  We report a systematic first-principles study based on density functional
theory (DFT) of the structural and ferroelectric properties of the
$R_2$Ti$_2$O$_{7}$ perovskite-related oxides with $R=$ La, Ce, Pr, and Nd. We
show that, in all cases, the ferroelectric distortion to the ground-state polar
$P2_1$ structure from its parent centrosymmetric $P2_1/m$ phase is driven by a
single polar soft mode consisting of rotations and tilts of the TiO$_6$
octahedra combined with displacements of the $R$ ions. A secondary
centrosymmetric distortion, which is stable in the parent structure,
contributes substantially to the energy lowering of the ground state. We
evaluate the trends in structure and polarization across the series as a
function of $R$ and reconcile discrepancies in reported values of polarization
in the literature. Our results confirm that the family of $R_2$Ti$_2$O$_{7}$
materials belong to the class of proper geometric ferroelectrics.
