The electronic structure of an iron- and indium-related center in silicon h
as been studied by Fourier transmission infrared spectroscopy, employing un
iaxial stress and polarization spectroscopy. The center gives rise to an ex
citation spectrum centered at about 6550 cm(-1), and is closely related to
the orthorhombic FeIn pair center with an excitation spectrum at about 6100
cm(-1). The two spectra are always observed in our Fe and In codoped sampl
es, and show metastability properties similar to those that have been repor
ted previously for the trigonal and orthorhombic FeIn pair centers. In this
paper, uniaxial stress results are reported which show unambiguously that
the 6550-cm(-1) spectrum involves a center with trigonal symmetry. The pola
rization properties of the stress split lines show that the initial state o
f the transitions transforms as the Gamma(4) irreducible representation of
the C-3v point group which is identical to the one previously deduced for t
he trigonal FeIn center by electron paramagnetic resonance. One of the exci
tation lines shows interesting polarization rules, which are discussed in d
erail and explained as a superposition of a sigma and a pi dipole at a C-3v
center. The origin of the spectrum is tentatively assigned to transitions
from the deep ground state to shallowlike hole states. The symmetry assignm
ent of the center and its ground state gives strong evidence that the cente
r studied is indeed the trigonal version of the FeIn pair center. [S0163-18
29(99)04023-0].