Structural phase transition accompanied by metal-insulator transition in PrRu4P12

A structural phase transition has been found using electron diffraction technique in PrRu4P12 accompanied by a metal-insulator (M-I) transition (TMI = 60 K). Weak superlattice spots appeared at (H,K,L) (H + K + L = odd) position at a temperature of T = 12 K and 40 K. Above T = 70 K, the spots completely vanished. The space group of the low-temperature phase is probably Pm. This is the first observation of a symmetry other than Im in skutterudite compounds.