XANES and crystal field spectroscopy of five-coordinated Nickel(II) in potassium-nickel phosphate

Abstract

The first spectroscopic data on KNiPO 4 used as reference compound for 5-coordinated nickel in crystalline oxide compounds are presented. The Ni-K edge X-ray Absorption Near Edge Structure (XANES) spectrum is compared to those of 4- and 6-coordinated Ni in NiCr 2O 4 and CaNiSi 2O 6, respectively. The intensity of the pre-edge increases from 6- to 4-coordination while that of the main-edge decreases. Both features are thus sensitive indicators of the Ni coordination state. Crystal field spectra are also different for the three nickel coordination states. The KNiPO 4 crystal field spectrum exhibits a position and relative intensity of the absorption bands similar to those found in the spectra of pentacordinated Ni complexes. They are interpreted using the electron energy level diagram of Ni(II), in a distorted square pyramid crystal field. Criteria for the use of XANES and crystal field spectroscopy to characterize the presence of 5-coordinated Ni in oxide compounds are given. The similar crystal field stabilization energy encountered in 5- and 6-coordinated nickel explains the stability of 5-coordinated nickel in phosphates.