PDF Study of Dynamic Correlated Disorder in the Hydride Ion Conductor Ca2NH

Calcium nitride-hydride (Ca2NH) has been shown to promote mild-condition ammonia synthesis as a co-catalyst (Kitano et al., Chem. Sc., 2016). Kitano et al. suggested Ca2NH acted as a hydrogen sink freeing up active sites on the transition metal (Ru) for further disassociation of N2 and H2 gases. EIS conducted at the University of St Andrews shows temperature dependent conductivities associated with type-II (thermally induced) ionic conductors (Boyce, Physics Reports, 1979). A previous QENS experiment (RB 1720393) showed significant Q-dependent broadening associated with this system, strongly indicating H- diffusion. Thus, it’s likely that the ionic conductivity of systems like Ca2NH play a significant role in promoting ammonia synthesis. A PDF experiment could explore the nature of this ionic conductivity and its relation to the structure of Ca2NH. Recent research has pin-pointed correlated disorder as a potential source for high ionic conductivities measured in solid-state systems (Keen & Goodwin, Nature, 2015). Analysis of our previous PDF experiment (RB 1610383) will soon be published in a Chemistry of Materials article outlining the relationship between correlated disorder and ionic conductivity in the superionic conductor BaH2. A clear understanding of the relationship between structure, correlated disorder, and ionic conductivity in the Ca2NH system will benefit future research into nitrogen-hydrogen based co-catalysts for mild-condition ammonia synthesis.

氮化氢钙（Calcium nitride-hydride，Ca₂NH）已被证实可作为助催化剂促进温和条件下的氨合成（Kitano等，《化学科学》，2016）。Kitano等认为，Ca₂NH充当储氢体，释放过渡金属（Ru）上的活性位点，以促进N₂和H₂气体的进一步解离。圣安德鲁斯大学开展的电化学阻抗谱（Electrochemical Impedance Spectroscopy，EIS）研究显示，其温度依赖性电导率与II型（热诱导）离子导体相关（Boyce，《物理报告》，1979）。先前的准弹性中子散射（Quasielastic Neutron Scattering，QENS）实验（RB 1720393）表明，该体系存在显著的Q依赖性展宽，强烈暗示氢离子（H⁻）扩散。因此，Ca₂NH这类体系的离子电导率可能在促进氨合成中发挥重要作用。对分布函数（Pair Distribution Function，PDF）实验可探索这种离子电导率的本质及其与Ca₂NH结构的关系。近期研究已明确关联无序是固态体系中高离子电导率的潜在来源（Keen与Goodwin，《自然》，2015）。我们先前PDF实验（RB 1610383）的分析结果即将发表于《材料化学》期刊，该文章将阐述超离子导体BaH₂中关联无序与离子电导率的关系。清晰理解Ca₂NH体系中结构、关联无序与离子电导率之间的关系，将有助于未来针对温和条件氨合成的氮氢基助催化剂研究。