Theory of Metal-Insulator Transition in Praseodymium Skutterudite Compounds

The metal-insulator transition of PrRu4P12 and the non-magnetic phase transition of PrFe4P12 are theoretically investigated. The nesting property of the Fermi surface with q = (1, 0, 0) is coupled, through Fe/Ru displacements, with anti-quadrupolar ordering at the phase transitions. Coupling between phonon modes of the displacements of the Fe/Ru ions and the Pr 4f(2) ground state doublet leads to a symmetry lowering (1, 1, 2) type distortion of the Fe/Ru positions and a simultaneous splitting of the 4f(2) doublet into cigenstates with. quadrupole moments 8. The sign of both the distortion and the quadrupole moment alternate from site to site, resulting in anti-quadrupolar ordering. The difference between these two compounds stems from another small Fermi surface in PrFe4P12; the presence of this hand would suppress the metal-insulator transition in PrFe4P12, even after the nesting effect is taken into account. Therefore, the low temperature state of PrFe4P12 is expected to remain metallic.