X-Ray Study of the 2H-12R Structural Transformation in PbI₂

Abstract

An X-ray diffraction study was made on a structural transformation between type 2H, the low temperature form, and type 12R, the high temperature form, in PbI₂ crystals. 2H crystals grown in gel at room temperature were changed to type 12R through the course 2H→faulted 12R→faulted 12R′→12R by successive heat treatments. The faulted 12R structures could be analyzed by using a simplest model in which the layer stacking was specified by only one parameter. It was revealed that in a faulted 12R structure there exist all n consecutive layers in the same proportion. It was found from the fact that the transformation from gel-grown 2H to 12R proceeds straightforwardly through a short route to 12R. A study on the reverse transformation from 12R to 2H in the gel-grown 2H crystals and on the 2H-12R transformation in 12R crystals grown from vapour has shown that these transformations occur directly.