Role of alkali field strength on the speciation of Ni2+ in alkali borate glasses: comparison with crystalline Ni-borates

This work presents a comparative spectroscopic study of Ni speciation in binary alkali borate glasses, with up to 35 mol% alkali oxides, using optical absorption spectroscopy and Ni K-edge X-ray Absorption Near Edge Structure (XANES) and taking advantage of comparison with previous structural information obtained with Ni K-edge Extended X-ray Absorption Spectroscopy (EXAFS) on the same samples. Nickel-bearing K, Na or Li borate glasses exhibit an exceptional diversity of coloration (from green to yellow, brown, and purple). This is due to a progressive coordination change of Ni2+ with increasing alkali content, from 6-, to 5- and 4-Ni coordination, each coordination state having a peculiar structural significance. [6]Ni2+ is predominant in low alkali borate glasses, where Ni octahedra cluster in ordered Ni-rich domains. [4]Ni2+ is formed at low alkali cation field strength: it predominates in high potassic glasses but it is only a minor coordination state in Na-borate glasses and it is absent in Li-borate glasses, in which, by contrast, [5]Ni2+ prevails. For these three coordination states, the comparison with the structure of crystalline borates suggests a diversity of linkages between Ni2+ and the borate polymeric network. Among the alkali borate glasses investigated, each Ni-coordination state shows similar spectroscopic properties. This implies that the modification of Ni speciation with glass composition arises only from a variation of the relative contribution of 6-, 5- and 4-coordinated Ni2+ without significant modification of the spectroscopic parameters characteristic of each site. The coordination number of Ni2+ is correlated with the field strength of the modifier cations present in the glass: higher the field strength of the modifier cations, higher the nickel coordination number. High-K borate glasses favor the formation of [4]Ni2+, as this coordination state is not observed in Li- and is only minority in Na borate glasses, because of Li and Na preference for charge compensating [4]B instead of [4]Ni2+. Due to the similarities between Ni2+ and Mg2+, the diversity of the structural scenarios around Ni2+ in alkali borate glasses may shed light on the structural behavior of Mg in the same glasses, providing additional evidence of the control of Mg coordination by the field strength of the alkalis.