  The effect of pressure on the optical-absorption edge of CdIn2S4, MgIn2S4,
and MnIn2S4 thiospinels at room temperature is investigated up to 20 GPa. The
pressure dependence of their band-gaps has been analyzed using the Urbach rule.
We have found that, within the pressure-range of stability of the low-pressure
spinel phase, the band-gap of CdIn2S4 and MgIn2S4 exhibits a linear blue-shift
with pressure, whereas the band-gap of MnIn2S4 exhibits a pronounced non-linear
shift. In addition, an abrupt decrease of the band-gap energies occurs in the
three compounds at pressures of 10 GPa, 8.5 GPa, and 7.2 GPa, respectively.
Beyond these pressures, the optical-absorption edge red-shifts upon compression
for the three studied thiospinels. All these results are discussed in terms of
the electronic structure of each compound and their reported structural
changes.
