Neutron diffraction study of the monoclinic to tetragonal structural transition in LaNbO4 and its relation to proton mobility

The fergusonite-scheelite structural transition of LaNbO4 was characterized by high-intensity variable temperature neutron powder diffraction in an effort to link structural changes to proton transport in the low temperature monoclinic and high temperature tetragonal polymorphs. The pronounced decrease in enthalpy of proton mobility with increasing temperature in monoclinic LaNbO4, identified by Fjeld et al. [1], was found to coincide with the decrease in a key inter-tetrahedral oxygen separation. At temperatures above the transition, this oxygen to oxygen distance remains constant, behavior that is consistent with the invariant nature of the calculated enthalpy of mobility for the tetragonal phase. This oxygen separation is therefore proposed as the key structural parameter determining the success rate of proton transfer and ultimately the proton diffusion in LaNbO4. Infrared measurements performed on acceptor-doped LaNbO4 show an absorption in the region 2500 to 3700 cm−1 attributed to the O–H stretching motion influenced by hydrogen bonding.

Graphical AbstractThe structural phase transition from monoclinic fergusonite to tetragonal scheelite crystal structure clearly influences the proton conductivity of acceptor-doped LaNbO4 (see Figure). The present article attempts to explain why. Figure optionsDownload as PowerPoint slideHighlights
► Transition from monoclinic to tetragonal structure studied using neutron diffraction.
► Change in proton mobility linked to contraction of a key oxygen separation.
► Infra-red spectra reveal intermediate strength hydrogen bonding.