  1T-TiSe2 has a semimetallic band structure at room temperature and undergoes
phase transition to a triple-q charge density wave (CDW) state with a
commensurate superlattice structure (2a * 2a * 2c) below Tc ~ 200 K at ambient
pressure. This phase transition is caused by cooperative phenomena involving
electron-phonon and electron-hole (excitonic) interactions, and cannot be
described by a standard CDW framework. By Cu intercalation or the application
of pressure, this phase transition temperature is suppressed and
superconductivity (SC) appears. However, it is not clear what kind of order
parameters are affected by these two procedures. We investigated the crystal
structure of CuxTiSe2 and pressurized 1T-TiSe2 around the SC state by
synchrotron x-ray diffraction on single crystals. In the high-temperature
phase, the variation of structural parameters for the case of Cu intercalation
and application of pressure are considerably different. Moreover, the
relationship between the critical points of the CDW phase transition and the SC
dome are also different for the two cases. The excitonic interaction appears to
play an important role in the P-T phase diagram of 1T-TiSe2, but not in the x-T
phase diagram.
