Heteroanionic hydrides are an emerging class of compounds with representatives showing hydride ion conductivity  or catalytic activity . Their properties are fundamentally linked to their anionic substructure; for example, might a difference between ordered and disordered anions change their behavior from a conducting to an isolating behavior .
From the solid-state reaction of Li2O, LaN and LaH3, we obtained the new nitride hydride oxide LiLa2NH2O as a black powder. It crystallizes in the K2NiF4 structure type (I4/mmm, a = 3.65431(6) Å, c = 13.3570(3) Å) and can be described as an aliovalent substitution product (2 O2- → N3- + H-) of the hydride ion conductor LiLa2HO3 . The analysis of the anion substructure demanded the combination of different methods. We therefore performed neutron powder diffraction measurements on both hydride LiLa2NH2O and deuteride LiLa2ND2O (coherent scattering lengths bc(H) = -3.7 fm, bc(D) = 6.7 fm) in addition to powder X-ray diffraction and nuclear magnetic resonance spectroscopic analysis.
H/D and N/O show a separation on two crystallographic sites 4d and 4e with only slight but significant mixing. Additionally, both sites are significantly underoccupied, resulting in a charge imbalanced composition. H/D occupies favorably the Li-rich site 4d, while N and O share the La-rich 4e position (Figure 1). Both sites are coordinated in a distorted octahedral fashion with Li situated on opposite vertices of the H-rich polyhedron with short Li-H distances (1.83 Å; LiH: 2.04 Å).
Figure 1. Crystal structure of LiLa2NH2O (left) and coordination polyhedra of the H-rich site (4d, middle) and N/O-rich site (4e, right).
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