  We theoretically investigate Sn(II) phosphates as optoelectronic materials
using first principles calculations. We focus on known prototype materials
Sn$_n$P$_2$O$_{5+n}$ (n=2, 3, 4, 5) and a previously unreported compound,
SnP$_2$O$_6$ (n=1), which we find using global optimization structure
prediction. The electronic structure calculations indicate that these compounds
all have large band gaps above 3.2 eV, meaning their transparency to visible
light. Several of these compounds show relatively low hole effective masses
($\sim$2-3 m$_0$), comparable the electron masses. This suggests potential
bipolar conductivity depending on doping. The dispersive valence band-edges
underlying the low hole masses, originate from the anti-bonding hybridization
between the Sn 5s orbitals and the phosphate groups. Analysis of
structure-property relationships for the metastable structures generated during
structure search shows considerable variation in combinations of band gap and
carrier effective masses, implying chemical tunability of these properties. The
unusual combinations of relatively high band gap, low carrier masses and high
chemical stability suggests possible optoelectronic applications of these
Sn(II) phosphates, including p-type transparent conductors. Related to this,
calculations for doped material indicate low visible light absorption, combined
with high plasma frequencies.
