On the yellow color of gamma‐irradiated brazilianite from Minas Gerais (Brazil).

Abstract

The origin of gamma irradiation-induced strong yellow color in brazilianite from Brazil is investigated. The irradiation- induced optical absorption band responsible for the color shows an onset in the blue spectral region at about 2.5 eV and

maximum centered in the UV at ~ 4.24 eV. From the ratio between the squared value of the line width (W2

) and the peak energy (M), a value of about 0.09 eV is estimated, which is consistent with an absorption band caused by a Schirmer ́s-type O− bound small polaron. By electron paramagnetic resonance (EPR), we are able to confrm its microscopic structure. This O− hole center is in fact the Al3+–O−X2+–P5+ hole center already identifed by EPR earlier in the literature (where X2+ stands for a nearby divalent cation with negligible abundance of magnetic isotopes). The EPR spectrum of the Al3+–O−X2+–P5+

hole center, along with the O− bound small polaron absorption band responsible for the yellow color, appears simultane- ously at high concentrations after gamma irradiation, and vanishes together for thermal annealing above 300 °C, returning

after re-irradiation in a reversible way. Their appearance is concomitant with the H0

centers and Ti3+ electron centers, and

possible charge-compensating centers.