  We report 45% enhancement in the thermoelectric figure of merit, ZT of Sr3SnO
via Ca substitution. First-principles calculations have been performed to study
the electronic and thermoelectric transport properties of Ca substituted Sr3SnO
(Sr3-xCaxSnO). The effects of Ca subtitution on bandgap are studied and
detailed mechanisms are proposed to explain the obtained results. We have found
that effective mass and thermopower of Sr3SnO redueces with the increase of
hole concentration. The optimum hole concentration has been obtained for
Sr2CaSnO and the corresponding Seebeck coefficient is 219 {\mu}V/K. The
electrical conductivity of Sr3SnO and its alloys exhibits semiconducting nature
which contradicts with experimental results in Ca3SnO. We have found that due
to the Ca-deficiency, the Ca3SnO shows the metallic conductivity and removes
this contradiction with our results. The lattice thermal conductivities (\kl)
of Sr3SnO and Ca3SnO have been calculated by using both PBE and GW functionals.
The lattice thermal conductivity obtained by PBE functional largely
underestimates the experimental value for Ca3SnO. The total thermal
conductivity (with kl obtained by GW) at 300K is 2.33 and 1.897 W/mK for Sr3SnO
and Ca3SnO, respectively, with excellent agreement with experimental value
1.707 W/mK for Ca3SnO. The dimensionless figure of merit (ZT) for Sr2CaSnO at
500 K is 0.6 and making it promising for thermoelectric applications.
