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.com
Volume 3, Issue 2
ISSN: 2470-9905
Crystallography 2017
October 16-17, 2017
2
nd
International Conference on
October 16-17, 2017 | Chicago, USA
Applied Crystallography
Notes:
Structure, design and function of pyroelectric crystals
P
yroelectricity, a phenomenon first discovered in 314 BC by the Greek philosopher Theophrastus, who noticed that a
mineral, presumably tourmaline, attracts or repels ash, when exposed to a temperature change. It was not until the 17
th
century, that it was determined that pyroelectricity is the ability of some crystals to generate a temporary voltage which is
followed by attraction of depolarization charges from the surrounding. It was generally considered, that such property is
confined exclusively to the polar directions of the 10 out of 32 crystal classes. Experimental observation of pyroelectricity
associated with surface polarity was thought impossible because surface polarity attracts very little depolarization charges
to be detectable. During the last decade; however, improvement in instrumentation provided means to measure accurately
pyroelectric currents of few pico-amperes and pyroelectric coefficients of the order of 10
-13
C/(cm
2
K), which is 1:10000 with
respect to commercially important materials. This opens interesting opportunities to apply the pyroelectric effect as a tool
for searching disorders in crystals or for acquiring structural information about near polar surfaces of non-polar crystals.
One of the important advantages of the pyroelectric technique is that it allows studying a large variety of materials including
rough surfaces. Mechanistic studies of the formation of mixed crystals by intentional doping or by occluded impurities had
demonstrated reduction in the symmetry of the non-polar hosts by converting the latter into mixed crystals composed from
polar sectors. Furthermore, the occluded guests create constrainedpolar-domainswithin the host crystals, whichdetermine their
macroscopic properties. The structure of such domains can be elucidated at the molecular level by pyroelectric measurements
combined by computational techniques such as DFT (density functional theory) calculations and MD (molecular dynamics)
simulations. Here are described the application of these concepts for the following examples: (1) The structure of the polar
domains of the doped centrosymmetric α-glycine crystals with other α-amino acids, in concentrations less than 1%, were
determined at the molecular level by pyroelectric measurements combined with theoretical calculations. (2) The riddle of the
anomalous pyroelectricity from the centrosymmetric α-glycine crystals is resolved by considering the landing of large clusters,
present in the supersaturated solutions, during the growth of the crystals, in keeping with the non-classical mechanisms of
crystal growth. (3) The detection of enantiomeric disorder, by pyroelectricity along the non-polar directions, in the racemic
crystals of D,L alanine and D,L aspartic acid, which is not detectable by the diffraction techniques. (4) Freezing experiments
performed on the surfaces of LiTaO
3
studied in a specially designed set-up, revealed that positively charged surfaces enhance,
whereas negatively charged surfaces delay freezing. Differences in the operation of the pyroelectric effect on inducing ice
nucleation on hydrophilic and hydrophobic surfaces will be presented.
Biography
Meir Lahav has completed his PhD and Postdoctoral and then joined the Weizmann Institute. His scientific interests comprise solid-state and surface chemistry,
stereochemistry, the properties of polar crystals and the emergence of homochirality on Earth. He shared with Prof. L. Leiserowitz the Prelog Medal for Stereo-
chemistry from ETH, the G. Aminoff Prize for Crystallography from the Swedish Academy of Science and the Israel Prize.
meir.lahav@weizmann.ac.ilMeir Lahav
Weizmann Institute of Science, Israel
Meir Lahav, Struct Chem Crystallogr Commun, 3:2
DOI: 10.21767/2470-9905-C1-001