Stability of the modulated structure of Ba 0.5em{}l kal lazurite and its recrystallization at a temperature of 600°C over a wide range of sulfur dioxide fugacities

The stability of three-dimensional incommensurate modulation in cubic lazurite from the Ba 0.5em{}l kal region is experimentally investigated at T = 600°C. It is found that the X-ray photoelectron spectra of the annealed samples exhibit a peak corresponding to sulfite and a split peak associated with the (Na,Ca)SO4 sulfate. It is assumed that the splitting is caused by the ordering of the complexes not involved in the framework. This assumption is confirmed by the presence of a similar split peak in the X-ray photoelectron spectra of triclinic lazurite. It is demonstrated that the initial modulation is retained at the fugacity fSO₂ = 8 × 10- 3 bar. The decisive factors responsible for the retention of the three-dimensional incommensurate modulation are the temperature and the fugacity of sulfur dioxide. The latter quantity should be close to the stability boundary of the basic lazurite structure. The growth and transformation mechanisms of the modulation formation are conidered.