THE STABILITY OF POLYMORPHS OF MGCL2 - AN AB-INITIO STUDY

Periodic Hartree-Fock calculations show that the layered structures of alpha- and beta-MgCl2 are stable at the HF level; earlier work has su ggested this was not so for beta-MgCl2. The neglect of correlation lea ds to a large overestimate of the Cl-Cl interlayer separation and henc e of the c lattice parameter. Structures optimised using density funct ional theory and the same basis set are in better agreement with exper iment. alpha- and beta-MgCl2 are close in energy at HF and DFT levels, and vibrational effects are likely to be important in determining the ir relative stability. Neglecting possible distorted variants of the m ore ionic rutile and fluorite structures, we estimate transition press ures of similar to 17 GPa for beta-MgCl2 --> rutile and similar to 77G Pa for rutile --> fluorite. (C) 1997 Elsevier Science B.V.