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

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, l-asparagine monohydrate crystal (space group P212121), 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 (01̅0) 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.