  Ternary chalcogenides have been of recent investigation for applications in
photovoltaic and thermoelectric devices. We study the structural, electronic,
optical, and thermoelectric properties of nine ternary chalcogenides,
NaAX$_{2}$, where A represents pnictogens (As, Sb, and Te) and X represents
chalcogens (S, Se, and Te). Calculations based on density functional theory
yield the following results: (i) phonon dispersion curves predict three of the
compounds, NaAsS$_{2}$, NaSbS$_{2}$, and NaSbSe$_{2}$, to be dynamically stable
in the monoclinic, C2/c, structure, (ii) the layered atomistic configuration
causes the corresponding electronic and optical properties to display a high
degree of anisotropy, (iii) A-X electronic bonding features vary significantly
with structural distortions arising from atomic size mismatch, therefore
directly influencing stability, (iv) strong absorption is observed in the
stable compounds, with coefficients ranging from 10$^{4}$ to 10$^{5}$ cm$^{-1}$
in the visible-UV range, and (v) remarkably high Seebeck coefficients exceeding
500 ${\mu}$V/K at carrier concentrations commonly achieved in such materials.
From these results, we conclude that NaAsS$_{2}$, NaSbS$_{2}$, and NaSbSe$_{2}$
are suitable candidates for both photovoltaic, particularly in tandem solar
cells, and thermoelectric applications. Experimental synthesis and verification
are suggested.
