Abstract
La{sub 2}NiO{sub 4+{delta}} ({delta}=0) has a high-temperature tetragonal (HTT) phase ({ital T}{gt}680 K), a low-temperature orthorhombic (LTO) phase, and a low-temperature tetragonal (LTT) phase ({ital T}{lt}70 K); that is, HTT{r arrow}LTO{r arrow}(LTT). These phases are isomorphic to those of La{sub 2{minus}{ital x}}Ba{sub {ital x}}CuO{sub 4} ({ital x}=0.12). Using Raman spectroscopy, we have measured the {ital c}-axis-polarized phonons. The {sup 2}{ital A}{sub 1{ital g}} modes of the HTT phase occur in all three phases at 155 and 445 cm{sup {minus}1}, with energies and widths only weakly dependent on temperature. The low-energy NiO{sub 6}-octahedra tilting modes (which drive the phase transitions) have been observed and clearly show the first-order nature of the LTO{r arrow}LTT phase transition. The lowest-energy Raman mode observed (at 70 cm{sup {minus}1} with {ital B}{sub 1{ital g}} symmetry), allowed only in the LTT phase, involves rocking motion of the undisplaced in-plane oxygen atoms.
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