Long-term structural surface modifications of mixed conducting La2NiO4+δ at high temperatures

Long-term annealing of La2NiO4+delta single crystals at 1,273 K in air leads to the formation of nickel-rich Ruddlesden-Popper phases at the single-crystal surfaces. Both the composition and the morphology of these phases depend on the surface orientation; whereas only crystallites of La4Ni3O10-delta were observed on (001) surfaces, both La3Ni2O7-delta and La4Ni3O10-delta were formed on (100) surfaces. The formation of the nickel-rich RP phases is believed to be due to surface segregation of nickel or evaporation of a volatile lanthanum species. The crystallographic (001) planes inside the La3Ni2O7-delta and La4Ni3O10-delta crystallites were found to be oriented in the direction of preferential crystallite growth, which indicates that the diffusion of lanthanum and nickel cations is faster along the crystallographic (001) planes than perpendicular to these planes.