Study of the ferroelectric phase transition of TlGaSe2 by dielectric, calorimetric, infrared and X-ray diffraction measurements

Abstract

The real part of the dielectric constant e′, the heat capacitycp, the infrared reflectivity, and the X-ray diffraction of TlGaSe2 have been measured in the temperature range from 12 K (30K) to 300 K. Both e′ andcp show two anomalies at about 110 K and 120 K. A study of the hysteresis loop as well as an investigation of the dielectric dispersion in the microwave region show that the phase below 110 K is ferroelectric. The crystal structure remains nearly unchanged in the course of the phase transition. The loss of the\(\bar 1\) symmetry (C2/c→Cc) results from small positional shifts of the T1 atoms in the ab plane accompanied by a discontinuity in the axial ratios. We suggest, that the ferroelectricity is caused by the stereochemically active electron lone pair configuration of the Tl+ ion. Thus TlGaSe2 may provide the first example for ferroelectricity caused by this mechanism.