We present the results of thermal expansion measurement (T = 1.5-300K) in p
olycrystalline PrRu4P12, which exhibits metal-insulator (M-I) transition at
around 60 K. There is no significant change in lattice volume at M-I trans
ition In the thermal expansion coefficient alpha(T). however. a slight jump
(similar to 5 x 10(-7) K-1) is observed at the transition. Magnetic field
has no effect on alpha(T) at around the transition. at least up to 70 kOe.
Those facts suggest that this M-I transition is a non-magnetic one of secon
d order. Much below M-I transition. we found that a broad peak emerges in a
lpha(T) under magnetic field. As magnetic held increases. this peak becomes
much broader and shifts toward higher temperature almost proportional to t
he field. Surprisingly, a negative magnetostriction is observed in this tem
perature region. (C) 2000 Elsevier Science B.V. All rights reserved.