Stripe order and vibrational properties of La₂NiO_4+δ for δ=2∕15: Measurements and ab initio calculations

The optical properties of the static charge- and stripe-ordered material La₂NiO_4+δ for δ=2/15 have been measured over a wide frequency and temperature range for light polarized within the a-b planes and along the c axis. Below the charge-ordering temperature, T_co≃319 K, a charge gap opens and the electronic background, upon which four strong infrared-active phonons are superimposed, drops towards zero. As the temperature decreases, many new spectral features are observed in response to the ordering of interstitial oxygen as well as the formation of a superlattice due to the charge order in the NiO₂ planes. In particular, the prominent mode at 354 cm⁻¹ splits into three components; while the frequencies do not shift below the magnetic-ordering transition at T_m=110 K, there is a transfer of oscillator strength in response to the change in registry of the charge stripes with respect to the underlying lattice. Ab initio calculations have been performed using density-functional theory, and the phonon dispersion curves were obtained using the direct method. Likely assignments of the new modes activated by stripe order are discussed. In some crystals, two antiresonances are observed in the conductivity for T≃T_co, which change to a resonant character for T≲T_m; these modes are shown to be due to longitudinal optic c-axis modes which appear as a result of surface misorientation.