Experimental electron-density-distribution study of potassium iron disulfide, a low-dimensional material

The experimental electron-density distribution of potassium iron disulfide, KFeS₂, has been determined from high-resolution single-crystal x-ray intensity measurements collected with Ag K{α} radiation to a resolution of (sin{θ})/{λ}=1.30 A^-1. The structure of KFeS₂ consists of parallel chains of edge-sharing FeS₄ tetrahedra extending along the c-axis. In addition to the conventional structural parameters, multipole deformation functions and third- and fourth-cumulant anharmonic thermal parameters were included in some least-squares refinements. In all cases, inclusion of anharmonic thermal vibration parameters yielded significantly better fits to the x-ray data. From the experimental multipole population parameters of the iron atom, apparent d-orbital occupancies and the number of unpaired 3d_↑ electrons have been estimated within the crystal-field approximation and the assumption that a minimal basis set of atomic d orbitals is adequate for the d electrons. The values obtained for the number of 3d_↑ electrons from various refinements of the x-ray data agree well with values deduced from previous magnetic-susceptibility measurements and from theoretical calculations. The crystal-field splitting of the iron d orbitals and the ensuing asphericity is clearly evident in the experimental electron-density maps. The electric field gradient at the iron nucleus, calculated from the x-ray multipole parameters due to the slightly distorted (D2d) tetrahedral crystal field, predicts a quadrupole splitting of {Δ}EQS=3.3(10) mm/sec, somewhat larger than observed in M\"ossbauer experiments ({Δ}EQS=0.53 mm/sec). The major contribution to the electric field gradient comes from the valence-electron distribution, which is consistent with the observed asphericity of electron density around iron.