Structure–property correlations in highly modified Sr, Mn-borate glasses

Highly modified borate glasses with the composition (1 − 2x)MnO–x(SrO–B2O3) (x = 0.46, 0.42, 0.36, 0.25, and 0.20) were prepared and investigated by Raman, infrared (IR), and electron paramagnetic resonance (EPR) spectroscopy. Optical properties were studied in regard to photoluminescence, optical absorption, and refractive index. The Mn2 +/Mn3 + equilibrium was shifted towards the divalent manganese ion as a result of the strongly reducing melting conditions employed in this work, which facilitate the preparation of transparent glasses with up to 80 mol% total SrO and MnO content. Changes in the optical and physical properties within this glass series were related to structural variations. The structure of glasses with relatively low MnO content was found to involve mainly trigonal [BØ2O]− and tetrahedral [BØ4]− metaborate groups, which are replaced progressively by pyroborate [B2O5]4 − and orthoborate [BO3]3 − triangular units upon increasing MnO content. At the highest modification level (x = 0.20) the structure is built of orthoborate isomeric species in triangular [BO3]3 − and tetrahedral [BØ2O2]3 − configuration. The latter species form [B3O9]9 − rings, which reestablish some degree of network connectivity, as they involve three bridging and six non-bridging oxygen atoms, and this is reflected by the increase of the glass transition temperature for x = 0.25 over x = 0.20. Micro-Raman measurements showed structural inhomogeneities in these glasses due to chemical isomerization processes involving short- and medium-range order structures. Also, increasing MnO content was shown to cause MnO-clustering in the glasses as revealed by luminescence and EPR measurements.