  Transition metal sulfide perovskites offer lower band gaps and greater
tunability than oxides, along with other desirable properties for applications.
Here we explore dimensional reduction as a tuning strategy using the
Ruddlesden-Popper phases in the Ba-Zr-S system as a model. The three
dimensional perovskite BaZrS$_3$ is a direct band semiconductor, with a band
gap of 1.5 eV suitable for solar photovoltaic application. However, the three
known members of the Ruddlesden-Popper series, are all indirect gap materials,
and additionally have lower fundamental band gaps. This is accompanied in the
case of by a band structure that is more favorable for carrier transport for
oriented samples. The layered Ruddlesden-Popper compounds, show significantly
anisotropic optical properties, as may be expected. The optical spectra show
tails at low energy, which may complicate experimental characterization of
these materials.
