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.