The ultrastiff crystals of mucic (galactaric) acid

The upper limits of the stiffness of organic crystals, a newly established class of engineering materials, are an underexplored, yet very exciting domain of the material property space, because it could potentially provide ordered materials composed of light atoms with mechanical properties comparable to those of light metals and alloys. At present, however, the discovery of mechanically stiff and hard crystals is entirely serendipitous, since the relationships between the stiffness and crystal structure remain elusive. Here, guided by the very high density of the crystals of mucic (galactaric) acid, we investigated and report that this material is an ultrastiff organic crystal, with a Young's modulus obtained by nanoindentation on the (100)/(1̄00) pair of faces of E = 50.25 ± 1.55 GPa and a hardness of H = 2.81 ± 0.15 GPa (n = 15). This value of the modulus, which exceeds those of very stiff crystals such as α-glycine, tartaric acid and l-threonine, is corroborated by density functional theory (DFT) calculations, which provide an expectedly even higher value of E = 68.5 GPa. The measured moduli on the other two accessible faces are also above the higher approximate limit (25 GPa) of values that are common for organic crystals, with E = 29.25 ± 0.43 GPa (n = 70) for the (001)/(001̄) faces and E = 31.3 ± 0.9 GPa (n = 15) for the (010)/(01̄0) faces. Comparison with other organic crystals using material property plots not only underscores the unique combination of high stiffness and high density in this material, rooted in its extensive network of intermolecular hydrogen bonds, but also confirms that it has the highest-to-date measured surface stiffness among the organic crystals. This result is expected to guide the discovery of other extraordinarily stiff organic crystals that could be of interest as mechanically robust, lightweight materials.