High-Purity Synthesis of Perovskite Oxynitride LaHfO2N Prepared via a Reduced-Ammonolysis Treatment Using an Active Metal and Amorphous Oxide-Based Precursor

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Abstract We present a novel synthesis method for single-phase LaHfO2N via the ammonolysis of amorphous oxide-based precursors in the presence of active metal powders such as Mg and MgH2. Hf-containing oxynitrides and nitrides are difficult to synthesize by conventional ammonolysis because oxygen removal from the thermodynamically stable Hf–O framework and subsequent nitridation require strongly reducing conditions. The present active-metal-assisted ammonolysis process provides an effective synthetic strategy for such Hf-containing compounds, as demonstrated by the successful formation of phase-pure LaHfO2N. The addition of Mg or MgH2 significantly enhanced the nitridation process; increasing the Mg or MgH2 content led to a higher proportion of the perovskite LaHfO2N phase and a corresponding decrease in the residual oxide-based precursor phase, La2Hf2O7. In contrast, excessive addition of active metals induced over-reduction, resulting in colored impurity phases and enhanced optical absorption in the visible and near-infrared regions. Therefore, careful control of the active-metal content was essential for obtaining high-purity LaHfO2N powders. Optimal phase purity was achieved at precursor-to-Mg or MgH2 mixing ratios of approximately 1.0:2.0. The optical absorption of the resulting LaHfO2N powders decreased when oxide-based precursors preheated at lower temperatures were used, indicating that amorphous oxide-based precursors are effective for suppressing undesired coloration. After treatment with a dilute HNO3 solution, the perovskite phase remained intact, with no detectable impurities such as HfNx and no significant Mg substitution at the Hf sites. The photocatalytic activity of the LaHfO2N samples was evaluated under ultraviolet irradiation. The powders exhibited water-splitting capability, predominantly producing oxygen with only trace amounts of hydrogen.

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