Engineering plastic phase transitions via solid solutions: the case of "reordering frustration" in ionic plastic crystals of hydroxyquinuclidinium salts

A family of salts of R-(+)-(3)-hydroxyquinuclidinium [QH](+), with SO42-, BPh4-, BF4- and PF6- counter-anions, have been prepared by the metathesis of [QH]Cl and metal salts of the corresponding anions. Solid solutions of formula [QH](PF6)(x)(BF4)(1-x) for x = 0.9, 0.8, 0.7 have also been obtained. The crystalline materials have been investigated by a combination of solid-state techniques, including variable temperature XRD, thermal analyses, multinuclear (B-11, C-13, N-15, F-19, and P-31) solid-state NMR spectroscopy, variable temperature wideline F-19 T-1 relaxation measurements, and micro-Raman spectroscopy to investigate their thermal stability and phase transition behaviors. It has been shown that the salts [QH]PF6 and [QH]BF4 undergo an order-disorder solid-solid phase transition to plastic phases, whereas [QH](2)SO4 center dot H2O and [QH]BPh4 do not display any plastic phase transition. Doping [QH]BF4 into the [QH]PF6 lattice up to 30% results in the formation of a solid solution that is plastic in an expanded thermal range, thanks to a phenomenon that we describe here for the first time as "reordering frustration".