  The interplay between crystal symmetry and charge stripe order in
Pr1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4 has been studied by means of single
crystal x-ray diffraction. In contrast to tetragonal La1.67Sr0.33NiO4, these
crystals are orthorhombic. The corresponding distortion of the NiO2 planes is
found to dictate the direction of the charge stripes, similar to the case of
diagonal spin stripes in the insulating phase of La2-xSrxCuO4. In particular,
diagonal stripes seem to always run along the short a-axis, which is the
direction of the octahedral tilt axis. In contrast, no influence of the crystal
symmetry on the charge stripe ordering temperature itself was observed, with
T_CO 240K for La, Pr, and Nd. The coupling between lattice and stripe degrees
of freedom allows one to produce macroscopic samples with unidirectional stripe
order. In samples with stoichiometric oxygen content and a hole concentration
of exactly 1/3, charge stripes exhibit a staggered stacking order with a period
of three NiO2 layers, previously only observed with electron microscopy in
domains of mesoscopic dimensions. Remarkably, this stacking order starts to
melt about 40K below T_CO. The melting process can be described by mixing the
ground state, which has a 3-layer stacking period, with an increasing volume
fraction with a 2-layer stacking period.

Strongly Correlated Electrons;Superconductivity