Ultrasonic investigation on a cage structure compound PrTi2Al20

We present experimental results of elastic constants as a function of temperature (T) and magnetic field (H) for a single crystal of PrTi2Al20 for the first time. A clear elastic softening was observed in the temperature dependence of elastic constants, C-L=(C-11 + 2C(12) + 4C(44))/3 and C-T = (C-11-C-12+C-44)/3 being, suggestive of a remarkable fluctuation of quadrupolar moments O-20 and O-22 with Gamma(3) symmetry. The elastic anomaly associated with a phase transition at 2.0 K exhibits the ferroquadrupolar (FQ) phase behavior under magnetic fields in accordance with the other physical properties such as magnetic susceptibility and specific heat. The softening can be explained well based on the crystalline electric field (CEF) effect model with a Gamma(3) ground state. Furthermore, it was found that a coupling constant of the inter-site interaction between relevant quadrupolar moments is positive, indicating the FQ interaction mediated in this system. We argue the FQ scenario and 4f multiplets of Pr3+ split by CEF effect in PrTi2Al20