ELECTRON MICROSCOPY STUDY OF THE NATURE OF THE LOW-TEMPERATURE PHASE TRANSITION IN α-MnSe

Electron microscopy and electron diffraction has been used to study the low temperature phase transition in α-MnSe by in situ investigations down to liquid nitrogen temperatures. Direct observation allowed to characterize the transformation from the cubic NaCl-structure to the hexagonal NiAs-structure as a shear transformation on the close packed planes. Various stacking schemes are discussed with the purpose of proposing a detailed mechanism for the shear transition in agreement with other physical measurements such as X-ray and neutron diffraction and Mossbauer spectroscopy. On the basis of diffraction contrast analysis of the images at the transformation faults in the cubic phase, a model is proposed whereby the NiAs-structure is obtained by the propagation of a suitable sequence of shockley partials. Intermediate faults leave anions in trigonal prismatic environment which could explain some features of the physical measurements