Multi-step structural phase transitions with novel symmetry breaking and inverse symmetry breaking characteristics in a [Ag₄I₆]^2- cluster hybrid crystal

In this study, a multi-step phase transition hybrid composed of (Pr-dabco)₂Ag₄I₆ clusters (Pr-dabco⁺ = 1-propyl-1,4-diazabicyclo[2.2.2]octan-1-ium) has been prepared and characterized by microanalysis, IR and UV-vis spectroscopy, TG and DSC techniques, etc. This hybrid is thermally stable up to ∼486 K with five phases in the temperature region below 486 K. The phase transition shows symmetry breaking (SB) character between phases II (space group P2₁/c) and III (space group Pa3), while inverse symmetry breaking (ISB) between phases II and I (space group Pbca), and it is rather exceptional for matter to exhibit simultaneously SB and ISB nature in two successive phase transitions. Most importantly, each phase transition is associated with a dielectric anomaly, and phase V appears to be a plastic crystal with extra high ac conductivity (>10^-2 S cm^-1). Our work opens up new avenues to find a multi-phase transition material in silver halide hybrids.