Phase separation near the charge neutrality point in FeSe1-xTex crystals with x < 0.15

Our study of FeSe1-xTex 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. We tentatively propose an explanation of the structural instability of the iron plane in the studied layered compounds in terms of the possible change in the bond polarity and the peculiarity of the direct d-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.