Puzzling Intergrowth in Cerium Nitridophosphate Unraveled by Joint Venture of Aberration-Corrected Scanning Transmission Electron Microscopy and Synchrotron Diffraction

Thorough
investigation of nitridophosphates has rapidly accelerated
through development of new synthesis strategies. Here we used the
recently developed high-pressure metathesis to prepare the first rare-earth
metal nitridophosphate, Ce4Li3P18N35, with a high degree of condensation >1/2. Ce4Li3P18N35 consists of an
unprecedented
hexagonal framework of PN4 tetrahedra and exhibits blue
luminescence peaking at 455 nm. Transmission electron microscopy (TEM)
revealed two intergrown domains with slight structural and compositional
variations. One domain type shows extremely weak superstructure phenomena
revealed by atomic-resolution scanning TEM (STEM) and single-crystal
diffraction using synchrotron radiation. The corresponding superstructure
involves a modulated displacement of Ce atoms in channels of tetrahedra
6-rings. The displacement model was refined in a supercell as well
as in an equivalent commensurate (3 + 2)-dimensional description in
superspace group P63(α, β,
0)­0­(−α – β, α, 0)­0. In the second
domain type, STEM revealed disordered vacancies of the same Ce atoms
that were modulated in the first domain type, leading to sum formula
Ce4–0.5xLi3P18N35–1.5xO1.5x (x ≈ 0.72) of the average structure.
The examination of these structural intricacies may indicate the detection
limit of synchrotron diffraction and TEM. We discuss the occurrence
of either Ce displacements or Ce vacancies that induce the incorporation
of O as necessary stabilization of the crystal structure.