Suppression of geometric frustration by magnetoelastic coupling in AuCrS2

We studied the structural, magnetic, and electronic properties of the geometrically frustrated layered AuCrS${}_{2}$ system by means of x-ray and neutron powder diffraction, specific heat, dc magnetization, and dc electrical resistivity measurements. The room-temperature structural refinement is consistent with a hexagonal centrosymmetric $R$-3$m$ symmetry and with formal valence states Au${}^{+}$ and Cr${}^{3+}$, where the Cr${}^{3+}$ ions form a regular triangular lattice within the hexagonal planes. On cooling, we observe a first-order structural phase transition to a monoclinic $C$2/$m$ symmetry concomitant to an antiferromagnetic order at ${T}_{N}$ $=$ 47 K. The atomic displacements associated with this transition stretch the triangular lattice, thus suppressing the geometric frustration. This accounts for the magnetic order observed and gives evidence of a large magnetoelastic coupling. The refined magnetic structure is commensurate and consists of double ferromagnetic chains along the stretching direction with \ensuremath{\mu} $=$ 2.54 ${\ensuremath{\mu}}_{B}$/Cr${}^{3+}$; the residual frustration stabilizes an elegant pattern of alternate ferromagnetic and antiferromagnetic intra- and interplane couplings between adjacent chains. The electrical transport of our sintered powder samples is found to be semiconducting-like with ${\ensuremath{\rho}}_{300\mathrm{K}}$ \ensuremath{\sim} 157 \ensuremath{\Omega}cm and an activation energy of 0.38 eV.