Metal-insulator transitions in NdNiO3 thin films

We present a study of the crystallography and transport properties of NdNiO3 thin films, grown by pulsed-laser deposition, on a variety of substrates and with a range of thicknesses. Results highlight the importance of epitaxy, and show that NdNiO3, with a sharp metal-insulator phase transition, can be fabricated without the need for high-pressure processing. The conductivity of the nickelate films was found to be well described by a linear sum of activated transport and Mott's variable range hopping in the entire measured temperature range of the semiconducting state, and this description was also found to provide an accurate fit for previously published transport properties of bulk ceramics. The transition was subsequently modeled using a percolative approach. It was found that the temperature of the metal-insulator phase transition, in both our films and in bulk, corresponded to a critical percolation threshold where the volume fraction of the semiconducting phase (V-s) was 2/3, as expected for a three;dimensional cubic lattice. For the thinnest films grown on NdGaO3, a possible crossover to two-dimensional percolation was indicated by V-s = 1/2.