Unidirectional Order and 3D Stacking of Stripes in Orthorhombic $\bf Pr_{1.67}Sr_{0.33}NiO_4$ and $\bf Nd_{1.67}Sr_{0.33}NiO_4$.

The crystal structure and charge stripe order in $\rm Pr_{1. 67}Sr_{0.33}NiO_{4}$ and $\rm Nd_{1. 67}Sr_{0.33}NiO_{4}$ was studied by means of single crystal x-ray diffraction in zero and high electric fields. In contrast to tetragonal $\rm La_ {1.67}Sr_{0.33}NiO_{4}$, these crystals are orthorhombic at room temperature. We find that the distortion of the $\rm NiO_2$ planes associated with the orthorhombic strain dictates the direction of the charge stripes. The critical temperature for charge stripe order is the same as in $\rm La_{1.67}Sr_ {0.33}NiO_{4}$ ($\rm T_{CO}\sim 245$~K), i.e., it does not depend on the crystal symmetry. A second structural transition observed only in $\rm Nd_{1.67}Sr_{0.33}NiO_{4}$ at temperatures $\rm T\sim 100$~K has no noticeable influence on the stripe order. In crystals with a hole content very close to 1/3 we observe a tripling of the charge stripe unit cell along the c-axis for temperatures $\rm T < 225$~K, which indicates a strong tendency towards a well defined three dimensional order. A high electric field applied to $\rm Nd_ {1.67}Sr_{0.33}NiO_{4}$ had no noticeable impact on the charge stripe order, i.e., a sliding of stripes was not observed. {\it The work at Brookhaven was supported by the Office of Science, U.S. Department of Energy under Contract No. DE-AC02-98CH10886.}