Page 115 - Materials Chemistry, Second Edition

102 2 Solid-State Chemistry

(AlPO 4 ), cane sugar, topaz (Al 2 SiO 4 (F, OH) 2 ), tourmaline group minerals (Ca, K,
Na)(Al, Fe, Li, Mn) 3 (Al, Cr, Fe, V) 6 (BO 3 ) 3 (Si, Al, B) 6 O 18 (OH, F) 4 , and even bone!
Synthetic examples include perovskites (e.g., BaTiO 3 , PbTiO 3 , Pb(Zr x Ti 1 x )O 3 –
PZT, KNbO 3 , LiTaO 3 , BiFeO 3 ) and polyvinylidene fluoride – PVDF. The latter
structure is not crystalline, but is comprised of a polymeric array of intertwined
chains that attract/repel one another when an electrical field is applied. This results
in a much greater piezoelectric effect than quartz.
Other noncentrosymmetric crystals that alter their shape in response to changes in
temperature are referred to as pyroelectric. These crystals are used in infrared
detectors; as an intruder passes the detector, the body warmth raises the temperature
of the crystal, resulting in a voltage that actuates the alarm. Even for such miniscule
temperature changes of a thousandth of a degree, a voltage on the order of 15 mV
may result, which is readily measured by electronic components. Crystals that
exhibit this effect are BaTiO 3 (barium titanate), PbTi 1 x Zr x O 3 (lead zirconate
titanate, PZT), and PVDF (polyvinylidene fluoride). Of the 32 crystallographic
point groups discussed earlier, 20 are piezoelectric and ten are pyroelectric
(Table 2.12).
The pyroelectric crystal classes are denoted as polar, each possessing a spontaneous
polarization that gives rise to a permanent dipole moment in their unit cells. If this
dipole can be reversed by the application of an external electric field (generating a
hysteresis loop), the crystal is referred to as ferroelectric. These crystals will exhibit
a permanent polarization even in the absence of an applied electrical field. As an
example, let us consider the pyroelectric crystal BaTiO 3 . Above the Curie temperature
(T c )of 130 C, BaTiO 3 is cubic (recall Figure 2.27). Since the positions of the positive

and negative charges balance one another, there is no net polarization. However, as the
crystal is cooled to temperatures below T c , the unit cell becomes distorted into a
tetragonal array where the Ti 4þ ion is moved from its central position. This yields
a finite polarization vector, resulting in ferroelectricity. It should be noted that all
ferroelectric crystals are both piezoelectric and pyroelectric, but the reverse is not
necessarily true.

Table 2.12. Piezoelectric and Pyroelectric Crystal Systems
Crystal Crystallographic point group
system
Centrosymmetric Non-centrosymmetric Non-centrosymmetric
Piezoelectric Pyroelectric
Triclinic 1 1 1
Monoclinic 2/m 2, m 2, m
Orthorhombic mmm 222, mm2 mm2
Tetragonal 4/m, 4/mmm 4, 4, 422, 4mm, 42m 4, 4mm
Trigonal 3, 3m 3, 32, 3m 3, 3m
Hexagonal 6/m, 6/mmm 6, 6, 622, 6mm, 6m2 6, 6mm
Cubic m 3, m 3m 23, 43m N/A