Organic Polar Crystals, Second Harmonic Generation, and Piezoelectric Effects from Heteroadamantanes in the Space Group R3m

Polar crystalline materials, a subset of the non‐centrosymmetric materials, are highly sought after. Their symmetry properties make them pyroelectric and also piezoelectric and capable of second‐harmonic generation (SHG). For SHG and piezoelectric applications, metal oxides are commonly used. The advantages of oxides are durability and hardness – downsides are the need for high‐temperature synthesis/processing and often the need to include toxic metals. Organic polar crystals, on the other hand, can avoid toxic metals and can be amenable to solution‐state processing. While the vast majority of polar organic molecules crystallize in non‐polar space groups, we found that both 7‐chloro‐1,3,5‐triazaadamantane, for short Cl‐TAA, and also the related Br‐TAA (but not I‐TAA) form polar crystals in the space group R3m, easily obtained from dichloromethane solution. Measurements confirm piezoelectric and SHG properties for Cl‐TAA and Br‐TAA. When the two species are crystallized together, solid solutions form, suggesting that properties of future materials can be tuned continuously. 7‐halo‐1,3,5‐triazaadamantanes with halo=chloro, bromo form polar crystals from solution, exhibiting non‐linear optical and piezoelectric effects. The chloro compound and the bromo compound form substitutional solid solutions.