Polytypism and phase transitions in TlInS2 and TlGaSe2 crystals

Low-temperature X-ray diffraction study of TlInS2 and TlGaSe2 crystals have been performed. The thermal expansion coefficients along the [001] directi on of the crystals are determined within the temperature range 80-250 K. It is shown that the incommensurate phase with the modulation wave-vector q(i ) = (1/4 +/- 6)c* in these crystals is formed in the temperature ranges 196 -214 and 107-114 K, respectively. The polytype modification of TlGaSe2 with the lattice parameter c approximate to 120 Angstrom is revealed. No supers tructural reflections were recorded for the long-period polytype modificati on.