Design of Pyroelectric Mixed Crystals Having a Varying Degree of Polarity: The l-Asparagine·H₂O/l-Aspartic Acid System

Abstract

As part of an ongoing program on the design of functional materials with a varying degree of polarity, we investigated the processes of conversion of a nonpolar host, Lasparagine monohydrate crystal (space group P2 1 2 1 2 1 ), into a conglomerate of mixed polar sectors when grown in the presence of varying amounts of L-aspartic acid guest at the glass−aqueous/solution interface. The structure, composition, and the reduction of symmetry of the mixed crystals were confirmed with pyroelectric coefficient measurements, X-ray diffraction, and HPLC analysis and supported by atom−atom potential energy computations. The pyroelectricity measured at the ( 010) and (01̅ 0) faces imply the formation of hybrid crystals with top and bottom parts having opposite polarities. Pyroelectric coefficients measured at these two faces as a function of the occluded guest concentration increase linearly up to 8 wt %/wt of guest, followed by an enhancement of polarity at 8−12 wt %/wt of guest and subsequent reduction upon an increase up to 16 wt %/wt of guest. An interpretation of the magnitude of the pyroelectric effect on guest concentration is proposed.

Nonclassical Crystal Growth as Explanation for the Riddle of Polarity in Centrosymmetric Glycine Crystals

“…Thus, presence of both surface and bulk pyroelectricity implies that the nonclassical crystal growth mechanism preserves stereospecificity. This conclusion is in full agreement with the earlier described mechanism of occlusion of “tailor-made” auxiliaries in doped crystals. ,, It also implies that, after the attachment, the clusters undergo site-selective structural transformation, involving the removal of any hydrated capping layer.…”

Origin and structure of polar domains in doped molecular crystals

“…Studies of crystal doping demonstrated that the formation of a mixed molecular crystal is determined by the structure of the dopants and the structure of surface sites at which the guest molecules bind before their occlusion into the host crystal567. According to this mechanism, the dopants are incorporated in a polar mode within the bulk of the host8910 and thus convert non-polar hosts into mixed polar crystals9111213. The dopant-induced lattice polarity of the mixed crystal originates from two sources.…”

Engineering Crystal Properties through Solid Solutions

“…Lahav and co-workers investigated and explained the origin of polarity and pyroelectricity in cinnamamide/thienylacrylamide, and asparagine/aspartic acid systems. ,,, As discussed above, the reduction of crystal symmetry was attributed to a non-homogeneous distribution of the component within the mixed crystal. Hulliger et al have discussed the polarity in solid solutions of polar 4-chloro-4′-nitrostilbene molecules and nonpolar 4,4′-dinitrostilbene …”