PHYSICAL REVIEW B 84, 094455 (2011) Suppression of geometric frustration by magnetoelastic coupling in AuCrS 2 S. J. E. Carlsson, 1,2,* G. Rousse, 1 I. Yamada, 1,3 H. Kuriki, 3 R. Takahashi, 3 F. L´ evy-Bertrand, 2 G. Giriat, 4 and A. Gauzzi 1 1 Institut de Min´ eralogie et de Physique des Milieux Condens´ es, Universit´ e Pierre et Marie Curie, Sorbonne Universit´ es and CNRS, 75005 Paris, France 2 Institut N´ eel, CNRS and Universit´ e Joseph Fourier, BP166, 38042 Grenoble, France 3 Department of Chemistry, Graduate School of Science and Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan 4 Centre for Science at Extreme Conditions and School of Engineering, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom (Received 4 July 2011; published 28 September 2011) 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-3m 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 C2/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 μ = 2.54 μ 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 ρ 300K ∼ 157 cm and an activation energy of 0.38 eV. DOI: 10.1103/PhysRevB.84.094455 PACS number(s): 75.25.−j, 61.05.F−, 64.70.K−, 75.80.+q I. INTRODUCTION During the last decades, the physics of spin frustration in triangular lattice antiferromagnets (TLA) has attracted a great deal of interest due to the interplay between lattice geometry and electronic properties. 1,2 In case the lattice and electronic degrees of freedom are coupled, the frustration can be suppressed by distorting the triangular lattice. In layered compounds, this coupling leads to a rich manifold of three-dimensional orderings that depend on the stacking of the layers and on the anisotropy. Recently, incommensurate spin structures and improper ferroelectricity have been reported. 3–5 Delafossite-like compounds ABO 2 offer an interesting play- ground for studying these orderings thanks to the simple structure where the B ions form a triangular lattice within the BO 2 layers, and the latter are linked by the A ion. If the B ion is magnetic, geometric frustration is then achieved, as the dominant intraplane interaction is antiferromagnetic (AFM). In the chromium system ACrO 2 (A = Ag, Cu, Pd, and Na), the most common spin ordering observed at low temperature is the 120 ◦ spiral structure with weak interplane coupling. This type of structure, common for TLA systems in general, 6–9 has been found to induce ferroelectricity in AgCrO 2 and CuCrO 2 (Refs. 10 and 11), which is promising for the design of multiferroic materials. The sulfide counterparts ACrS 2 (A = Ag, Cu, Na, Li, and K) 12–16 of the above oxides share with the latter a similar structure with triangular Cr layers. All compounds are found to order AFM at low temperatures. However, the orderings are very different from one another. They range from an incommensurate helical structure in NaCrS 2 (Ref. 12) and CuCrS 2 (Ref. 15) to the 120 ◦ spin structure in LiCrS 2 (Ref. 16) and the commensurate structures in KCrS 2 (Ref. 12) and AgCrS 2 . 17 A recent work has reported that the latter compound is a multiferroic, 18 where ferroelectricity is driven by the AFM order, similarly to the case of AgCrO 2 and CuCrO 2 . It turns out that the magnetoelectric coupling in AgCrS 2 is concomitant to large magnetoelastic effects previously reported in NaCrS 2 (Ref. 19) and CuCrS 2 (Ref. 15), but not in their oxide counterparts. This difference may be due to a comparatively small contribution of the electrostatic energy to the total energy, a consequence of the reduced ionicity of sulfides as compared to oxides. In geometrically frustrated systems, this situation may lead to a large magnetoelastic response, for the magnetic ordering requires a significant distortion of the lattice. In light of the above considerations, the study of magnetic frustration in sulfides with geometric frustration may provide new hints for the effective design of multiferroic materials. In order to address this point, in this paper, we have studied the correlation between the structural distortions and the magnetic properties in AuCrS 2 . A previous study has shown that, at room temperature, AuCrS 2 crystallizes in the hexagonal space group R3m and becomes AFM below the N´ eel temperature (T N ) of 55 K. 20 It exhibits a layered structure with a triangular lattice of Cr 3+ ions similar to that of other ACrS 2 compounds. However, the stacking sequence of the CrS 2 layers is different, as the Au atoms are linearly coordinated with two S atoms of adjacent layers. In light of this, AuCrS 2 is a candidate for unusual orderings and possibly multiferroicity. No low temperature studies of the crystal and magnetic structure of AuCrS 2 have previously been made. Therefore, we have performed neutron and synchrotron x-ray powder diffraction and physical properties measurements between room temperature and 2 K. First, we have redetermined the room temperature nuclear structure. Our data suggest a centrosymmetric R-3m space group, contrary to the R3m 094455-1 1098-0121/2011/84(9)/094455(8) ©2011 American Physical Society