Ferroquadrupolar ordering in PrTi ₂Al ₂₀

The origin of the nonmagnetic phase transition in PrTi ₂Al ₂₀, reported earlier in a macroscopic study, has been asserted microscopically using elastic and inelastic neutron scattering techniques. It has been shown spectroscopically that the crystalline-electric-field ground state is a nonmagnetic Γ ₃ doublet, whereas the excited states are two triplets (Γ ₄ and Γ ₅) and a singlet (Γ ₁). The diffraction experiment under external magnetic field shows that the nonmagnetic transition is indeed ferroquadrupolar ordering, which takes place as a consequence of cooperative removal of the ground-state-doublet degeneracy. It is therefore concluded that PrTi ₂Al ₂₀ is another rare example of Pr compounds exhibiting nonmagnetic quadrupolar order.