…This can also contribute to the enhanced resistivity and T MIT by creating local disproportionation of NiO 6 octahedra. 45 The n = 2 SL exhibits insulating behavior in the entire temperature range measured. The temperature dependent resistivity curve can be well fitted with the two-dimensional variable range hopping conduction model ( Figure S6 ), attesting to the dimensional crossover.…”

Anisotropic chemical expansion due to oxygen vacancies in perovskite films

“…This strain leads to unusual phases and functional responses, not available in bulk prototypes 3 – 5 . Additionally, first-principles calculations showed that strain can reduce the formation energy for oxygen vacancies in epitaxial films, where, in turn, the vacancies can lead to significant changes of electronic properties 6 – 10 . It is worth noting that the majority of thin-film growth techniques allow for varying oxygen content 1 .…”

“…Anisotropic lattice distortions around oxygen vacancies are inherent to many other AB O 3 perovskites 6 – 10 , 18 , 19 . This general property points to the possible universal presence of vacancy-misfit interactions in epitaxial AB O 3 -type films.…”

“…NNO is an A 3+ B 3+ O3-type perovskite, whose charge transport properties and metal–insulator transition attract much interest. Oxygen vacancies are believed to drive the NNO properties towards the insulating state 9 , 10 . Similar to the Ti-Vo elongation compared to the Ti–O distance in SrTiO 3 , the Ni-V O distance is longer than the Ni–O distance in NNO 10 .…”