Local Distortion Induced Metal-to-Insulator Phase Transition in PrRu$_4$P$_{12}$

Extended X-ray Absorption Fine Structure (EXAFS) experiments have been carried out on PrRu$_4$P$_{12}$ and PrOs$_4$P$_{12}$ to study the metal-to-insulator (MI) phase transition in PrRu$_4$P$_{12}$. No Pr displacement was observed across the MI transition temperature from the EXAFS data. Instead, our EXAFS data clearly show that a Ru displacement is associated with this MI transition. The very high Debye temperature for the Ru-P bond ($\Theta_D$=690 K) suggests that a slight rotation/displacement of relatively rigid RuP$_6$ octahedra leads to this small Ru displacement, which accompanies the MI transition at 62 K in PrRu$_4$P$_{12}$.