Rigid-Ion Model of Ionic Crystal
To investigate the possibility of a 3-D intrinsic gap mode in ionic
crystals we use the tabulated rigid ion potential for NaI from Table's
2 and 4 of Ref..
The potential has the following form
The lattice constant for the fcc structure is chosen to
be 6.35A, within 1% of the experimental value . The
calculated TO frequency is WTO=2.51*1013rad/s,
within 5% of the experimental value of
The gap between the optic and acoustic branches extends from
In carrying out the MD calculations on a 216-ion cube with periodic
boundary conditions the method of Sangster and Dixon 
has been used to evaluate the Ewald sum. For this method the cut-off
distance in real space is approximately half the length, L/2, of the
cube (the interaction of sixth neighboring shell is counted), and the
reciprocal lattice is summed with the convergence parameter of
In order to investigate the role of point group symmetry on the
intrinsic gap mode parameters the zinc blende structure has also been
tested with the same potential. A local minimum of the lattice
energy occurs at the slightly larger lattice constant, 7.00A.
 J.Michielsen, P.Woerlee, F.V.D.Graaf, and J.A.A.Ketelaar,
J. Chem. Soc. Faraday Trans. II 71, 1730 (1975).
 M.J.L.Sangster, U.Schrooder, and R.M.Atwood,
J. Phys. C: Solid State Phys. 11, 1523 (1978).
 M.J.L.Sangster and M.Dixon, Adv. Phys. 25, 247 (1976).
Last modified: April 3, 1997