Quadrupole order in clathrate compound Pr3Pd20Si6

In order to investigate ordered phases revealed at low temperatures in clathrate compound Pr3Pd20Si6, we have performed ultrasonic measurements using a single crystal in magnetic fields at low temperatures. Temperature dependence of elastic constant C11 and CL[10] = (C11 + C12 + 2C44)/2 shows the anomalies at about 150 mK in zero field. Magnetic phase diagrams are determined by measuring C11 in H || [001] and CL[l0] in H || [111]. The magnetic phase diagrams along the three principal axes indicate a closed loop and a strong magnetic anisotropy. This result suggests that antiferroquadrupole ordering appears at low temperatures in Pr3Pd20Si6.