Article pubs.acs.org/crystal
Design of Pyroelectric Mixed Crystals Having a Varying Degree of Polarity: The L‑Asparagine·H2O/L‑Aspartic Acid System A. Belitzky,† I. Weissbuch,† Y. Posner-Diskin,‡ M. Lahav,*,† and I. Lubomirsky*,† †
Department of Materials and Interfaces, ‡Chemical Research Support Unit, Weizmann Institute of Science, Rehovot 76100, Israel S Supporting Information *
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 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.
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INTRODUCTION Polar crystals are of topical importance in the pure and applied sciences as a result of their unique electrical, optical, and chemical properties.1−5 We have previously reported that polar crystals have the ability to induce freezing of supercooled water.6,7 The results of these studies raised a fundamental question about the possible role that might be played by the magnitude of a pyroelectric effect created at a given crystal surface on the freezing temperature of supercooled water. Consequently, we report here the design of single polar crystals, where the value of their pyroelectric coefficient can be varied on the same face in a controlled manner.8 Ten of the 32 traditional crystal classes are polar.9−11 Mechanistic studies on the formation of mixed crystals, however, have demonstrated that the repertoire of polar mixed crystals can be substantially augmented by reducing the symmetry of nonpolar crystals when grown in the presence of tailor-made auxiliaries.11−20 In addition, one of the advantages of such crystals is that their degree of polarity can be tuned by monitoring the amount of occluded guest molecules. However, previous studies on the reduction in symmetry have not investigated the correlation between the degree of polarity of a mixed crystal versus the amount of occluded guest molecules. For that purpose, we reinvestigated mixed crystals of Lasparagine·H2O (L-Asn)/L-aspartic acid (L-Asp)12,13 by applying a multidisciplinary approach of pyroelectricity10,11,18−20 and diffraction methods. This system is most appropriate for these studies since it yields large single crystals needed for the pyroelectric measurements and up to 16 wt %/wt of guest LAsp that can be occluded within the host.12,13 A nonlinear © XXXX American Chemical Society
correlation between the degree of polarity of the mixed crystals versus the amount of occluded guest molecules is reported and discussed below.
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EXPERIMENTAL SECTION
L-Asp·H2O powder with 98% purity was purchased from SigmaAldrich Israel. The mixed crystals were grown, from seeds set at the bottom of the vessel, using the slow evaporation method from oversaturated solutions at various ratios of L-Asn/L-Asp in Millipore clean water (18 MΩ). The growth experiments were performed at 23 °C and 45% relative humidity. After 3−7 days, few millimeter-wide plate-like crystals grew in the oversaturated solutions. The crystals were much larger compared to the seeds. The growth of the hexagonal, nontransparent plate-like crystals indicates that the crystals have occluded L-Asp. 21 Pyroelectric Measurements. Pyroelectric measurements were performed by a periodic temperature change technique with a setup shown in Figure 1. This method measures the pyroelectric effect of the surface layer, the thickness of which (0.1−400 μm) can be varied by the periodicity of the temperature change. The plate-like crystals of LAsn/L-Asp were glued to a copper plate using SPI silver paint. The measurements were performed using a modulated 800 mW, 1342 nm semiconductor laser. The crystals were given enough time to cool between laser pulses. The laser beam was spread to a spot much larger than the crystal size using a scattering lens. The exact heat flux at the surface was measured using a power meter. The pyroelectric current was measured by a low-impedance (
