Bismuth centers in alkali halides

Bismuth enters into the alkali halide lattice in its trivalent and/or divalent state if it is introduced by diffusion of bismuth vapors at appropriate temperatures. In its trivalent state its ground state is ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ and transitions from this level to the ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$, ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$, and ${}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ states are the well known $A$, $B$, $C$ bands of the $n{s}^2$ configuration. Such bands are observed in NaCl, KCl, KBr, and RbCl crystals doped with bismuth. In its divalent state bismuth has one intense band at ∼ 390 nm. On radiation damage or additive coloration it is possible to reduce the valence state, and by controlled experiments it has thus been possible to introduce and study the properties of ${\mathrm{Bi}}^{+++}$, ${\mathrm{Bi}}^{++}$, ${\mathrm{Bi}}^{+}$, ${\mathrm{Bi}}^0$, and even colloids of atomic bismuth in these crystals. When the colloids of bismuth are formed, the centers give the characteristic optical absorption band in the uv region (∼ 275 nm), give enhanced conductivity which can be attributed to thermionic emission of electrons from bismuth colloidal particles, and give characteristic conduction-electron spin-resonance signals (18 G in KBr). These features persist till the melting point of the crystal and their intensity is a function of concentration. Electron microscopic studies lend support to this interpretation.