An Investigation of the Metaborate Monomer and Trimer Ions in Alkali Halide Crystals

Résumé du livre

The infrared spectra of the metaborate monomer and trimer ions in NaC1, KC1, and KBr were studied. These spectra contain bands due to the trimer in solid solution, in a second phase of the appropriate potassium or sodium salt, and perhaps on adjacent lattice sites. Furthermore, complex equilibria exist between the trimer in these three forms and the monomer. These equilibria depend upon the lattice, the boron concentration, and the thermal history of the crystal. The presence of the second phase and its relationship to the infrared bands was confirmed with light scattering experiments. A model for the trimer in solid solution is proposed in which the planar trimer, (BO2−)3, replaces three next nearest neighbor halide ions that form an equilateral triangle. Moreover, the trimer is most soluble in KC1 consistent with the argument that the solubility should be greater when the fixed distances between the three negative charges on the trimer more closely approach the anion-anion distance in the lattice. In contrast, the solubility of the monomer increases as the size of the lattice increases from NaCl to KBr. A normal coordinate analysis which employes a general valence force field predicts eigenvalues which are in fair agreement with the assigned values of the fundamentals, even though most of the force constants were either borrowed from related compounds or estimated. Values for the absorption coefficients of the monomer (w3) in KC1 and KBr, and the trimer (w6 and w-- ) in KC1 were determined.