High Sodium-Ion Conductivity in Interlocked Quaternary Chalcogenides Built with Supertetrahedral Building Units

Herein,
we report the syntheses, structure, Na-ion conductivity,
and theoretical investigation of two moisture stable quaternary compounds,
Na3ZnGaQ4 (Q = S, Se). These compounds are synthesized using high-temperature
solid-state synthesis routes employing polychalcogenide flux or by
metathesis reactions. The crystal structure of these compounds is
built up of a three-dimensional (3-D) network of corner-shared supertetrahedral
(T2) units, where two such 3-D networks are interlocked. The d-block metal and the main group metal, Ga, occupy the same
crystallographic site with a 1:1 ratio, making it a rare form of building
unit. Band structure calculations show that both the compounds are
wide band gap semiconductors with band gaps of 2.25 and 1.61 eV, respectively,
for Na3ZnGaS4 (I) and Na3ZnGaSe4 (II), which are slightly underestimated
compared to experimentally determined band gaps of 3.0 and 1.90 eV,
respectively. I and II possess ionic conductivities
of 3.74 × 10–4 and 0.12 mS/cm with activation
energies of 0.42 and 0.38 eV, respectively, at 30 °C. Interestingly, I shows a significantly high ionic conductivity of 0.13 mS/cm
at 30 °C upon exposure to air, which could be due to water adsorption
on the surface or occlusion in the grain boundaries. Assuming the
vacancy-assisted diffusion mechanism for ionic conductance, this difference
is consistent with the difference on vacancy formation energies in
these compounds, as predicted by DFT calculations. The bond valence
sum map indicates that in both structures, the lowest energy diffusion
path is one dimensional and it is along the c axis
of the unit cell.