Interplay between electronic and crystallographic instabilities in the low-dimensional ferroelectric CuInP2Se6

Temperature-dependent high-resolution ultra-violet photoemission spectroscopy (PES) measurements combined with tight-binding linear muffin-tin orbital (LMTO) atomic-sphere approximation (TB-LMTO-ASA) band-structure calculations are carried out to study the polar ordering in the low-dimensional ferroelectric CuInP2Se6. In the paraelectric phase, the energy-dependent PES spectra are in good agreement with our LMTO calculations. Below T-c approximate to 230 K, evidence for a strong redistribution of the experimental density of occupied Sates (DOS) is given. The LMTO calculations, accounting for the off-centre position of Cu atoms in the low-T phase, allow us to interpret this behaviour in terms of 4 redistribution of the partial Cu 3d DOS. This strong interplay between electronic and cystallographic instabilities is analysed in the light of a second-order Jahn-Teller effect acting as the driving force of the ferroelectric transition.