Observation of Symmetry Lowering Associated with the Metal–Insulator Transition in SmRu4P12 by 101Ru-NQR

We report the results of NQR (nuclear quadrupole resonance) study on SmRu 4 P 12 , one of a few candidates for materials showing octupolar ordering. In this study, we firstly succeeded in observing the 101 Ru-NQR signal in filled skutterudite compounds R Ru 4 P 12 ( R : rare earth). A single 101 Ru-NQR signal splits into two resonance lines, having intensity ratio of 3:1, just below the metal–insulator ( M – I ) transition temperature of 16.5 K. This result gives convincing evidence for symmetry lowering in the ordered state of this compound, namely the cubic I m \bar3 space group for the filled skutterudite compounds is lowered to the trigonal R \bar3. The model of the ordered structure characterized by a wave vector q =(1,0,0) satisfactorily explains the observed phenomena in the present study and the M – I transition in this compound.