We have determined the antiferromagnetic structure of monoclinic ${\mathrm{U}}_2{\mathrm{Rh}}_3{\mathrm{Si}}_5$ and the temperature dependence of the magnetically ordered moment $\mu$ via single-crystal neutron diffraction. The value of $\mu$ exhibits a sharp discontinuity at the ordering temperature $T_N=25.5\mathrm{}$ K, jumping from $1.6{\mu }_B$ to zero within a narrow temperature interval $[\Delta T_N=0.2\mathrm{}$ K, $\mu \left(9\mathrm{}\mathrm{\hspace{0.5em}}\mathrm{K}\right)=2.35\mathrm{}{\mu }_B$]. This demonstrates the first-order nature of the transition. The magnetically ordered structure is a noncollinear arrangement of the eight uranium magnetic moments within the crystallographic unit cell. The moments are oriented parallel to nearest-neighbor U-Rh bonds, creating uniaxial (Ising-type) magnetic behavior, with canted local easy axes. The temperature dependence of the lattice parameters, as determined via high-resolution x-ray diffraction, exhibit pronounced discontinuities at $T_N.$ These data demonstrate a strong magnetoelastic coupling.

• Received 5 August 1997

DOI:https://doi.org/10.1103/PhysRevB.56.13693