Infrared Study Of The Low-Temperature-Orthorhombic Low-Temperature-Tetragonal Structural Phase-Transition In La2nio4

We have studied the polarized infrared reflectance of a stoichiometric single crystal of La{sub 2}NiO{sub 4} through the {ital Abma} ({ital D}{sub 2{ital h}}{sup 18}) (low-temperature orthorhombic) {r arrow}{ital P}4{sub 2}/{ital ncm} ({ital D}{sub 4{ital h}}{sup 16}) (low-temperature tetragonal) structural phase transition. This transition, which occurs near 70 K, involves a rigid tilt of the Ni-O{sub 6} octahedra and produces distinct changes in the infrared-active phonons. Our observations are consistent with a first-order phase transition. This phase transition also occurs in La{sub 1.88}Ba{sub 0.12}CuO{sub 4}, and has a profound influence on superconductivity in that material.