Investigation of the system La_2-xSr_xNiO_4+δ via measurements of thermodynamic and transport properties

Stripe order in cuprates and nickelates has been the subject of intense research activity over the last few decades. The potential connection between stripe order and high-temperature superconductivity makes elucidating the underlying mechanism that drives stripe order an important problem. Stripe order has been observed in the system La_2-xSr_xNiO_4+δ for 0.25 ≤ x ≤ 0.8. Chemical substitution of Sr²⁺ for La³⁺ dopes holes into the NiO₂ planes and induces a variation of the Ni²⁺/Ni³⁺ ratio. In the stripe-ordered state, homogeneous striped regions of Ni³⁺ ions bisect regions of antiferromagnetic order associated with magnetic moments on the Ni²⁺ ion sites. To investigate the system
La_2-xSr_xNiO_4+δ, polycrystalline samples with x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6 and 0.8 were synthesized via solid state reaction. Phase purity and sample quality were characterized via powder x-ray diffraction measurements and iodometric titration. Measurements of the transport and thermodynamic properties of these samples as a function of temperature were used to construct a detailed T vs. x phase diagram.