Charge disproportionation in TlGaSe₂ crystals detected by dielectric spectroscopy

The electrical properties of TlGaSe₂ ferroelectric-semiconductor with layer crystalline structure in the direction parallel to the layers plane is studied by impedance spectroscopy in the frequency range 30 Hz-15 MHz. The impedance spectra were analyzed together with earlier results for perpendicular direction. Low frequency dispersion due to the interfacial polarization from Maxwell-Wagner type space charge effect has been observed. It was established that anomalous rise of relaxation time in the 145-190 K temperature range far from the known phase transitions temperatures is characteristic only for the polarization perpendicular to the layers plane. The model of a specific phase transition taking place in 145-190 K temperature region assuming carriers accumulation was proposed. Charge disproportination is the main characteristic feature of this phase transition which leads to the charge density modulation in one crystallographic direction perpendicular to the layers plane of the crystal.