Crystal structures of two new high-pressure oxynitrides with composition SnGe₄N₄O₄, from single-crystal electron diffraction

SnGe 4 N 4 O 4 was synthesized at high pressure (16 and 20 GPa) and high temperature (1200 and 1500°C) in a large-volume press. Powder X-ray diffraction experiments using synchrotron radiation indicate that the derived samples are mixtures of known and unknown phases. However, the powder X-ray diffraction patterns are not sufficient for structural characterization. Transmission electron microscopy studies reveal crystals of several hundreds of nanometres in size with different chemical composition. Among them, crystals of a previously unknown phase with stoichiometry SnGe 4 N 4 O 4 were detected and investigated using automated diffraction tomography (ADT), a three-dimensional electron diffraction method. Via ADT, the crystal structure could be determined from single nanocrystals in space group P 6 3 mc , exhibiting a nolanite-type structure. This was confirmed by density functional theory calculations and atomic resolution scanning transmission electron microscopy images. In one of the syntheses runs a rhombohedral 6 R polytype of SnGe 4 N 4 O 4 could be found together with the nolanite-type SnGe 4 N 4 O 4 . The structure of this polymorph was solved as well using ADT.