Congrugent crystallisation of antimony sulphoiodide (SbSI) glass of stoichiometric composition, which is prepared successfully for the first time using rapid melt-quenching, has been investigated using differential scanning calorimetry (DSC). The results for glass powder show a glass transition at 127 deg C and two separate exothermal peaks with maxima around 140 deg C and 190 deg C. The ratio of the intensities of the exothermal peak at ~190 deg C to the peak at ~140 deg C increases as the particle size and heating rate are increased, but their total enthalpy remains constant at 62 +/- 2 J/g for all DSC runs. Surface heating of the glass induced by a 520nm CW laser shows two contracted regions: needle-like crystalline formations at low temperature and bulk crystallisation at high temperature. The observed phenomena and DSC results suggest two different kinds of crystallisation of the SbSI phases: one-dimensional crystallisation at low temperature which starts from the sampel surface and three-dimensional bulk crystallisation that contains the transformation to crystalline state at higher temperatures. The origin of the two different crystallisations can be traced to the strong anisotropy of the SbSI crystal structure due to the weak van der Waals interaction between covalent-ionic chains.
