The electronic structures of the skutterudite-type phosphides CoP 3 and NiP 3 have been investigated by means of first-principles linear muffin-tin orbital-atomic sphere approximation band-structure calculations. The presence of P 4 rings in the skutterudite structure is of great importance in determining the nature of the electronic bands around the Fermi level, composed mainly of -type molecular orbitals of these units. The metallic character found for NiP 3 should be ascribed to the phosphorus framework rather than to the metal atoms.The skutterudite ͑i.e., CoAs 3 -type͒ structure is found for a number of binary transition-metal pnictides M X 3 ͑Ref. 1͒, with interesting electronic properties that suggest their use as advanced thermoelectric materials.2,3 Except for NiP 3 , all other known compounds in this family ͑M X 3 with M ϭCo, Rh, Ir and XϭP, As, Sb͒ are isoelectronic and have been found to behave as diamagnetic p-type semiconductors. [3][4][5][6][7][8][9][10][11][12][13] While some of them seem to have substantial optical band gaps ͑0.45 and 1.4 eV for CoP 3 and IrSb 3 , respectively͒, 3,7 no optical gap is found for CoAs 3 and CoSb 3 .7 NiP 3 , the only known skutterudite 14,15 of a group-10 metal, shows metallic behavior with Pauli-type paramagnetism.
13Closely related crystal structures are found for the ternary compounds RM 4 X 12 ͑Rϭrare-earth metal, M ϭFe, Ru, Os, and XϭP, As, Sb͒. In these, the M 4 X 12 subnet adopts the skutterudite structure, [16][17][18][19] while the rare-earth metal atoms occupy the cubic positions that are empty in the binary phases. The physical properties of the ternary phosphides LaM 4 P 12 are specially interesting, being superconductors with transition temperatures T c ϭ4.1, 7.2, and 1.8 K for M ϭFe, Ru, and Os, 20 respectively. Although these compounds show interesting physical properties, relatively little is known about their electronic structure, probably because of the complex crystal structure with a relatively open unit cell. So far, only two bandstructure calculations have been reported for the skutteruditetype compounds. In the first study, 21 the electronic structures of LaFe 4 P 12 and related compounds were analyzed by using the extended Hückel tight-binding ͑EHTB͒ method. Recently, self-consistent band-structure calculations, using the linear augmented plane-wave method for IrSb 3 , CoAs 3 , and CoSb 3 , have been published by Singh and Pickett. 22 In this contribution, we report first-principles bandstructure calculations using the linear muffin-tin orbital method in the atomic sphere approximation ͑LMTO-ASA͒ ͑Refs. 23-25͒ to explore the band structure of CoP 3 and NiP 3 .
CRYSTAL STRUCTUREThe nature of the highest occupied bands, responsible for the electronic properties of a material, is strongly dependent on its crystal structure. A detailed knowledge of the crystal structure is, therefore, indispensable before analyzing the details of the electronic structure. In the case of the skutterudite structure ͑space-group Im3, n. 204͒, 26 the convention...