Neutron diffraction study of the monoclinic to tetragonal structural transition in LaNbO{sub 4} and its relation to proton mobility

The fergusonite-scheelite structural transition of LaNbO{sub 4} 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...

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Bibliographic Details
Published in:Journal of solid state chemistry 2012-03, Vol.187
Main Authors: Huse, M., Skilbred, A.W.B., Karlsson, M., Eriksson, S.G., Norby, T., Haugsrud, R., Knee, C.S.
Format: Article
Language:English
Subjects:
ABSORPTION
BONDING
CONTRACTION
DIFFUSION
DOPED MATERIALS
ELECTROLYTES
ENTHALPY
FUEL CELLS
HYDROGEN
INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY
MOBILITY
MONOCLINIC LATTICES
NEUTRON DIFFRACTION
NEUTRON TEMPERATURE
OXYGEN
PHASE TRANSFORMATIONS
POWDERS
PROTON CONDUCTIVITY
PROTON TRANSPORT
PROTONS
SPECTRA
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Summary:The fergusonite-scheelite structural transition of LaNbO{sub 4} 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 LaNbO{sub 4}, identified by Fjeld et al. , 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 LaNbO{sub 4}. Infrared measurements performed on acceptor-doped LaNbO{sub 4} show an absorption in the region 2500 to 3700 cm{sup -1} attributed to the O-H stretching motion influenced by hydrogen bonding. - Graphical Abstract: The structural phase transition from monoclinic fergusonite to tetragonal scheelite crystal structure clearly influences the proton conductivity of acceptor-doped LaNbO{sub 4} (see Figure). The present article attempts to explain why. Highlights: Black-Right-Pointing-Pointer Transition from monoclinic to tetragonal structure studied using neutron diffraction. Black-Right-Pointing-Pointer Change in proton mobility linked to contraction of a key oxygen separation. Black-Right-Pointing-Pointer Infra-red spectra reveal intermediate strength hydrogen bonding.
ISSN:0022-4596
1095-726X
DOI:10.1016/J.JSSC.2011.12.025