Thermal Equilibrium of Color Centers in Doped KCl Crystals

Abstract

In additively colored crystals of KCl: Sr a thermal reaction between $F$ and $Z_2$ centers is observed. The reaction is reversible and approaches equilibrium in a few hours in the range of temperatures between 70°C and 170°C. The equilibrium value of the ratio between the concentrations of $F$ and $Z_2$ centers, ($\frac{x_F}{x_{Z2\mathrm{}}}$), does not depend upon the total amount of coloration nor upon the past history of the crystal, and for a given impurity content is a function only of temperature. For crystals containing 2.5×${10}^{-4\mathrm{}}$ molar fraction of Sr, the experimental points fit the equation: $\frac{x_F}{x_{Z2\mathrm{}}}=0.8\mathrm{}\times {10}^{-4\mathrm{}}\exp (-\frac{0.28}{\mathrm{kT}}), \mathrm{kT} \mathrm{in}\mathrm{ev}.$ A comparison of the latter empirical equation with those that can be derived through the application of the mass-action law to some specific models of the $F\leftrightarrows Z_2$ conversion is discussed.