The structure of the paraelectric and incommensurate phases of TlGaSe₂

The structure of TlGaSe₂ has been studied by triple-crystal X-ray scattering techniques. At room temperature in the paraelectric phase the structure is believed to be C2/c, but the authors found that some of the (h, h, l) Bragg reflections, those with h odd, are broadened along the c* direction. This is shown to arise because of faults in the stacking of the atomic sheets perpendicular to c*. A simple model is developed for the scattering which suggests that on average a stacking fault occurs in the single crystal about once every four planes. Between 117 and 110 K the crystal has an incommensurate structure with a wavevector ( delta , delta , ¹/₄) where delta approximately=0.02 reciprocal lattice units. At 110 K, delta jumps discontinuously to zero to produce a commensurate phase. These results confirm the earlier suggestions of an incommensurate phase in TlGaSe₂.