In-situ low-temperature transmission electron microscopy of the structural phase transitions in a thiospinel CuIr2S4 compound

The structural phase transition in a thiospinel CuIr2S4 compound which is associated with a metal-to-insulator transformation at approximately 230 K was studied by in-situ low-temperature transmission electron microscopy. It was directly revealed that the structural transition is characterized by the formation of fine {110} twin lamellae and the appearance of additional electron diffraction spots along the [100]*, [110]* and [111]* directions of the room-temperature cubic CuIr2S4. On the basis of results of electron diffraction analysis, the lattice structure and symmetry of the CuIr2S4 low-temperature insulating phase were determined. Furthermore, we found that the characteristic additional diffraction spots for the insulating phase became quite faint or disappeared when the temperature decreased below approximately 50 K. This suggests a subsequent structural change in the insulating CuIr2S4 at extremely low temperatures.