Structural Considerations and Polarization Energy of Some PbFCl-Type Compounds

Some ionic compoundsMXYof the matlockite family (PbFCl) are characterized by the symmetry of the anionsXandYsurroundings. If the cage around the ionsXandYare regular, the crystal is said to be "regular" and these conditions of symmetry give four equations adequate to compute the lattice parameters. Moreover, the symmetries play a decisive part on some physical properties because they are at the origin of electronic distorsions that produce dipolar moments. More precisely, it should be mentioned thatXhas no dipolar moment because the field acting onXis zero and that only the cationMand the anionYmay have dipolar moments. Both the interactions of the induced moments as well as the polarization energy contribute to stabilize the lattice. Relations between local symmetries and polarization energy and its variation with pressure will be investigated. In the case of PbFI, with an increase in pressure the ion I moves in the cell which becomes more regular. At the same time, opposite variations of the fields and of the induced moments on the ions I and Pb occur so that the polarization energy decreases. This effect is only observable with the highly polarizable ion I and is negligible for Cl.