  The problem of localization of the resonant impurity states is discussed for
an illustrative example of Titanium doped Lead Telluride. Electronic structure
of PbTe:Ti is studied using first principles methods, densities of states and
Bloch spectral functions are analyzed. We show that Ti creates resonant states
in the conduction band of PbTe, however, spectral functions of the system
strongly suggest localization of these states and show poor hybridization with
PbTe electronic structure. The contrast between results presented here and
previously reported spectral functions for PbTe:Tl correlate very well with the
different effect of those impurities on thermopower (S) of PbTe, which is large
increase is S for PbTe:Tl and almost no effect on S for PbTe:Ti. Moreover,
magnetic properties of the system are studied and formation of magnetic moments
on Ti atoms is found, both for ordered (ferromagnetic) and disordered
(paramagnetic-like) phases, showing that PbTe:Ti can be a magnetic
semiconductor.
