Strongly emerging octahedral modifications with Al doping and its influence on the electronic transport in NdNiO₃ thin films

The temperature-dependent electronic transport and the metal-insulator transition in ABO₃perovskite-type NdNiO₃are highly sensitive to chemical substitutions and lattice-mismatch induced strain in thin films. We synthesized two series of NdNi_1-<i>x</i>Al<i>_x</i>O₃(<i>x</i>= 0-0.50) thin films on LAO (001) single crystal substrates using the pulsed laser deposition technique: one with varied Al doping level (<i>x</i>= 0-0.50) and the other with fixed doping (<i>x</i>= 0.5) but different thickness (10-200 nm). Substituting Al at Ni-site modifies the Ni oxidation state, as observed by x-ray photoelectron spectroscopy. A Raman mode around 693 cm^-1emerges with just 2% Al-doping at the Ni-site, which is otherwise absent in undoped NdNiO₃film. This mode is associated with an anti-stretching of BO₆octahedra in perovskites. With increasing doping, this mode strongly emerges in a dominating manner in the spectra, clearly indicating a further enhancement of octahedral anti-stretching. This mode exhibits a blue shift with increasing Al doping and a red shift with increasing thickness at<i>x</i>= 0.50. Temperature-dependent Raman spectra, analyzed using Balkanski and Grüneisen models, reveal anharmonicity and Red-shifting behaviors. The undoped film shows a metal-to-insulator transition temperature (<i>T</i>_MI) of ∼77 K, which systematically increases with greater anti-stretching of octahedra and Al doping. At higher doping levels (<i>x</i>= 0.20 and 0.50), the films show insulating behavior below room temperature. As the thickness of NdNi_0.50Al_0.50O₃films varies from 10 nm to 200 nm, the anti-stretching mode got affected, resulting in systematic decrease in resistivity. Our findings confirm that the anti-stretching mode arises by Al doping, and it is associated with the insulating behavior of Al-doped NdNiO₃thin films.