The NaCl- to CsCl-type phase transition discussed on the basis of the cP to cI deformation with the symmetry Cmcm 4(c) m2m
A structure forming a cubic primitive lattice cP may be deformed into a structure forming a cubic body-centred lattice cI in the space group Cmcm at position 4(c) m2m 0,y,1/4. If in related structures the sites are alternately occupied by unlike atoms, the NaCl and the CsCl types occur, respectively. The corresponding phase transition can be described as a deformation of a heterogeneous sphere packing in the subgroup Pmmn (a,-c,b) of Cmcm. All sphere configurations with symmetry Cmcm 4(c) m2m were derived. On the basis of this information, further possibilities for phase transitions that also correspond to sphere-packing deformations were found with this symmetry. Two of them possibly may take place in metals. The first one leads from a primitive hexagonal lattice to a hexagonal close packing, the other from a cubic body-centred lattice also to a hexagonal close packing.
Sowa
H
Hahn-Meitner-Institut, Bereich Strukturforschung NE, Berlin, Germany. heidrun.sowa@t-online.de