The structural role and coordination environment of cobalt in 45P2O5–CaO–Na2O phosphate glasses: thermal properties and Raman, UV–vis-NIR, and EPR spectroscopy

Abstract

Cobalt-containing materials are of interest for a wide range of applications, from biomaterials to solid-state lasers in optics. For instance, Co²⁺ is known to trigger the formation of new blood vessels, i.e. angiogenesis. Here, the use of phosphate glasses as a vehicle for local release of Co²⁺ ions is an attractive strategy to overcome the vascularisation limitation in tissue engineering. This study aimed to establish structure–property correlations as a function of the coordination environment of cobalt in 45P₂O₅–(30 − x)CaO–25Na₂O–xCoO (x: 0.01 to 10 mol%) glasses. Constant polymerization and O/P ratio, resulting ultimately in constant basicity, were shown by ICP-OES and Raman spectroscopy. The latter, combined with EPR analysis, indicated that Co²⁺ was the predominant oxidation state and the presence of Co³⁺ can be excluded. UV–vis–NIR absorption spectra showed that the ratio between Co²⁺ in four- and six-fold coordination remained constant throughout the glass series. Their thermal properties measured by DSC and heating microscopy did not change much in the substitution range studied here. The steady trend in T_g values suggests a compensation between two opposite effects caused by the presence of four and six-fold coordinated Co²⁺, both being present at a constant ratio throughout the glasses. Accordingly, the higher field strength of Co²⁺ compared to that of Ca²⁺ is expected to strengthen the glass network. In contrast, four-fold coordinated cobalt is expected to weaken the network by connecting fewer fragments of the phosphate glass network than six-fold coordinated cobalt. These results indicate that the structural properties of the glasses with constant basicity are influenced by the coordination number of Co²⁺.