Variable Temperature In Situ Neutron Powder Diffraction and Conductivity Studies of Undoped HoNbO4and HoTaO4

Neutron powder diffraction data has been used to quantify the monoclinic (space group I2/a) to tetragonal (I41/a) phase transition that occurs at 775 °C in HoNbO4and 1300 °C in HoTaO4. In both cases, deviation from second-order behavior is evident. The LnTaO4(Ln = Tb–Er) family of oxides has the potential to adopt one of monoclinic, I2/aor P2/c, structures depending on the synthesis conditions. The monoclinic P2/cpolymorph of HoTaO4undergoes an irreversible first-order phase transition to the high-temperature I41/ascheelite-type structure upon heating, with the monoclinic I2/aphase recovered upon cooling. This is the first direct evidence of this irreversible phase transition and implies a maximum heating temperature to synthesize the P2/cphase for potential ionic conductivity applications. Heating a green powder mixture of Ho2O3+ Ta2O5revealed a complex series of phase transformations, including the observation of a weberite-type Ho3TaO7intermediate 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 LnTaO4materials that can be used to identify dopants and improve their ionic conductivity and applicability. Undoped HoNbO4and HoTaO4are poor conductors, and the activation energy of tetragonal HoNbO4is greater than that of the monoclinic polymorphs. Oxygen ion and proton conductivities of the undoped structures occur via interstitial oxygen sites (∼10–6S cm–1at 800 °C), providing a potential avenue to improve their application in practical devices such as solid oxide fuel cells.