Crystallisation and annealing behaviour of two caesium borosilicate glass samples with approximate boroleucite composition were characterised by means of differential scanning calorimetry measurements as well as X-ray powder diffraction investigations with subsequent profile fitting of the observed patterns according to the Rietveld and/or Pawley method. While one sample crystallised primarily in an orthorhombic phase (a = 6.592(3) Å, b = 11.825(5) Å, c = 12.620(6) Å; possible space groups: Pmc21 ((26), setting: P21am), Pma2 (28) and Pmma ((51), setting: Pmam)) which has been unknown until now and transformed irreversibly into the well-known cubic boroleucite phase at T ≈ 850 °C, cubic caesium boroleucite (space group: Ia3̅d (230)) was crystallised directly from the other sample. In this case, the orthorhombic phase was metastable. At the onset temperature of glass crystallisation Tx, on = 770 °C, it existed only for about 75 min. The change of the lattice parameter a of the cubic unit cell of caesium boroleucite in the course of the thermal treatments was explained by the alteration of the size of the TO4 tetrahedra (T = Si, B).
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