Phase transition, crystal structure, and magnetic order in VOCI

VOCI develops magnetic order below T-N=80.3(3) K. Employing single-crystal x-ray diffraction and powder neutron diffraction it is shown that the magnetic phase transition is accompanied by a small, temperature-dependent monoclinic lattice distortion with gamma=90.212(2)degrees at T=3.2 K. The phase transition presumably is of second order with a critical exponent beta=0.099(12)degrees. The monoclinic distortion lifts the frustration of interchain interactions along 1/2a +/- 1/2b of the orthorhombic room-temperature structure with the result that chains of antiferromagnetic order exist along 1/2a +/- 1/2b while ferromagnetic chains exist along 1/2a +/- 1/2b. Both antiferromagnetic interchain coupling (if stronger along 1/2a +/- 1/2b than along 1/2a +/- 1/2b) and ferromagnetic interchain coupling (if stronger along 1/2a +/- 1/2b than along 1/2a +/- 1/2b) can explain the observed magnetic superstructure with magnetic space group F2'/d' on the 2a X 2b X 2c supercell (standard setting C2'/c') and with magnetic moments parallel to a. Remarkably, the lattice distortion is not accompanied by a structural distortion so that the driving force for the lattice distortion is completely governed by the magnetic interactions.