Variable Temperature In Situ Neutron Powder Diffraction and Conductivity Studies of Undoped HoNbO<sub>4</sub> and HoTaO<sub>4</sub>
Bryce G. Mullens, Matilde Saura‐Múzquiz, Giulio Cordaro, Frederick P. Marlton, Helen E. Maynard‐Casely, Zhaoming Zhang, Gianguido Baldinozzi, Brendan J. Kennedy
Abstract
Neutron powder diffraction data has been used to quantify the monoclinic (space group I 2/ a ) to tetragonal ( I 4 1 / a ) phase transition that occurs at 775 °C in HoNbO 4 and 1300 °C in HoTaO 4 . 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, I 2/ a or P 2/ c, structures depending on the synthesis conditions. The monoclinic P 2/ c polymorph of HoTaO 4 undergoes an irreversible first-order phase transition to the high-temperature I 4 1 / a scheelite-type structure upon heating, with the monoclinic I 2/ 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 P 2/ c phase for potential ionic conductivity applications. Heating a green powder mixture of Ho 2 O 3 + Ta 2 O 5 revealed a complex series of phase transformations, including the observation of a weberite-type Ho 3 TaO 7 intermediate between 1200 and 1390 °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 HoNbO 4 and HoTaO 4 are poor conductors, and the activation energy of tetragonal HoNbO 4 is greater than that of the monoclinic polymorphs. Oxygen ion and proton conductivities of the undoped structures occur via interstitial oxygen sites (∼10 –6 S cm –1 at 800 °C), providing a potential avenue to improve their application in practical devices such as solid oxide fuel cells.