Anomalous structural phase transition in stoichiometricLa2NiO4

Abstract

A detailed neutron-powder-diffraction experiment as a function of temperature has revealed the existence of a reversible structural transition at about 80 K in stoichiometric ${\mathrm{La}}_2$Ni${\mathrm{O}}_4$. This phase transition is characterized by the anomalous behavior of the orthorhombic strain with no apparent change of the space group (Cmca). Structure refinements demonstrate that the elastic anomaly arises from a sudden rotation of the equatorial plane of Ni octahedra [Ni-4O(1)] which becomes orthogonal to the elongation axis [O(2)-Ni-O(2)], thus increasing the effective local symmetry of Ni sites at temperatures below 80 K. This anomalous structural behavior is strongly dependent on the ${\mathrm{Ni}}^{+3\mathrm{}}$ content and, up to now, has been observed only in the stoichiometric brown compound ${\mathrm{La}}_2$Ni${\mathrm{O}}_{4.00}$, which is three-dimensional-antiferromagnetically ordered below room temperature. These findings demonstrate that the similar transition observed in the parent copper oxides is a feature common to a number of layered perovskites and probably not directly related to superconductivity.