Study of Anomalous Metal-Insulator Transition of Filled Skutterudite-Type Compounds

PrRu_4P_3<12>和SmRu_4P_4<12>在立方填充方钴矿结构（Im-3空间群）中分别在62 K和16 K发生金属-绝缘体（M-I）相变<MI>。对于PrRu_4P_<12>，各种实验表明M-I跃迁是非磁性的二级跃迁。为了阐明M-I转变的机制，我们用<MI>电子衍射和同步辐射X射线衍射研究了T_2以下的晶体结构。在低于T_的温度下观察到超晶格斑点<MI>。对SmRu_4P_（1-x），低温相空间群为Pm-3<12>。磁化率在T_1处明显异常，<MI>与PrRu_4P_1相反<12>。详细测量了高磁场下的比热、电阻率和磁化强度。详细的测量表明，M-I跃迁是分两步发生的。这两个反常的场依赖性可达30 T，表明这两个连续的跃迁分别是反铁四极序和反铁磁序。

PrRu_4P_<12> and SmRu_4P_<12> crystallized in the cubic filled skutterudite structure (space group Im-3) undergo a metal-insulator (M-I) transition at T_<MI>=62K and 16K, respectively. For PrRu_4P_<12>, various experiments suggest that the M-I transition is non-magnetic one of second order. To clarify the mechanism of the M-I transition, we have studied the crystal structure below T_<MI> using electron and synchrotron radiation X-ray diffraction. The superlattice spots were observed below T_<MI>. It is suggested that the space group of low temperature phase is Pm-3 For SmRu_4P_<12>. magnetic susceptibility clearly shows an anomaly at T_<MI> in contrast to PrRu_4P_<12>. We have measured in detail the specific heat, electrical resistivity and magnetization under high magnetic field. The detailed measurements have revealed that the M-I transition occuers in two successive steps. The field dependence up to 30T of the two anomalies suggests that the two successive transitions are antiferro-quadrupolar ordering and antiferro-magnetic order.