  Motivated by the peculiar behavior of FeSi and FeSb2 we study the effect of
local electronic correlations on magnetic, transport and optical properties in
a specific type of band insulator, namely a covalent insulator. Investigating a
minimum model of covalent insulator within a single-site dynamical mean-field
approximation we are able to obtain the crossover from low temperature
non-magnetic insulator to high-temperature paramagnetic metal with parameters
realistic for FeSi and FeSb2 systems. Our results show that the behavior of
FeSi does not imply microscopic description in terms of Kondo insulator
(periodic Anderson model) as can be often found in the literature, but in fact
reflects generic properties of a broader class of materials.
