ANTIFERROMAGNETISM IN MN5SI3 - THE MAGNETIC-STRUCTURE OF THE AF2 PHASE AT 70 K

The antiferromagnetic structure of the AF2 phase of Mn5Si3, stable bet ween the Neel point at 99 K and the transition to a second antiferroma gnetic AF1 phase at 66 K, has been determined from unpolarized neutron integrated intensity measurements on single-crystal and powder sample s. The paramagnetic phase adopts the hexagonal D8(8) structure, Z = 2, with Mn1 and Mn2 atoms in a fourfold and a sixfold site respectively. With the onset of long-range magnetic order the symmetry is reduced t o orthorhombic. The orthorhombic cell dimensions at 70 K are a = 6.898 56(1), b = 11.89120(2) and c = 4.793 30(1) Angstrom and these are rel ated to those of the hexagonal cell, a(h) etc, by a approximate to a(h ), b approximate to root 3a(h) and c approximate to c(h). The orthorho mbic cell is C-face centred, space group C cmm, with Z = 4. Magnetic r eflections indexed on this cell have h + k odd corresponding to a (010 ) magnetic propagation vector. Zero-field neutron polarimetry on a sin gle crystal shows that there are no components of moment parallel to t he c axis. A magnetic structure in which the Mn1 and one-third of the Mn2 atoms have no ordered moment and the remaining Mn2 atoms a moment of 1.48(1)mu(B) directed parallel and antiparallel to b gives a good f it to both the powder and single-crystal intensities. This structure d iffers significantly from previous models, but it is coherent with cur rent ideas about the stability of Mn moments in compounds with B-subgr oup elements. Its relation to the recently revised structure of the lo w-temperature AF1 phase is discussed.