Octahedral modifications by Ga doping in NdNiO3 thin films and its systematic influence on electronic transport

NdNiO₃, a perovskite nickelate, exhibits a first-order metal–insulator transition (MIT) with thermal hysteresis. The low temperature insulating behavior is associated with a charge disproportion of Ni²⁺ and Ni⁴⁺ ions resulting from a convergence of Ni³⁺ ions. To understand the effects of isovalent Ga³⁺ doping for Ni³⁺ on the charge distribution and insulating behavior, we prepared thin films of NdNi_1−xGa_xO₃ (x = 0–0.20) on LaAlO₃ (001) (LAO) single crystal substrates using the pulsed laser deposition (PLD) technique. Raman spectra show an emergence of a new phonon mode around 697 cm⁻¹ by just 5% Ga-doping at the Ni-site, which strongly intensifies and shows a red shift with further increase in the Ga-doping. This mode belongs to the breathing vibration of NiO₆ octahedra and remains observable down to a low temperature of 93 K. The NdNiO₃ film shows an MIT around 62 K, and the resistivity of these thin films increases systematically with the increase in Ga doping. However, for x = 0.20, the film shows completely insulating behavior without any transition. These films (x = 0–0.10) show non-Fermi liquid (NFL) behavior in the metallic state, with a temperature exponent n of 4/3. Our results demonstrate that the breathing mode arises by Ga doping, and it enhances the insulating behavior of Ga doped NdNiO₃ thin films.