*Article* **Hydrogen-Induced Order–Disorder Effects in FePd3**

**André Götze 1, Siobhan Christina Stevenson 2, Thomas Christian Hansen <sup>3</sup> and Holger Kohlmann 1,\***


**Abstract:** Binary intermetallic compounds, such as FePd3, attract interests due to their physical, magnetic and catalytic properties. For a better understanding of their hydrogenation properties, both ordered FePd3 and disordered Fe0.25Pd0.75 are studied by several *in situ* methods, such as thermal analysis, X-ray powder diffraction and neutron powder diffraction, at moderate hydrogen pressures up to 8.0 MPa. FePd3 absorbs small amounts of hydrogen at room temperature and follows Sieverts' law of hydrogen solubility in metals. [Pd6] octahedral voids are filled up to 4.7(9)% in a statistical manner at 8.00(2) MPa, yielding the hydride FePd3H0.047(9). This is accompanied by decreasing longrange order of Fe and Pd atoms (site occupancy factor of Fe at Wyckoff position 1*a* 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 FePd3 under isobaric conditions (*p*(D2) = 8.2(2) MPa) are consistent with a 3D Ising or Heisenberg model (critical parameter *β* = 0.28(5)). The atomic and magnetic order and hydrogen content of FePd3 show a complex interplay.

**Keywords:** intermetallics; metal hydrides; neutron diffraction; in situ diffraction; order–disorder effects; interstitial hydrides; deuterides; magnetism
