The Mössbauer spectra of ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}{\mathrm{H}}_x,$ where $x=0,$ 1, 2, 3, 4, and 5, have been measured and analyzed between 4.2 and 295 K. Because ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$ exhibits a helical magnetic order between 225 and 90 K and a fan magnetic order below 90 K, its Mössbauer spectra were fit with a distribution of hyperfine fields and θ angles for four of the eight magnetically inequivalent sites. Because the hydrides exhibit a magnetization within the basal plane of the hexagonal cell, their Mössbauer spectra were fit with seven sextets. The four isomer shifts correlate with the Wigner-Seitz cell volume, the hyperfine fields correlate with the number of iron near neighbors and give estimates of the individual iron magnetic moments ranging from $0.91{\mu }_B$ to $2.13{\mu }_B,$ and the quadrupole splittings are in agreement with a point charge calculation. The temperature dependence of the hyperfine fields in ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$ is insensitive to the transition from the fan to the helical magnetic structure, a transition which does not modify to any extent the iron electronic structure. As a result of the sudden ferromagnetic ordering which results from the presence of as little as one hydrogen per formula unit, the increase of 70 kOe in the weighted average hyperfine field upon hydrogenation is the largest between $x=0\mathrm{}$ and 1. The temperature dependence of the weighted average hyperfine field in the hydrides shows Brillouin behavior, a behavior which is slightly different for ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}.$ The temperature dependence of the isomer shift yields a Debye temperature of 345 K. The variation of the hyperfine parameters upon hydrogenation confirms that first the $9e$ octahedral site is filled by hydrogen, and second the $18g$ tetrahedral hydrogen site is filled.

• Received 24 April 2000

DOI:https://doi.org/10.1103/PhysRevB.62.11731