Structural parameters of intercalation compounds based on titanium dichalcogenides near the temperature of the polaronic band collapse

Using in-situ X-ray and synchrotron powder diffraction, the crystal structure of two types of materials, AgxTiTe2 and Fe0.2TiTe2, with charge carriers localized in form of small polarons, has been investigated in a temperature range including the point of polaronic band collapse (PBC). We have found that in case when the Fermi level is situated above the middle of the polaronic band, the PBC leads to lattice softness because of the increase of screening of interatomic interaction by localized electrons. When the Fermi level is situated between the middle and bottom of the polaronic band, the effect of PBC leads to a complicated temperature dependence on lattice properties due to several competitive factors: formation of a gap at the Fermi level because of impurity ordering, change of the screening condition because of electron localization and change of the vibration mode number because of structural phase transition of the order–disorder type.