Phase transition, crystal structure, and magnetic order in VOCl

Abstract

VOCl develops magnetic order below TN =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 γ=90.212 (2) ° at T=3.2 K. The phase transition presumably is of second order with a critical exponent β=0.099 (12). The monoclinic distortion lifts the frustration of interchain interactions along 1 2 a± 1 2 b of the orthorhombic room-temperature structure with the result that chains of antiferromagnetic order exist along 1 2 a+ 1 2 b while ferromagnetic chains exist along 1 2 a- 1 2 b. Both antiferromagnetic interchain coupling (if stronger along 1 2 a+ 1 2 b than along 1 2 a- 1 2 b) and ferromagnetic interchain coupling (if stronger along 1 2 a- 1 2 b than along 1 2 a+ 1 2 b) can explain the observed magnetic superstructure with magnetic space group F 2′ / d′ on the 2a×2b×2c supercell (standard setting C 2′ / 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. © 2009 The American Physical Society.