Order-Disordered Phase Transitions and Site Preference in Fe1.5Ni1.5Ga4 Unravelled by Resonant Scattering

<jats:p> The insufficient X-ray scattering contrast and similar neutron scattering cross-sections of Fe and Ni render the accurate determination of ‘coloring scheme’ ambiguous in the vacancy ordered 4 <jats:sup>3</jats:sup> −superstructure of CsCl-related phase Fe <jats:sub>1.5</jats:sub> Ni <jats:sub>1.5</jats:sub> Ga <jats:sub>4</jats:sub> . Therefore, to overcome this limitation, X-ray resonance diffraction near the K-absorption edge of Fe has been utilized for the accurate site occupancy pattern of Fe and Ni in the structure of Fe <jats:sub>1.5</jats:sub> Ni <jats:sub>1.5</jats:sub> Ga <jats:sub>4</jats:sub> . Measurements on three independently annealed samples with progressively longer annealing times revealed no evidence of atomic ordering, instead, Fe prefers to partially occupy the Ni site (48𝑔 Wyckoff site) in the structure of binary Ni <jats:sub>3</jats:sub> Ga <jats:sub>4</jats:sub> (𝐼𝑎3̅𝑑 space group, 𝑎 =11.52 Å). The vacancy ordering present at the 16𝑏 site drives the lattice distortion in the structure. Upon heating to ~955 K, the room-temperature (RT) structure transforms to the 2.9 Å CsCl-type structure (𝑃𝑚3̅𝑚 space group), where Fe, Ni and vacancy are randomly distributed suppressing the lattice distortions. DFT based calculations on the hypothetical ordered model predict a ferromagnetic ground state with non-magnetic and antiferromagnetic states lying very close to it in energy. This near-degeneracy points to a highly complex magnetic energy landscape with competing ferromagnetic and antiferromagnetic interactions, which explains the experimentally observed spin-glass behavior. </jats:p>