“…Here, the transverse modulus C 66 (C 44 ) is the linear responses to the ε xy and ε xx − ε yy (ε yz and ε zx ) strains which couple to the quadrupoles O xy and O 2 2 (O yz and O zx ), respectively, in the hexagonal symmetry. The CEF analyses based on the elastic properties in addition to the magnetic properties make the results more precise [20,22,24,33,34,36]. In this work, to clarify the 4 f -electronic state under the CEF and an influence of the quadrupole interaction in Tb 3 Ru 4 Al 12 , we measured the elastic moduli using ultrasonic technique at zero magnetic field and under fields, and determined the CEF level scheme and the H-T phase diagrams.…”