Quadrupolar ordering and magnetic properties of tetragonal

Magnetic susceptibility, magnetization process, specific heat, ultrasonic velocity, and inelastic neutron scattering experiments were performed on powdered and single-crystalline (Formula presented) samples. (Formula presented) with a simple body-centered tetragonal (Formula presented)-type structure undergoes a transition to an antiferromagnetic state at (Formula presented) of 3.2 K. An anomaly at 7.0 K is observed in the temperature dependence of specific heat and magnetic susceptibility. A very large softening beyond 50% from 300 down to 7.0 K is also observed in the temperature dependence of the elastic constant of the (Formula presented) mode. The results reveal clearly that the anomaly at 7.0 K (Formula presented) originates from a ferroquadrupolar ordering with an orthorhombic (Formula presented)-symmetry-lowering mode. Separation between the ground and the first excited singlet states of (Formula presented) levels is so small in the quadrupolar ordered phase and a transition probability between these states is so extremely large that there occurs a Van Vleck antiferromagnetic transition at 3.2 K by exchange-induced magnetic moments arising from the transition between the ground and the first excited singlet states.