Infrared study of the low-temperature-othorhombic–low-temperature-tetragonal structural phase transition inLa2NiO4

We have studied the polarized infrared reflectance of a stoichiometric single crystal of ${\mathrm{La}}_2$ ${\mathrm{NiO}}_4$ through the Abma (${\mathit{D}}_{2\mathit{h}}^{18}$) (low-temperature orthorhombic) →P$4_2$/ncm (${\mathit{D}}_{4\mathit{h}}^{16}$) (low-temper- ature tetragonal) structural phase transition. This transition, which occurs near 70 K, involves a rigid tilt of the Ni-${\mathrm{O}}_6$ 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 ${\mathrm{La}}_{1.88}$ ${\mathrm{Ba}}_{0.12}$ ${\mathrm{CuO}}_4$, and has a profound influence on superconductivity in that material.