  A structural phase transition has been found using electron diffraction
technique in PrRu4P12 accompanied by a metal - insulator (M - I) transition
(TMI = 60K). Weak superlattice spots appeared at (H, K, L) (H + K + L = 2n + 1;
n is an integer) 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 Pm3. This is the first observation of a symmetry other than Im3 in
skutterudite compounds.
