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Al3+ ions readily substitute for
Si4+ ions in quartz. When
such substitution occurs, a charge compensator is needed for charge
neutrality. Four compensators are
known in quartz: H+, Li+, Na+ and a hole
trapped at an oxygen ion. Above is a
schematic representation of the aluminum associated centers: Al-OH-
center, Al-M+ center (where M is an interstitial alkali, either Li+,
or Na+), and Al-hole center.
The Al-OH- center is formed when an interstitial proton
bonds to an oxygen ion.
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The aluminum related centers are related to
acoustic losses and radiation induced frequency shifts in quartz
resonators. These effects can be
reduced by an electrodiffusion process called “sweeping,” a process that removes
the interstitial cations (H+, Li+, Na+) from
the quartz lattice - see next two pages.
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The Al-hole center consists of a hole,
i.e., a missing electron, trapped in a nonbonding p orbital of an oxygen ion
located near a substitutional Al.
These are so weakly bound (0.03 eV) that, at room temperature, the
hole is rapidly jumping among the four oxygens surrounding the Al. Al-hole centers affect the optical
absorption (smoky coloration) of quartz.
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Oxygen vacancy centers, called E’ centers,
are another class of point defects.
These centers affect the ultraviolet absorption of quartz.
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L. E. Halliburton,
J. J. Martin & D. R. Koehler, “Properties of Piezoelectric Materials,” in
E. A. Gerber and A. Ballato, Precision Frequency Control, Vol. 1, pp.
1-45, Academic Press, 1985.
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L. E. Halliburton,
M. E. Markes, and J. J. Martin, “Point Defects in Synthetic Quartz: A Survey of Spectroscopic Results With
Application to Quality Assurance,” Proc. 34th Annual Symposium on Frequency
Control, pp. 1-8, 1980, AD-A213670. Proc. copies available from NTIS.
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L. E. Halliburton,
N. Koumvakalis, M. E. Markes, and J. J. Martin, “Radiation Effects in
Crystalline SiO2: The Role
of Aluminum,” J. Appl. Phys., Vol. 52, pp. 3565-3574, 1981.
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