Structural Complexities in Sodium Ion Conductive Antiperovskite Revealed by Cryogenic Transmission Electron Microscopy

We use low-dose cryogenic transmission electron microscopy
(cryo-TEM)
to investigate the atomic-scale structure of antiperovskite Na2NH2BH4 crystals by preserving the room-temperature
cubic phase and carefully monitoring the electron dose. Via quantitative
analysis of electron beam damage using selected area electron diffraction,
we find cryogenic imaging provides 6-fold improvement in beam stability
for this solid electrolyte. Cryo-TEM images obtained from flat crystals
revealed the presence of a new, long-range-ordered supercell with
a cubic phase. The supercell exhibits doubled unit cell dimensions
of 9.4 Å × 9.4 Å as compared to the cubic lattice structure
revealed by X-ray crystallography of 4.7 Å × 4.7 Å.
The comparison between the experimental image and simulated potential
map indicates the origin of the supercell is a vacancy ordering of
sodium atoms. This work demonstrates the potential of using cryo-TEM
imaging to study the atomic-scale structure of air- and electron-beam-sensitive
antiperovskite-type solid electrolytes.