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 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.