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

An X-ray diffraction study was made on a structural transformation between type 2 H , the low temperature form, and type 12 R , the high temperature form, in PbI 2 crystals. 2 H crystals grown in gel at room temperature were changed to type 12 R through the course 2 H →faulted 12 R →faulted 12 R ' →12 R by successive heat treatments. The faulted 12 R 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 12 R structure there exist all n consecutive layers in the same proportion. It was found from the fact that the transformation from gel-grown 2 H to 12 R proceeds straightforwardly through a short route to 12 R . A study on the reverse transformation from 12 R to 2 H in the gel-grown 2 H crystals and on the 2 H -12 R transformation in 12 R crystals grown from vapour has shown that these transformations occur directly.