Concerning the use of standards for identifying coordination environments in glasses

It is an established methodology to use crystallographically well-defined standard materials for understanding site geometries in glasses. Here we discuss the benefits and the limitations of this approach for the investigation by Mössbauer and EXAFS of Fe2+ Fe3+ in aluminosilicate glasses. As a case study we specifically consider [5]Fe2+ [5]Fe3+ sites and whether these exist in our glasses; and if so, whether they have defined site geometries or occur simply as a consequence of the site distortions and Fe-O bond length distributions. Results are consistent with the existence of [5]Fe2+ and [5]Fe3+ but do not prove this because site distortion and a mixture of 4- and 6-coordinated sites can produce comparable results. This exemplifies the need for caution when interpreting glass data based on standards.