The Atomic Position and Size of the Thallium Ions in KCl(Tl) Phosphors

A new X-ray investigation has been made of the KCl(Tl) phosphor in view of the discrepancy between the early theory of Fromherz involving formation of complexes in the KCl(Tl) phosphor and the recent theoretical work of Williams, in which substitutional entry of the thallium ions is assumed. The present study has now shown ( a ) that the expansion of the potassium chloride structure due to the incorporation of thallium is in agreement with Vegard's law and ( b ) that Tl + ions enter the structure substitutionally. The present results differ from those obtained by Stasiw and Saur in a less detailed X-ray study. The intensities of X-ray reflections have also been used to establish the atomic position of the activator atoms in the phosphor and the results fully confirm the interpretation of the observed lattice expansions. A new value of 1.42 7 Å has been obtained for the ionic radius of the Tl + ion in six-fold coordination with Cl - ions. This is of importance in the detailed calculations of the luminescence of KCl(Tl) which have been made by Williams and others.