Low-temperature structural phase transition in La2NiO4+

La2NiO4+ (=0) has a high-temperature tetragonal (HTT) phase (T>680 K), a low-temperature orthorhombic (LTO) phase, and a low-temperature tetragonal (LTT) phase (T<70 K); that is, HTTLTOLTT). These phases are isomorphic to those of La2-xBaxCuO4 (x=0.12). Using Raman spectroscopy, we have measured the c-axis-polarized phonons. The 2A1g modes of the HTT phase occur in all three phases at 155 and 445 cm-1, with energies and widths only weakly dependent on temperature. The low-energy NiO6-octahedra tilting modes (which drive the phase transitions) have been observed and clearly show the first-order nature of the LTOLTT phase transition. The lowest-energy Raman mode observed (at 70 cm-1 with B1g symmetry), allowed only in the LTT phase, involves rocking motion of the undisplaced in-plane oxygen atoms. © 1990 The American Physical Society.