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 (01¯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.