Inter-ionic potentials in solid cubic alkali iodides

Abstract

A non-empirical fully ionic description, with the anion wavefunctions in their compressed but still spherically symmetrical states optimal for the crystal, is presented 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 the RELCRION program which takes full account of relativistic effects. Both the dispersive attractions and energies arising from electron correlations of short range are computed.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 around 22 kJ mol −1 for KI and RbI are reduced to around 13 kJ mol −1 for CsI, with overestimations of some 0.2 au in the equilibrium cation-anion separations R decreasing as the metal becomes more electropositive. The prediction that the B2 phase of CsI is more stable (by 6 kJ mol −1 ) than the B1 polymorph agrees with experiment. For both KI and RbI, the zinc-blende polymorph is predicted to lie some 37 kJ mol −1 in energy above the B1 polymorph.An additional potential, plausibly ascribed to slight covalency, correcting these underestimations is derived semi-empirically.

Theoretical study of the structures and electronic properties of all-surface KI and CsI nanocrystals encapsulated in single walled carbon nanotubes

“…The system of ions plus nanotube is treated as a collection of mononuclear species interacting mainly through twobody interactions depending only on internuclear separations. Although, for no crystal to wall charge transfer, many of the methods [10][11][12][13][14] and some potentials 15 have been reported, a summary is needed. The intraionic interactions derived using an ionic description, supported for alkali halides by extensive evidence, 16,17 provide an excellent description of the structures and energetics of nonencapsulated bulk crystals.…”

“…27 The C 6 ͑GH͒ coefficients were calculated via the Slater-Kirkwood formula 29 from the polarizabilities ͑␣ G ͒ and electron numbers ͑P G ͒. The C 6 ͑XY͒ coefficients were derived 15 from the data in Table I using free ion ␣ C because there is extensive evidence 10,17,[30][31][32][33][34] that the properties of cations having p 6 outermost electronic configurations are essentially unaffected by their environment in crystal. The derivation of each P X from data for the isoelectronic noble gas should ensure that the C 6 ͑XY͒ coefficients are correct to within 5%.…”

“…14 The dipole-quadrupole dispersion coefficients were computed from the C 6 ͑GH͒ coefficients by using the Starkschall-Gordon formula. 38 The expectation values ͑Table II͒ for the outermost electrons were derived, as justified, 10,39 from free cation and free carbon atom wavefunctions but with anion functions computed for the equilibrium geometry of the bulk rocksalt structured crystal taking account of the in-crystal induced modifications 10,11,15 of the anion properties. The introduction of scaling factors into the calculation of the purely intracrystal coefficients ensures that these are correct to within 5%.…”

“…The latter computations, performed for the rocksalt structured bulk crystal over a range of closest cation-anion separations, yielded uncorrelated cation-anion V sCA 0 ͑r CA ͒, anion-anion V sAA 0 ͑r AA ͒, and cation-cation V sCC 0 ͑r CC ͒ potentials 15 which are exact given the wavefunctions of the two species. Use of the OHSMFS model introduces into the total crystal energy, the rearrangement energy needed to convert a free anion into its form optimal for the crystal having the considered geometry.…”

“…For all pairs, V sGH corr ͑r͒ was evaluated as the difference between the correlation energies of the interacting and noninteracting species using the local DFT of a uniform electron gas with the electron density in the interacting pair taken to be the sum of those of the noninteracting species, the Gordon-Kim method. 47 The correlation contribution to the rearrangement energy needed to calculate the V sCA corr ͑r CA ͒ was derived 15 by scaling 12 the prediction of local uniform electron-gas DFT introducing the Cowan modification 48 which eliminates terms describing the interaction of each electron with itself.…”