Superlattice formation in titanium diselenide

Experimental results are presented on the interesting $2a_0$ by $2c_0$ superlattice which forms in ${\mathrm{Ti}}_{1+x}{\mathrm{Se}}_2$ at low temperature. A second-order phase transition has been found at $T_c\cong 150$°K in crystals grown by iodine vapor transport at ∼800°C. This transition has been studied by electron and x-ray diffraction, magnetic susceptibility, resistivity, and infrared reflectivity. It appears to be a normal to lock-in transition with instability and strong diffuse scattering above $T_c$. It is suggested that the instability is Fermi-surface driven and that strong electron scattering occurs involving a zone-boundary phonon. A far-infrared lattice mode is observed which breaks up into several additional modes below the phase transition.