The theoretical description of quantum phase transition, induced by the external magnetic field, into antiferromagnetic state in the van Vleck - singlet - magnet with a single-ion anisotropy of easy-plane type and ion spin S=1 is proposed. It is show
n that the spin polarization of the ground non-degenerated state proves to be the order parameter of such a transition and that the Landau thermodynamic approach can be employed for its (transition) description. The magnetic properties which include the field behavior of the magnetization and magnetic susceptibility of the antiferromagnetic phase in the fields of different directions are studied. The peculiarities of induced magnetostriction in van Vleck antiferromagnet, which as well as magnetization has a singularity in the phase transition point, are investigated. An attempt is made for qualitative comparison of results obtained with avaliable experimental data.
