  Our study of FeSe$ _ {1-x}$Te$ _ {x}$ crystals with x $<$ 0.15 shows that the
phase separation in these compositions occurs into phases with a different
stoichiometry of iron. This phase separation may indicate structural
instability of the iron plane in the studied range of compositions. To explain
it, we discuss the bond polarity and the peculiarity of the direct $d$ exchange
in the iron plane in the framework of the basic phenomenological description
such as the Bethe-Slater curve. With this approach, when the distance between
iron atoms is close to the value at which the sign of the magnetic exchange for
some $d$ orbitals changes, structural and electronic instability can occur.
Anomalies in the crystal field near the point of charge neutrality can also be
a significant component of this instability. A similar instability of the iron
plane may also be an important factor for other series of iron-based
superconductors.
