Lattice and magnetic instabilities in CaFe₂As₂: A single-crystal neutron diffraction study

Neutron diffraction measurements of a high-quality single crystal of CaFe₂As₂ are reported. A sharp transition was observed between the high-temperature tetragonal and low-temperature orthorhombic structures at T_S=172.5 K (on cooling) and 173.5 K (on warming). Concomitant with the structural transition, the Fe moments order in a commensurate antiferromagnetic structure with a saturated moment of 0.80(5)μ_B/Fe directed along the orthorhombic a axis. The hysteresis of both the structural and magnetic transitions is 1 K between cooling and warming, consistent with previous thermodynamic, transport, and single-crystal x-ray studies and provides clear evidence of the coupling between the structural and magnetic transitions in this material.