Temperature-dependent evolution of RbBSi2O6 glass into crystalline Rb-boroleucite according to X-ray diffraction data

The temperature-dependent evolution of the glass into a crystalline phase is studied for a rubidium borosilicate glass of composition 16.7 Rb2O center dot 16.7 B2O3 center dot 66.6 SiO2 employing X-ray diffraction (XRD) data. A glass sample was prepared by melt quenching from 1500 degrees within 0.5 hour. The glass sample was step-wise annealed at 13 distinct temperatures from 300 degrees C up to 900 degrees C for 1 h at every annealing step. To investigate changes in the glass structure, angle-dispersive XRD was applied by using an energy-resolving semiconductor detector. The radial distribution functions (RDFs) were calculated at every stage. For polycrystalline states the crystal structure of the samples with different thermal history was refined using the Rietveld method. Comparing correlation distances estimated from RDFs of glass and polycrystalline samples and mean interatomic distances calculated for polycrystalline samples by using atomic coordinates after Rietveld refinement, it is concluded that the borosilicate glass under study is converted into the crystalline state in the temperature range of 625-750 degrees C (i.e. in the temperature range close to the glass transition range 620-695 degrees C as determined by differential scanning calorimetry by using of heating rate of 20 K/min) at an average heating rate of about 0.35 K/min. When the heating rate is increased up to 10 or 20 K/min, the crystallisation temperature shifts sharply up to 831-900 degrees C and 878-951 degrees C, respectively. XRD data give evidence that distinctive traces of cubic RbBSi2O6 appear from glass at about 625 degrees C and a two-phase range exists up to 750 degrees C. After annealing at higher temperatures (800-900 degrees C) the crystal structure practically does not change any more.