An analysis is made, in terms of the superposition model, of the second- and fourth-order spin-Hamiltonian parameters for the Eu2+ ion in orthorhombic C2v symmetry sites of the alkali halides. Values for the intrinsic parameters b̄2 and b̄4 and for the power law exponents t2 and t4 are reported for the divalent europium ion surrounded by fluorine, chlorine, bromine, and iodine ions as ligands. It is found that the b̄4 value of Eu2+ decreases as the polarizability of the ligand increases in agreement with the results previously reported for the isomorphic Gd3+ ion and for the transition metal ions Mn2+ and Fe3+. It is established, for the first time, that the value for the intrinsic parameter b̄2 of Eu2+ also decreases as the polarizability of the ligand increases. In order to carry out the analysis mentioned above, a theoretical calculation was made of the lattice distortion around the impurity ion caused by its substitution for the host alkali ion in all the alkali halides.

The superposition model is used to analyze for the first time the second-order spin–Hamiltonian parameters b02 and b22 for the Mn2+ ion in orthorhombic C2v symmetry sites of the alkali chlorides LiCl, NaCl, KCl, and RbCl. Values for the intrinsic parameter ?2 and for the power-law exponent t2 are reported for the divalent manganese ion surrounded by chlorine ligands. In order to test the applicability of the superposition model a calculation was made of the quadrupole b02 term describing the 6S5/2 ground state splitting of the Mn2+ ion in axial symmetry sites of the alkali chlorides, using the intrinsic parameters ?2 and the power-law exponent t2 previously obtained from the analysis of the spin–Hamiltonian parameters corresponding to the orthorhombic symmetry site. In order to carry out the analysis mentioned above, a theoretical calculation was made of the lattice distortion around the impurity ion caused by its substitution for the host alkali ion in all the alkali halides. Values for the binding energy of the ground and first excited states of the impurity Mn2+ -cation vacancy pair are also reported.

Experimental and theoretical study of the F absorption band were carried out in mixed ionic crystals as a function of temperature and molar composition. Both the point-ion and pseudopotential methods of Gourary -Adrian and Bartram-Stoneham-Gash, respectively, have been used to obtain the F band shift effect in KCII_,Br, mixed crystals with molar fraction x and the temperature T of the crystal. The theoretical values were compared with the experimental ones, and a good agreement between experimental and theoretical calculations has been found.

Values for the binding energy of the ground and first excited states of the impurity Eu2+–cation vacancy pair are reported for the alkali halides.

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