Chemical Pressure-Induced Anion Order–Disorder Transition in LnHO Enabled by Hydride Size Flexibility

While
cation order–disorder transitions have been achieved
in a wide range of materials and provide crucial effects in various
physical and chemical properties, anion analogues are scarce. Here
we have expanded the number of known lanthanide oxyhydrides, LnHO
(Ln = La, Ce, Pr, Nd), to include Ln = Sm, Gd, Tb, Dy, Ho, and Er,
which has allowed the observation of an anion order–disorder
transition from the anion-ordered fluorite structure (P4/nmm) for larger Ln3+ ions (La–Nd)
to a disordered arrangement (Fm3̅m) for smaller Ln3+ (Sm–Er). Structural analysis
reveals that with the increase of Ln3+ radius (application
of negative chemical pressure), the oxide anion in the disordered
phase becomes too under-bonded, which drives a change to an anion-ordered
structure, with smaller OLn4 and larger HLn4 tetrahedra, demonstrating that the size flexibility of hydride anions
drives this transition. Such anion ordering control is crucial regarding
applications that involve hydride diffusion such as catalysis and
electrochemical solid devices.

尽管在多种材料中实现了阳离子有序-无序相变，并对其物理和化学性质产生了关键影响，但阴离子类似物却相对罕见。本研究中，我们扩展了已知镧系元素氧氢化物（LnHO，其中Ln = La, Ce, Pr, Nd）的种类，新增了Ln = Sm, Gd, Tb, Dy, Ho 和 Er，从而观察到从较大镧系阳离子（La–Nd）的阴离子有序萤石结构（P4/nmm）到较小镧系阳离子（Sm–Er）的无序排列（Fm3̅m）的阴离子有序-无序相变。结构分析表明，随着镧系阳离子半径的增加（即负化学压力的应用），无序相中的氧化物阴离子过度键合，从而驱动相变至阴离子有序结构，其中包含较小的OLn4和较大的HLn4四面体，这表明氢化物阴离子的尺寸灵活性驱动了这一相变。这种阴离子有序控制对于涉及氢化物扩散的应用至关重要，如催化和电化学固态设备等。