The specific heat of the filled skutterudite PrRu4P12 in magnetic fields

We have investigated the field dependence of the metal-insulator (MI) transition displayed by the filled skutterudite PrRu4P12 by means of specific heat measurements. The MI transition is not caused by a crystallographic transformation with breaking cubic symmetry, a valence transition nor a magnetic phase transition. Although the origin of the MI transition is still unclear, it is possible that an antiferro-quadrupolar ordering or a local-structure change of P atom is related to this transition. A lambda-like second order-type anomaly is observed at T-MI = 62.3 K. No field dependence of the MI transition is detected up to 12 T. A held-induced peak is observed much below T-MI (below 10 K). The magnetic entropy approaches the value similar to R In 2 above 10 K, which suggests that the crystalline-electric-field ground state of PrRu4P12 is the non-Kramer's doublet Gamma(3). (C) 2000 Elsevier Science B.V. All rights reserved.