Crystal structures and magnetic structures of some metal(I) chromium(III) sulfides and selenides

The crystal structures of the rhombohedral compounds ACrX2 (A = Na, Ag, Cu; X = S, Se) at 300
and 4 K have been refined from neutron-diffraction powder data. The structures are based on cubic
close-packings of the anions with Cr in half of the octahedral holes, so that CrX2 sandwiches are formed•
Na occupies the octahedral holes between these sandwiches, Ag and Cu half of the tetrahedral holes;
Ag and Cu have anomalously large (and anisotropic) thermal parameters, probably indicating a statistical
distribution of these atoms around their average positions. The Ag and Cu compounds undergo reversible
phase transitions at 397°C (AgCrS2), 402°C (CuCrS2), or 202°C (AgCrSe2); in the high-temperature
forms the Ag and Cu atoms are randomly distributed over all tetrahedral holes between the CrX2
sandwiches.
The magnetic structures of NaCrS2, NaCrSe2, and AgCrSe2 at 4.2 K have been determined by neutron
diffraction. The magnetic moments of Cr 3+ (about 15 % less than the spin-only value) are perpendicular
to the c axis. The moments in pseudohexagonal (110) planes are collinear, spins at a distance (-], ~, ~) being
antiparallel, spins at a distance (½, ~,{0 parallel. The spins in adjacent (110) planes make angles % where
= 0 ° for NaCrSe2, ~ - 14 ° for AgCrSe2, ~ = 33 ° for NaCrS2. Thus, NaCrSe2 consists of ferromagnetic
layers perpendicular to c, the spins in adjacent layers being antiparallel. In NaCrS2 and AgCrSe2 the
layers are helimagnetic. These magnetic structures and those of KCrS2 and LiCrS2 are discussed in terms
of antiferromagnetic direct Cr-Cr interactions and ferromagnetic Cr-X-Cr interactions between Cr
neighbors in the layers; the former interaction predominates for small, the latter for large Cr-Cr distances.