| Summary: | Binary intermetallic compounds, such as FePd<sub>3</sub>, attract interests due to their physical, magnetic and catalytic properties. For a better understanding of their hydrogenation properties, both ordered FePd<sub>3</sub> and disordered Fe<sub>0.25</sub>Pd<sub>0.75</sub> are studied by several <i>in situ</i> methods, such as thermal analysis, X-ray powder diffraction and neutron powder diffraction, at moderate hydrogen pressures up to 8.0 MPa. FePd<sub>3</sub> absorbs small amounts of hydrogen at room temperature and follows Sieverts’ law of hydrogen solubility in metals. [Pd<sub>6</sub>] octahedral voids are filled up to 4.7(9)% in a statistical manner at 8.00(2) MPa, yielding the hydride FePd<sub>3</sub>H<sub>0.047(9)</sub>. This is accompanied by decreasing long-range order of Fe and Pd atoms (site occupancy factor of Fe at Wyckoff position 1<i>a</i> decreasing from 0.875(3) to 0.794(4)). This trend is also observed during heating, while the ordered magnetic moment decreases up to the Curie temperature of 495(8) K. The temperature dependences of the magnetic moments of iron atoms in FePd<sub>3</sub> under isobaric conditions (<i>p</i>(D<sub>2</sub>) = 8.2(2) MPa) are consistent with a 3D Ising or Heisenberg model (critical parameter <i>β</i> = 0.28(5)). The atomic and magnetic order and hydrogen content of FePd<sub>3</sub> show a complex interplay.
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