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 Sa. Sb) 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+-O2?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+- O2--Fe3+
for the 11500 cm-1 absorption and first neighbours for the 10000 om-1
absorption.
Heisenberg-exchange-free nanoskyrmion mosaic
