Effect of lanthanides on phase transformation and structural properties of LnNbO4 and LnTaO4 thin films

Abstract Lanthanide niobate LnNbO4 (LnNO) and tantalate LnTaO4 (LnTO) thin films (~ 100 nm) were prepared by sol-gel/spin-coating process on PZT/Al2O3 substrates and annealing at 1000 °C (Ln = Nd, Sm, Eu and Gd). The LnNO (NNO, SNO, ENO and GNO) and LnTO (NTO, STO, ETO and GTO) precursors of films were synthesized using Nb or Ta tartrate complexes. The different phase transformations were identified in LnNO or LnTO precursors from fluorite T′-structure via tetragonal T-LnNbO4 (NNO, SNO and GNO) or T-EuNb5O14 and T-NdTaO4, T-SmTa7O19, hexagonal H-EuTa7O19 and cubic C-GdTa7O19 to monoclinic M-LnNbO4 or M′-LnTaO4 during annealing at 700–1200 °C. The XRD results of LnNO or LnTO films confirmed the coexistence of monoclinic LnNbO4 or LnTaO4 phase in addition to the tetragonal as main phase at 1000 °C. In STO film, the single monoclinic M′-SmTaO4 phase was revealed. The surface morphology and topography of thin films were investigated by the SEM and AFM analysis. STO film was smoother with roughness of 3.23 nm in comparison with SNO (6.35 nm). Effect of lanthanides can contribute to the formation different polymorphs of these films for the application to environmental electrolytic thin film devices.