Page 114 - Materials Chemistry, Second Edition

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2.3. The Crystalline State

display three colors (trichroic, Figure 2.66b). It should be noted that cubic crystals
do not exhibit pleochroism, since all unit cell axes are equivalent.
Crystals that do not possess a center of symmetry (i.e., noncentrosymmetric)
exhibit interesting properties when exposed to pressure – a phenomenon known as
piezoelectricity, from the Greek word piezein (to squeeze). As pressure is applied,
the crystal changes shape slightly by the movement of ions. The ionic migration
causes some of the positive and negative ions to move in opposite directions,
causing a polarization of charge. Conversely, if a piezoelectric crystal is placed in
an electric field, the ions move toward opposite electrodes, thereby changing the
shape of the crystal. All noncentrosymmetric crystallographic point groups will
exhibit piezoelectricity. It should be noted that 432 is also noncentrosymmetric,
but does not exhibit piezoelectricity.
At a temperature greater than the Curie temperature, T c , the lattice atoms can
migrate and cancel the effects of an external stress. However, at T < T c , the cubic
perovskite crystal becomes tetragonal; therefore the central cation (e.g., Ti 4þ
in BaTiO 3 ) becomes displaced, resulting in a net dipole moment and breaking the
charge symmetry of the crystal. The magnitude of the piezoelectric effect is not
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trivial; for a 1 cm quartz crystal exposed to a 2 kN (450 lb-force) external force will
generate 12 kV!
Piezoelectricity is the operating principle of quartz watches. In these devices, a
tiny crystal of quartz oscillates at a frequency of 32 kHz in response to an electrical
charge generated from the battery. In general, the overall size and composition of a
piezoelectric crystal will affect its oscillation frequency. Since quartz loses very
little energy upon vibration, the integrated circuit (IC, see Chapter 4) within a watch
is used to reduce the repeatable oscillations into electric pulses, which are displayed
as hours, minutes, and seconds on the watch face. The loud pop one hears when the
ignition button is pressed on a gas grill is the sound generated by a small spring-
loaded hammer hitting a piezoelectric crystal, generating thousands of volts across
the faces of the crystal – actually comparable to the voltage generated by an

automotive spark plug! Room humidifiers also operate via the induction of ca.
2,000,000 vibrations/s of a piezoelectric crystal, which is strong enough to cause
atomization of water molecules.
In a microphone, as one speaks, small changes in air pressure surrounding the
piezoelectric crystal cause tiny structural distortions that generate a very small
voltage. Upon amplification, these voltage changes are used to transmit sounds;
this principle has also been exploited for SONAR applications (“SOund Navigation
And Ranging”). The military has also investigated piezoelectric crystals mounted in
the boots of soldiers, whereby simple walking/running would provide current to
power electronic devices embedded in warfighter uniforms such as sensors, IR
shields, artificial muscles, etc.
Natural crystals that exhibit piezoelectricity include quartz (point group: 32),
Rochelle salt (potassium sodium tartrate; orthorhombic space group, 222), berlinite