  Traditionally, the formation of off-stoichiometric compounds is believed to
be the growth effect rather than the intrinsic tendency of the system. However,
here, using the example of La3Te4, we demonstrate that in n-type gapped metals
having a large internal gap between principal band edges and the Fermi level
inside of the principal conduction band, Fermi-level instability can develop,
resulting in a reduction of formation energy for acceptor defects.
Specifically, La vacancies in La3Te4 form spontaneously to produce the acceptor
states and remove a fraction of free carriers from the principal conduction
band via electron-hole recombination. Such a unique self-doping mechanism
allows to stabilize a range of off-stoichiometric La3-xTe4 compounds, which
have different electronic properties. Moreover, we thus show how controlling
synthesis conditions can be used as a knob to reach the target functionality,
including controllable metal-to-insulator transition.
