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

立方填充Skutterudite结构(Im-3空间群)中的PrRu_4p_&lt；12&gt；和SmRu_4p_&lt；12&gt；分别在62K和16K经历了金属-绝缘体(M-I)相变。对于PrRu_4p&t；12&gt；，各种实验表明M-I相变是非磁性的二级相变。为了阐明M-I转变的机制，我们用电子和同步辐射X射线衍射法研究了T&lt；mI&gt；以下的晶体结构。在T&lt；MI&gt；下观察到超晶格斑点。提出了SMRU_4p&lt；12&gt；的低温相空间群为Pm-3。与PrRu_4p；12&gt；相反，磁化率在T&lt；MI&gt；有明显的异常。我们详细测量了在强磁场下的比热、电阻率和磁化强度。详细的测量表明，M-I相变分两个连续步骤发生。这两个反常在30T以下的场依赖关系表明，两个相继相变是反铁-四极有序和反铁磁有序。

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.