3d-orbital determination of Ni2+ in NiP2 crystals by X-ray AO analysis

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=dx2-y2, b2=dz2, b3=dyz, b4=dzx and b5=dxy) 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.