Charge-Density-Wave Ordering in the Metal-Insulator Transition Compound   PrRu4P12
  X-ray and electron diffraction measurements on the metal-insulator (M-I) transition compound PrRu$_4$P$_{12}$ have revealed the emergence of a periodic ordering of charge density around the Pr atoms. It is found that the ordering is associated with the onset of a low temperature insulator phase. These conclusions are supported by the facts that the space group of the crystal structure transforms from Im$\bar{3}$ to Pm$\bar{3}$ below the M-I transition temperature and also that the temperature dependence of the superlattice peaks in the insulator phase follows the squared BCS function. The M-I transition could be originated from the perfect nesting of the Fermi surface and/or the instability of the $f$ electrons. 
C. H. Lee, H. Matsuhata, H. Yamaguchi, C. Sekine, K. Kihou, T. Suzuki,   T. Noro, I. Shirotani