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 HoNbO₄ and 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/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 I4₁/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 °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 LnTaO₄ 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 (∼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.

中文翻译：

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未掺杂 HoNbO4 和 HoTaO4 的变温原位中子粉末衍射和电导率研究


中子粉末衍射数据已用于量化 HoNbO ₄ 中 775 °C 时发生的单斜晶（空间群 I2/a）到四方晶（I4 ₁ /a）的相变，以及1300 °C，HoTaO ₄ 。在这两种情况下，二阶行为的偏差都是显而易见的。 LnTaO ₄ (Ln = Tb–Er) 系列氧化物有可能采用单斜晶系、I2/a 或 P2/c 结构之一，具体取决于合成条件。 HoTaO ₄ 的单斜晶系 P2/c 多晶型在加热时会发生不可逆的一级相变，转变为高温 I4 ₁ /白钨矿型结构，其中单斜晶系 I2/冷却后恢复相。这是这种不可逆相变的第一个直接证据，并且暗示了合成 P2/c 相用于潜在离子电导率应用的最高加热温度。加热 Ho ₂ O ₃ + Ta ₂ O ₅ 的绿色粉末混合物揭示了一系列复杂的相变，包括观察1200 至 1390 °C 之间存在韦氏石型 Ho ₃ TaO ₇ 中间体，在冷却时未观察到。结合电化学阻抗谱测量，该衍射数据提供了一个结构模型，解释了 LnTaO ₄ 材料中载流子的较高迁移率，可用于识别掺杂剂并提高其离子电导率和适用性。未掺杂的HoNbO ₄ 和HoTaO ₄ 是不良导体，四方HoNbO ₄ 的活化能大于单斜晶型。 未掺杂结构的氧离子和质子电导率通过间隙氧位点发生（800°C时~10 ^–6 S cm ^–1 ），为改善其在实际器件中的应用提供了潜在途径例如固体氧化物燃料电池。