Variable Temperature In Situ Neutron Powder Diffraction and Conductivity Studies of Undoped HoNbO4 and HoTaO4

Neutron powder diffraction data has been used to quantify the monoclinic (space group I2/a) to tetragonal (I4(1)/a) phase transition that occurs at 775 degrees C in HoNbO4 and 1300 degrees C in HoTaO4. In both cases, deviation from second-order behavior is evident. The LnTaO(4) (Ln = Tb-Er) family of oxides has the potential to adopt one of monoclinic, I2/a or P2/c, structures depending on the synthesis conditions. The monoclinic P2/c polymorph of HoTaO4 undergoes an irreversible first-order phase transition to the high-temperature I4(1)/a scheelite-type structure upon heating, with the monoclinic I2/a phase recovered upon cooling. This is the first direct evidence of this irreversible phase transition and implies a maximum heating temperature to synthesize the P2/c phase for potential ionic conductivity applications. Heating a green powder mixture of Ho2O3 + Ta2O5 revealed a complex series of phase transformations, including the observation of a weberite-type Ho3TaO7 intermediate between 1200 and 1390 degrees C that was not observed upon cooling. Coupled with electrochemical impedance spectroscopy measurements, this diffraction data provides a structural model that explains the higher mobility of charge carriers in LnTaO(4) materials that can be used to identify dopants and improve their ionic conductivity and applicability. Undoped HoNbO4 and HoTaO4 are poor conductors, and the activation energy of tetragonal HoNbO4 is greater than that of the monoclinic polymorphs. Oxygen ion and proton conductivities of the undoped structures occur via interstitial oxygen sites (similar to 10(-6) S cm(-1) at 800 degrees C), providing a potential avenue to improve their application in practical devices such as solid oxide fuel cells.