Crystal Fields of (NH₄)₂SO₄ Crystal

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The Madelung energies and the first and second derivatives of the crystal potentials at two unequivalent NH₄⁺ ion sites and one SO₄²⁻ ion site in (NH₄)₂SO₄ crystals have been calculated by the point charge model at various temperatures from 25 to −140°C. The Ewald’s method improved by Honma has been used for lattice sums. This new method has been less influenced by the choice of a type of a unit cell and has given more rapid convergence than the method in which summations are taken only over real space. The temperature dependence of the optical absorption spectra of (NH₄)₂SO₄: Tl is analyzed by using the crystal fields of the host crystal, and fairly good agreements with the experiment are obtained.