The noncollinear hexagonal antiferromagnet UPdSn exhibits two magnetic
phase transitions, at 35.5 and 23 K The first transition is from a he
xagonal paramagnetic state to a noncollinear antiferromagnetic state w
ith a doubled unit cell (phase 1). The second 23-K transition is to a
monoclinic magnetic structure (phase II). Ever since these transitions
were discovered, the question has been whether the moments simply rot
ate at 23 K, or whether the y and z components of the moment order at
35.5 K while the x component orders out of incipient fluctuations at t
he lower 23-K transition. While previous powder studies were rather in
conclusive on this point, in this study new single-crystal neutron-dif
fraction results are presented that show the second picture to be corr
ect. In addition, the structural distortions that accompany the change
in symmetry are discussed and show that there is phase-II type magnet
ic short-range order between 23 and 35.5 K