The origins of the colours of natural yellow, blue, and green sapphires

Abstract

An analysis of the absorption spectra of natural yellow sapphires shows that the absorption is due to single Fe³⁺ ions and pairs of ions Fe³⁺-O₂--Fe³⁺. Assignments of all levels from the 4G, 4P, and 4D have been made as well as four simultaneous electronic excitations of a pair of ions. The temperature dependence of the intensity of the pair absorption shows that one pair is mainly involved, probably the fourth- nearest neighbour pair. The value of the Heisenberg exchange parameter (J S_a. S_b) lies in the range 30-40 K. Estimates of this parameter for various excited states have been made. Synthetic yellow sapphires have spectra which duplicate the natural specimens. Blue and green natural sapphires have, in addition to the bands present in the spectra of yellow sapphires, spectra with bands at 17800 (┴C), 14200 (//c), 11500 (┴C), and 10000 om^-1 (//c). The first two can be linked to Fe,Ti pairs and the evidence favours the nearest neighbour pair Ti⁴⁺-O^2?Fe²⁺ for the 17800 cm^-1 band and possibly the first neighbour pair for the 14200 cm^-1 band. The second two can be produced in synthetic crystals by growth from fluoride-containing flux and the evidence supports an explanation involving second-nearest neighbour pairs Fe²⁺- O^2--Fe³⁺ for the 11500 cm^-1 absorption and first neighbours for the 10000 om^-1 absorption.