Spontaneous rearrangement of the checkerboard charge order to stripe order in ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{NiO}}_4$

The charge ordering in ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{NiO}}_4$ has been studied using the neutron-diffraction technique. An interesting rearrangement of the charge ordering was observed as a function of temperature. With decreasing temperature, a checkerboard-type charge order is formed below $T_{\mathrm{CO}}^{\mathrm{C}}\sim 480\mathrm{K}.$ It is, however, taken over by the stripe-type charge order with an incommensurability twice as large as that of the spin order below $T_{\mathrm{CO}}^{\mathrm{IC}}\sim 180\mathrm{K}.$ Surprisingly, the stripe phase persists up to $x=0.7\mathrm{}$ for highly hole-doped samples of ${\mathrm{Nd}}_{2-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_4$ with $0.45<~x<~\mathrm{0.7.}$