EXAFS Study of the Structure of Amorphous Nickel Borate

In this work, two amorphous hydrated nickel borates were synthesized with nickel chloride hexahydrate and borax as reactants at mole ratios of 1:2 and 1:8, respectively. The chemical compositions of these nickel borates were determined to be NiO center dot 0.8B(2)O(3)center dot 4.5H(2)O and NiO center dot B2O3 center dot 3H(2)O through thermogravimetry-derivative thermogravimetry (TG-DTG) and chemical analysis, in which the main anions were determined to be B3O3(OH)(5)(2-) and B2O(OH)(6)(2-), respectively, by Raman spectroscopy. The local structure of these samples was studied by synchrotron radiation extended X-ray absorption fine structure (EXAFS). Based on the processing and fitting of the EXAFS experimental data, the neighboring coordination atoms type, the interatomic distances, and the atom-pair numbers of Ni were determined. The fitting results of the amorphous nickel borate show that the local structure around the Ni atom has a similar structure to that of the Ni3B2O6 crystal. The neighboring coordination atoms of Ni in these amorphous nickel borates are O, B, and Ni. For NiO center dot 0.8B(2)O(3)center dot 4.5H(2)O, the interatomic distances for Ni-O, Ni-B, and Ni-Ni are 0.208, 0.263, and 0.311 nm, and the atom-pair numbers are 5.7, 3.8, and 3.8, respectively. The interatomic distances of NiO center dot B2O3 center dot 3H(2)O are 0.207, 0.262, and 0.310 nm, and the atom-pair numbers are 6.0, 4.0, and 4.0, respectively. The first shells of Ni2+ in NiO center dot 0.8B(2)O(3)center dot 4.5H(2)O and NiO center dot B2O3 center dot 3H(2)O are octahedral with six oxygen atoms.