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.