Material Science and Engineering, Nagoya Institute of Technology^* Muroran Institute of Technology^**
○ZAW WIN^* Kiyoaki Tanaka^* Ichimin Shirotani^**

NiP₂ has a pyrite type structure (Pa3, a=5.46522(5) A) synthesized by one of the authors under high pressure. Electron density distribution (EDD) of NiP₂ was measured at 100K and 3d-orbital functions of Ni²⁺ in the trigonal crystal field was determined keeping the orthonormal relationships among d-orbitals by X-ray AO analysis. In the X-ray AO analysis each atom is divided into sub-shell orbitals, which are treated as pseudo atoms. It enables us analyze the inter-atomic electron transfer even in the non-stoichiometric complexes keeping the electro-neutrality of the unit cell. The data was refined by QNTAO program (K.T.). In order to analyze 3p-orbitals of P atoms on the 3-fold axis sp²-like hybridized orbitals were employed. Asphericity of EDD was refined shifting electronic populations, kappas and 3d-orbital functions as well as anharmonic thermal vibration (AHV) terms for 1362 reflections. The R factor reduced to 0.0109 and all parameters converged well. The localized electrons and a contraction of the outermost 3p orbitals change the effective atomic potentials and enhance AHV of constituent atoms. Peaks around Ni²⁺ on the deformation density maps were well explained after the AO analysis and d-orbitals, fi, expressed in terms of the five basis functions,(b₁=d_x2-y2, b₂=d_z2, b₃=d_yz, b₄=d_zx and b₅=d_xy) are determined as,
f₁=b₂, f₂=pb₃+qb₄+rb₅, f₃=rb₁+qb₃-pb₄, f₄=-pb₁-rb₄+qb₅, f₅=qb₁-rb₃+pb₅,
where p=0.714(5), q= 0.203(5), r=-0.670(4). Orbitals f₁ to f₃ are fully occupied and the electron population on the degenerated orbitals f₄ and f₅ is 1.77(2), indicating that 1.54 electrons are donated from P₂ moiety to Ni making Ni and P₂ almost neutral.