Perovskite rare-earth nickelates in the thin-film epitaxial state

We have succeeded in the preparation of thin films of rare-earth nickelates RNiO3 (R = Pr, Nd, Sm, and Gd) under reduced oxygen pressure < 0.02 bar by metalorganic chemical-vapor deposition owing to their epitaxial stabilization on perovskite substrates. The film-substrate lattice mismatch is critical for the epitaxial stabilization of RNiO3 phases. Increase of the lattice mismatch or film thickness results in the deposition of rare-earth oxides and NiO instead of RNiO3. The epitaxial films of nickelates were strained and consisted of 90 degrees domains with the orthorhombic Pnma structure. The transport properties of the strained films on LaAlO3 were similar to those of the bulk material of the same composition under applied pressure of 9 kbar but they were different from the properties of the bulk material under ambient pressure. The result implies that transport properties of RNiO3 films with sharp metal-to-insulator transition can be effectively tuned by the control of the lattice strain. (C) 2000 American Institute of Physics. [S0003-6951(00)02715-7].