Polymorphs and solid solutions: materials with new luminescent properties obtained through mechanochemical transformation of dicyanoaurate(I) salts.

We report the use of mechano- and thermochemical methods to create new solid-state luminescent materials from well-known inorganic salts, potassium dicyanoaurate(I) KAu(CN), and potassium dicyanocuprate(I) KCu(CN). In particular, manual grinding or ball milling of commercial samples of KAu(CN) led to the formation of a novel polymorph of the salt, herein termed -KAu(CN), evident by a significant change in color of the fluorescence emission of the solid material from orange to violet. The formation of -KAu(CN) is reversible upon addition of small amounts of solvents, and powder X-ray diffraction analysis indicates that the structure of -KAu(CN) might be related to that of pristine KAu(CN) through a change in ordering of Au(CN) ions in a layered structure. Thermal treatment of KAu(CN) led to the discovery of another polymorph of this well-known salt, herein termed -KAu(CN), making KAu(CN) a rare example of a system in which thermochemical and mechanochemical treatments lead to the formation of different, in each case previously not reported, polymorphic forms. The thermally-induced transformation from KAu(CN) to -KAu(CN) takes place around 250 °C and proceeds in a crystal-to-crystal fashion, which enabled the preliminary structural characterisation through single crystal X-ray diffraction, revealing the retention of the layered structure and a change in ordering of Au(CN) ions. Milling of the simple salt KAu(CN) in the presence of equimolar amounts or less of its copper(I)-based analogue coordination polymer KCu(CN) leads to the formation of a series of solid solution materials, isostructural to -KAu(CN) and with visible fluorescence emission distinct from KCu(CN) or any herein investigated forms of KAu(CN).