Magnetoelastic coupling in the incommensurate antiferromagnetic phase of FeOCl

The antiferromagnetic phase transition of FeOCl has been studied by temperature-dependent x-ray-diffraction experiments, magnetic susceptibility, heat capacity, and dielectric measurements. The magnetic phase transition is found to be accompanied by a monoclinic lattice distortion, affecting the angle $\ensuremath{\gamma}$ between crystallographic axes parallel to the layers comprising the quasi-two-dimensional magnetic system. The temperature-dependent magnitude of $\ensuremath{\gamma}$ shows the phase transition to be of second order. Satellite reflections occur in x-ray diffraction with twice the magnetic modulation wave vector. These positions are temperature dependent, providing evidence for an incommensurate character of the magnetic order. The observed N\'eel temperature is ${T}_{N}=81.0\phantom{\rule{0.16em}{0ex}}(2)$ K.