Transmission-electron-microscopy study of charge-stripe order in La1.725Sr0.275NiO4

We characterize the local structure and correlations of charge stripes in La1.725Sr0.275NiO4 using transmission-electron microscopy. We present direct evidence that the stripe modulation is indeed one-dimensional within each NiO2 plane. Furthermore, we show that individual stripes tend to be either site centered or bond centered, with a bias towards the former. The spacing between stripes often fluctuates about the mean separation, contributing to a certain degree of frustration of the approximate body-centered stacking along the c axis. These results confirm ideas inferred from previous neutron-diffraction measurements on doped nickelates, and demonstrate that charge-stripe order is quite different from the conventional concept of charge-density-wave order.