3d-Metal Induced Magnetic Ordering on U(IV) Atoms as a Route toward U(IV) Magnetic Materials

Uranium­(IV)
5f2 magnetism is dominated by a transition
from a triplet to a singlet ground state at low temperatures. For
the first time, we achieved magnetic ordering of U­(IV) atoms in a
complex fluoride through the incorporation of 3d transition
metal cations. This new route allowed us to obtain an unprecedented
series of U­(IV) ferrimagnetic materials of the new composition Cs2MU3F16 (M = Mn2+, Co2+, and Ni2+), which were comprehensively characterized
with respect to their structural and magnetic properties. Magnetic
susceptibility measurements revealed ferromagnetic-like phase transitions
at temperatures of ∼14.0, 3.5, and 4.8 K for M = Mn2+, Co2+, and Ni2+, respectively. The transition
is not observed when the magnetic M cations are replaced by a diamagnetic
cation, Zn2+. Neutron diffraction measurements revealed
the magnetic moments of 0.91(6)–1.97(3) μB on the U atoms, which are only partially compensated by antiparallel
moments of 1.53(14)–3.26(5) μB on the 3d cations. This arrangement promotes suppression of the
transition to a diamagnetic ground state characteristic of U­(IV),
and in doing so, induces magnetic ordering on uranium via 3d–5f exchange coupling.