Observation of strong magnetoelastic coupling in a first-order phase transition of CrOCl

The phase transition of CrOCl toward a state of antiferromagnetic order below T-N=13.5 K has been identified as a first-order phase transition. The transition is accompanied by a lattice and structural distortion toward a twofold, 2b nuclear superstructure with a-axis unique monoclinic symmetry, as evidenced by temperature-dependent x-ray diffraction experiments. Magnetic-susceptibility and magnetization measurements indicate a transition with strong magnetoelastic coupling to a uniaxial antiferromagnet with ordered moments along c. A second transition is discovered at T-c approximate to 27.2 K that is presumably of purely magnetic origin and might indicate the formation of an incommensurate magnetic superstructure. The different behaviors of TiOCl, VOCl, and CrOCl are the result of the different symmetries of the filled 3d orbitals, which lead to different exchange interactions on the MO double layers of these isostructural compounds.