Electronic structure and bonding in skutterudite-type phosphides

Abstract

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...

Comparison of theoretical and experimental dielectric functions: Electron energy-loss spectroscopy and density-functional calculations on skutterudites

“…It is also evident that this pseudogap is largest for CoP 3 , and decreases in width in the skutterudites with the heavier pnicogens and larger unit cells. These results are in good agreement with previous work 32,34,35 and will later be used in the interpretation of experimental and theoretical spectra obtained in this study.…”

“…31 Several band structure calculations have been performed on these and similar materials using DFT. [32][33][34] However, apart from measurements of the electronic conductivity and Seebeck coefficient, there are few direct comparisons between calculations and experimental results for these materials, particularly when it comes to detailed information about their electronic structure. [35][36][37][38] The aim of the present work is to compare DFT calculated and EELS extracted dielectric functions for crystals with more complex bonding than has previously been approached.…”

Linear bands, zero-momentum Weyl semimetal, and topological transition in skutterudite-structure pnictides

“…Thus there is tension between the Zintl picture, which does not lead to integral formal charges on both of the primary units, and the presence of the gap that specifies an integral number of occupied bonding states. Several papers 14,23,24,[29][30][31][32][33][34][35] have presented results and some analysis for empty and for a few filled skutterudites. We have benefited from the previous work as we proceed on a deeper analysis.…”

“…1 of Ref. 24. Below we make use of this transformation to follow the opening of the (pseudo)gap between occupied and unoccupied states.…”

Transition metald-band occupancy in skutterudites studied by electron energy-loss spectroscopy

“…2 Tailoring their electronic and thermal properties is therefore possible, and modified skutterudite materials are indeed among the most promising of a new generation of thermoelectric materials. [2][3][4] Several theoretical [5][6][7][8] and experimental [9][10][11][12][13] studies of the electronic structure of Cobased skutterudites have been performed. A particular interesting signature of the bonding and electronic structure is the occupancy of the 3d states of Co. Anno et al 12 used x-ray photoelectron spectroscopy ͑XPS͒ to study the occupied states of CoAs 3 and CoSb 3 .…”