Charge modulations inLa2−xSrxNiO4+y: Ordering of polarons
Electron diffraction of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4+\mathit{x}}$ (0\ensuremath{\le}x\ensuremath{\le}1) reveal at low temperatures the presence of sharp, quasi-2D superlattice peaks around the (\ensuremath{\pi},\ensuremath{\pi}) point. We show that the modulation wave vector \ensuremath{\delta} measured from the (\ensuremath{\pi},\ensuremath{\pi}) point decreases with increasing hole concentration. However, a plateau with \ensuremath{\delta}\ensuremath{\sim}1/3 was observed for 0.075\ensuremath{\le}x\ensuremath{\le}0.2 and the superlattice peaks were found to collapse into a single spot at the (\ensuremath{\pi},\ensuremath{\pi}) point with a truly 2D ordering for x=0.5 where anomalous change of physical properties has been previously reported. Our results can be understood in terms of the ordering of polarons.