X-RAY STUDY ON THE ^|ᵃlpha;-^|ᵇeta; TRANSITION OF Ag2S

The refinement of the structure of α-Ag2S was undertaken together with an accurate determination of the lattice constants. The final structure gave the R-factor of 16.8% including unobservable reflections. The crystals found in nature are almost invariably twinned, and a theoretical consideration on the modes of twinning led to the conclusion that twenty-eight twin laws are possible. Heat treatments of the specimens were carried out in order to know the details of the progress of twin formation premonitory to the α-β transition. The specimens remain substantially unaltered up to 152-156°C, the only change being observed in the β -angle. Above this temperature range, individuals of twins in various orientations appear, and the X-ray reflections take an appearance of trains of spots. At about 165°C these spots along each line fuse into one another, giving an arc-like intensity distribution. Some nodes of strong intensities along these arcs are converted into sharp reflections of the high-temperature form at the transition temperature, 173°C. As the rise of temperature, therefore, the structure of Ag2S makes the first approach to the β-form by giving itself more maked cubicity through the change in the β-angle, at the next step simulates the symmetry of the high-temperature form by twinning, and finally at the transition temperature rearranges into parallel orientation the twin individuals which have been deranged owing to their obliquity.