CHARGE MODULATIONS IN LA2-XSRXNIO4+Y - ORDERING OF POLARONS

Electron diffraction of La2-xSrxNiO4+y (0 less-than-or-equal-to x less-than-or-equal-to 1) reveals at low temperatures the presence of sharp, quasi-2D superlattice peaks around the (pi,pi) point. We show that thermodulation wave vector delta measured from the (pi,pi) point decreases with increasing hole concentration. However, a plateau with delta approximately 1/3 was observed for 0.075 x less-than-or-equal-to 0.2 and the superlattice peaks were found to collapse into a single spot at the (pi,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.