Temperature evolution of the crystal structure of La2-xSrxNiO4 (x=(1/4) and (1/3)) as revealed through neutron powder diffraction

The crystal structure of La2-xSrxNiO4 (x=1/4 and 1/3) has been investigated by neutron powder diffraction. These compositions are well known to exhibit charge and spin ordering at Tco and Tso, respectively. The samples remain tetragonal with space group I4/mmm from 300 K down to 20 K. Small changes in the apical Ni-O1 and La/Sr-O1 bond lengths occur at the charge-ordering transition Tco. No unusual effects are found in the NiO2 plane. An analysis of the oxygen atom thermal parameters reveals on enhanced octahedral tilting above Tso, suggesting that charge or spin ordering relaxes the tilting. A static component to the tilting remains at 20 K, but is absent in the long-range symmetry of the structure. These observations suggest the existence of local structural distortions