Structural Changes above the Glass Transition and Crystallization in Aluminophosphate Glasses:  An in Situ High-Temperature MAS NMR Study

We present an in situ high-temperature nuclear magnetic resonance study on the structural changes in aluminophosphate glasses occurring in the temperature range between the glass transition temperature T_g and the crystallization temperature T_c, T_g < T < T_c. Decisive changes in the network organization between T_g and T_c in potassium aluminophosphate glasses in the compositional range 50K₂O−xAl₂O₃−(50 − x)P₂O₅ with 2.5 < x < 20 could be monitored for the first time employing 1D ³¹P- and ²⁷Al-MAS NMR. Accompanying ex situ NMR experiments (³¹P-RFDR NMR and ³¹P-{²⁷Al} CP-HETCOR NMR) on devitrified samples were performed at room temperature to further characterize the phases formed during the crystallization process. The structural role of boronwhich is known to inhibit the crystallization process in these aluminophosphate glasseson short and intermediate length scales was analyzed employing ¹¹B-MQMAS, ¹¹B-{²⁷Al} TRAPDOR and ¹¹B-{³¹P} REDOR NMR spectroscopy.