Inter-ionic potentials in solid cubic alkali iodides

Abstract

A non-empirical fully ionic description, with the anion wavefunctions in theircompressed but still spherically symmetrical states optimal for the crystal, ispresented for the cohesive energetics of two cubic phases of three solid iodides,KI, RbI and CsI. The non-correlated part of the energy is computed using theRELCRION program which takes full account of relativistic effects. Both thedispersive attractions and energies arising from electron correlations of short range arecomputed.For each polymorph stable under ambient conditions, the rock-salt (B1)phases of KI and RbI and the eightfold coordinated (B2) phase of CsI,the cohesion is slightly underestimated. The lattice energy deficits of around22 kJ mol−1 for KI and RbI arereduced to around 13 kJ mol−1 for CsI, with overestimations of some 0.2 au in the equilibrium cation–anion separationsR decreasing as the metal becomes more electropositive. The prediction that the B2 phase of CsI is morestable (by 6 kJ mol−1) than the B1 polymorph agrees with experiment. For both KIand RbI, the zinc-blende polymorph is predicted to lie some37 kJ mol−1 in energy above the B1 polymorph.An additional potential, plausibly ascribed to slight covalency, correcting theseunderestimations is derived semi-empirically.