Theory of the metal-insulator transition in PrRu4P12 and PrFe4P12 -: art. no. 245112

All symmetry-allowed couplings between the 4f(2)-electron ground state doublet of trivalent praseodymium in PrRu4P12 and PrFe4P12 and displacements of the phosphorus, iron, or ruthenium ions are considered. Two types of displacements can change the crystal lattice from body-centred cubic to simple orthorhombic or to simple cubic. The first type lowers the point group symmetry from tetrahedral to orthorhombic, while the second type leaves it unchanged, with corresponding space group reductions Im (3) over bar -->Pmmm and Im (3) over bar -->Pm (3) over bar, respectively. In former case, the lower point group symmetry splits the degeneracy of the 4f(2) doublet into states with opposite quadrupole moment, which then leads to antiquadrupolar ordering, as in PrFe4P12. Either kind of displacement may conspire with nesting of the Fermi surface to cause the metal-insulator or partial metal-insulator transition observed in PrFe4P12 and PrRu4P12. We investigate this scenario using band-structure calculations, and it is found that displacements of the phosphorus ions in PrRu4P12 (with space-group reduction Im (3) over bar -->Pm (3) over bar) open a gap everywhere on the Fermi surface.