The only commercially available TlGaSe2 vdW crystals have been synthesized at our facilities through float zone technique. TlGaSe2 semiconductors belong to the thallium chalcogenides family with the chemical formula TlBX2 (where B=In or Ga, X=S, Se or Te). Members of this family have both layered (TlGaSe2, TlGaS2, TlInS2) and chain (TlInSe2, TlInTe2, TlGaTe2) structures. Stacking of the atoms in TlGaSe2, TlGaS2 and TlInS2 layered crystals is arranged in the form of two twisted anionic layers with the weak bonded Tl(+1) cations located in the trigonal cavities between them. As a result, these crystals exhibit layered structure with weak vdW coupling between the adjacent layers. In particular, TlGaSe2 is a direct gap semiconductor with optical band gap of 1.87 eV, it exhibits unique structural phase transition from normal (N) to a commensurate (C) state via an intermediate incommensurate (I) phase at 100K and 120K. They exhibit para-to-ferroelectric phase transitions, and also shows ferroelectric and ferroelastic behavior. Crystals come with guaranteed structural phase transitions.

Properties of TlGaSe2 layered crystals