Local structure of Ni2+ ions in fluorochloro- and fluorobromo-zirconate glasses

Nickel doped fluorochloro- and fluorobromo-zirconate glasses have been prepared following and modifying standard methods. Electron probe microanalysis has been carried out to obtain the actual chlorine and bromine content of the samples. X-ray and optical absorption measurements have been performed in order to study the Ni2+ neighborhood. Extended x-ray absorption fine structure and x-ray absorption near edge structure spectra at the Ni K edge show the progressive substitution of the fluorine ions in the first coordination shell of Ni2+ by chlorine or bromine ions when their concentration increases in the glasses. From the analysis of the absorption features due to the (3)A(2g)(F)-->E-1(g)(D) spin forbidden transition, up to four Ni2+ environments have been identified and assigned to: 6F(-), 5F(-) and 1Cl(-) or 1Br(-),1F(-) and 5Cl(-) or 5Br(-) and 6Cl(-) or 6Br(-). It is deduced that less than 10% of fluorine ions substitution is enough to have all the Ni2+ ions hexacoordinated to chlorine or bromine ions. NiCl2 and NiBr2 crystals have been found in the samples with an intermediate content of chlorine and bromine ions, respectively; Ni2+ ions tetrahedrally coordinated with Cl- or Br- ions are present in the most concentrated samples in chlorine or bromine ions, respectively. Values for the crystal field and Racah parameters, have been estimated from the absorption data for the hexa and tetracoordinated vitreous environments of Ni2+. (C) 1999 American Institute of Physics. [S0021-9606(99)70207-2].