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We report single crystal X-ray diffraction measurements onNa$_2$Ti$_{2}Pn_{2}$O ($Pn$ = As, Sb) which reveal a charge superstructure thatappears below the density wave transitions previously observed in bulk data.From symmetry-constrained structure refinements we establish that theassociated distortion mode can be described by two propagation vectors, ${\bfq}_{1} = (1/2, 0, l)$ and ${\bf q}_{2} = (0, 1/2, l)$, with $l=0$ (Sb) or $l =1/2$ (As), and primarily involves in-plane displacements of the Ti atomsperpendicular to the Ti--O bonds. The results provide direct evidence forphonon-assisted charge density wave order in Na$_2$Ti$_{2}Pn_{2}$O and identifya proximate ordered phase that could compete with superconductivity in dopedBaTi$_{2}$Sb$_{2}$O.

Commensurate lattice distortion in the layered titanium oxypnictides Na$_{2}$Ti$_{2}Pn_{2}$O ($Pn =$ As, Sb) determined by X-ray diffraction