Effect of modifier cation size on the structure, properties and nickel speciation in BK7 type alkali borosilicate glasses

Alkali-borosilicate glasses 16M2O-10B2O3-74SiO2 were studied for M = Li to Cs. 11B NMR showed 57% four-fold coordinated boron for Li, but 88 to 97% for M = Na to Cs. The fraction of neutral borate triangles decreases from Na to Cs. Vibrational spectroscopy shows significant non-bridging oxygen on silicate species for Li, decreasing to a low level from Na to Cs. The glass transition temperature is highest for M = Cs and correlates with increasing network connectivity. Sub-liquidus phase separation and homonuclear bonding was observed for M = Li, while M = Na to Cs show good linking of silicate with borate tetrahedra. Raman spectroscopy reveals a maximum of danburite rings for M = K, the glass with the highest packing density. Glasses doped with Ni2O3 changed color from yellow (Li) to purple (Na), to blue in samples of higher optical basicity (K, Rb, Cs). Tetrahedral Ni2+ likely coexists with trivalent nickel, Ni3+, in blue glasses, while higher coordinated Ni2+ prevails in the yellow Lisample.