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 (¹¹B, ¹³C, ¹⁵N, ¹⁹F, and ³¹P) solid-state NMR spectroscopy, variable temperature wideline ¹⁹F T₁ 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]₂SO₄·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”.