Electronically driven structural transitions in A10(PO4)6F2 apatites (A = Ca, Sr, Pb, Cd and Hg).

It is shown that there is a dynamic lattice instability in the aristotype P63/m structure of A10(PO4)6F2 apatites containing divalent A-site Cd or Hg cations with (n - 1)d(10)ns(0) electronic configurations. The distortion to a low-symmetry P\bar{1} triclinic structure is driven by an electronic mechanism rather than from ionic size mismatch. Our theoretical work provides key insights into the role of the electronic configurations of A cations in fluorapatites.