Optical second harmonic generation (SHG) has been studied in TlMeX2 (Me=In,Ga, X=S,Se,Te) compounds with layered and chain crystalline structures. For layered TlInS2 and TlGaSe2, SHG signal has been observed in a range of temperatures below the point of phase transition into ferroelectric phase for which asymmetry is prominent. On the other hand, the chain TlInSe2 and TlGaTe2 have shown SHG signal at a temperature above 270 and 293K, respectively. Both the TlInSe2 and TlGaTe2 have symmetry centre at room temperature and second order optical non-linearity is an artefact incompatible with their bulk structure. Surface structure is speculated to be responsible for SHG signal observed in these materials for the first time to our knowledge. The details of temperature behaviour of SHG in TlInSe2 and TlGaTe2 lead to a phase-transition based model for explanation of the observed phenomenon. © 2013 WILEY-VCH Verlag GmbH &amp
Co. KGaA, Weinheim.