A dangling-silicon-bond defect in topaz

Abstract

Fast neutron bombardment of natural topaz crystals creates an optically active point defect with an absorption peak near 2 eV, coloring the crystal blue. This blue color anneals near 500 °C. The optical absorption is strongly dichroic, with a maximum when E is along the a direction in the orthorhombic lattice. We identify the defect as an oxygen vacancy in an isolated SiO4 tetrahedron, in the form of a doubly occupied dangling silicon bond. The defect is nonmagnetic, but electron spin resonance of its Frenkel partner implies an average oscillator strength f=0.012. A natural blue topaz is colored by this same defect, and electron and gamma irradiations produce a similar blue, but with strikingly different polarization dependences.