Aluminum-related acoustic loss in AT-cut quartz crystals

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Abstract
The substitutional aluminum ions present in all quartz requires charge compensation. In as-grown synthetic quartz, interstitial alkali ions serve as the charge compensators. Irradiation at temperatures above 200 K replaces the alkali with either a proton or a hole trapped on an adjacent oxygen. Electrodiffusion (sweeping) can be used to replace the alkali with either a proton or another specific alkali. A study of the acoustic loss spectra of a series of 5-MHz fifth overtone AT-cut resonator blanks as a function of sweeping and irradiation has been completed. No acoustic loss peaks were observed at temperatures less than 100 °C which could be attributed to either the Al-Li+ or Al-OH− centers. Na-swept samples showed the expected large loss peak at 53 K while unswept samples had only very small 53 K peaks. This shows that in as-grown synthetic quartz lithium is the primary charge compensating ion. Irradiation removed the 53 K peak and introduced new peaks at 23, 100 K, and 135 K. Annealing studies show that these peaks can be attributed to the Al-hole center. The anneal study also shows that the decay of the Al-hole center is matched by the return of the alkali to the aluminum site.

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